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HYGIENIC COFFEE PROCESSING AND TECHNOLOGIES


Conselho Diretor do Consórcio Pesquisa Café

Membros: Pedro Antonio Arraes Pereira – Presidente – Embrapa Elionaldo de Faro Teles – EBDA Antônio Lima Bandeira – Epamig Halmilton Humberto Ramos – IAC Florindo Dalberto – Iapar Evair Vieira de Melo – Incaper José Gerardo Fontelles – MAPA Silvio José Elia Galvão – Pesagro Antônio Nazareno Guimarães Mendes – Ufla Nilda de Fátima Ferreira Soares – UFV

Coordenadores Institucionais: Ramiro Neto Souza do Amaral – EBDA Paulo Cesar Afonso Junior – Embrapa César Elias Botelho – Epamig Terezinha de Jesus Garcia Salva – IAC Marcos Antônio Pavan – Iapar Romário Gava Ferrão – Incaper Edilson Martins Alcântara – MAPA/DCAF Wander Eustáquio de Bastos Andrade – Pesagro Rubens José Guimarães – Ufla Laércio Zambolim – UFV Comitê Local de Publicações – Embrapa Café Maria Isabel de Oliveira Penteado – Presidente Adriana Macedo – Secretária Executiva Paulo Cesar Afonso Junior Carlos Henrique Siqueira de Carvalho Mauricio Sérgio Zacarias


HYGIENIC COFFEE PROCESSING AND TECHNOLOGIES

Juarez de Sousa e Silva Pedro Amorim Berbert Roberto Precci Lopes

Brasília – DF 2011


© Embrapa Café Todos os direitos reservados. A reprodução não autorizada desta publicação, no todo ou em parte, constitui violação dos direitos autorais (Lei n º 9610)

Revisão Técnica e Lingüística: Todo o conteúdo técnico e a revisão ortográ�ca e gramatical são de inteira responsabilidade dos autores.

Produção e editoração: Thiago Farah Cavaton

1ª edição, tiragem: 500 exemplares Aquisição de exemplares: Embrapa Café Pq. Estação Biológica s/n – Ed. Sede – 3º andar 70770-901 – Brasília - DF Telefone: (61) 3448 4566 negocios.cafe@embrapa.br www.embrapa.br/cafe

Ficha catalográ�ca: H995 Hygienic Coffee Processing and Technologies / Juarez de Sousa e Silva... [et al]. Brasília, DF: Embrapa Café, 2011.

98 p.: ill (some col.). Several authors. Includes bibliographical references. ISBN 978-85-61519-02-5. 1. Coffee. 2. Coffee growing. 3. Coffee – Production – Technology. I. Sousa e Silva, Juarez de. II. Título. CDD: 633.73


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TABLE OF CONTENTS 1. Introduction...........................................................................7 2. Coffee Preparation..............................................................11 2.1. Separation and classification systems............................11 2.2. Dry method for coffee processing....................................17 2.3. Wet method for coffee processing....................................18 3. Natural Coffee Drying.......................................................20 4. Washed Coffee Drying.........................................................21 5. Pulped Coffee Drying...........................................................22 6. Coffee Quality.......................................................................23 7. Coffee Drying Methods.......................................................24 7.1. Sun drying...................................................................................25 7.1.1. Summary of terrace drying procedures...................31 7.2 Drying terrace construction..............................................32 7.2.1. Locating and building.....................................................32 7.3. Other sun drying methods..................................................35 7.3.1. Drying with solar energy..............................................35 7.3.1.1. The solar energy collector....................................36 7.3.1.2. Other dryers using solar energy.........................38 7.4. Coffee drying in “hybrid terrace� terraces....................39 7.4.1. Hybrid terrace drying operation...............................42 7.5. Table dryer and suspended terrace drier.......................45 7.6. Movable terrace dryer...........................................................45 8. High Temperature Coffee Drying.....................................46 8.1. Natural convection dryer..................................................46 8.2. Flex dryer....................................................................................47 8.3. Fixed-bed drying.......................................................................48

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8.4. Pneumatic coffee dryer..........................................................50 8.5 Counter flow coffee drying.................................................52 8.6. Concurrent flow coffee drying.........................................53 8.7. Dryeration..................................................................................56 8.8. Combination drying (High temperature plus ambient-air drying).........................57 8.8.1. New combination System (Pre-dryer, dryer and Silo-dryer).............................................58 8.8.2. Combination drying procedures.................................60 8.8.3. Recommendations.............................................................62 8.8.4. Combination drying management.............................62 9. Coffee Storage.......................................................................65 9.1. Green coffee bag storage......................................................65 9.2. Warehouse floor......................................................................67 9.3. Bulk Storage...............................................................................68 9.4. Good practices...........................................................................69 10. Silo construction on Coffee farm................................72 10.1. Selection of the area..........................................................72 10.2. Silo with screen frame........................................................73 10.3. Internal plastic sheet..........................................................75 10.4. Silo Loading.............................................................................76 10.5. Silo external finishing.......................................................77 11. Silo with Masonry Walls.................................................78 12. Silo Isolation and Fumigation......................................79 13. Hulling and Classification.............................................80 14. Coffee Moisture Content Determination...................83 14.1. Moisture content determination methods...............85 14.2. How to use the (EDABO) DEWOB......................................87 15. Important Coffee Properties and Notation..............89 16. References............................................................................92

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Introduction

1. Introduction

C

offee is a unique agricultural commodity. Because of the many successive layers enclosing the seed, its drying characteristics are different from those of other agricultural products such as cereal grains and oilseeds. The outermost layer of a ripe coffee cherry is a thin – the pericarp - which covers the pulp or fibrous fruit flesh. Next, there is a layer of mucilage, approximately 0.8 mm thick, which is translucent and colourless. The mucilage has no definite cellular structure and resembles an amorphous gel. Then, there is a thin yellowish parchment layer - the endocarp - and, finally, a silver skin covering the green coffee seed (Figure 1). Thus, the drying process, which involves heat and mass transfer, is different in each part of the coffee fruit and different of other crops like wheat, soybeans and maize, which have remarkably different seed structures (Berbert & Silva, 1999). Cleaning after threshing is sometimes unnecessary with the other mentioned products, but coffee must pass through cleaning machines before drying and storage.

1. Outer Skin

1 2

2. Silver Skin

3

3. Parchment 4. Pulp

5

4

5. Coffee Bean

Figure 1 – Cross sectional view of a coffee cherry

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Hygienic Coffee Processing and Technologies

Ripe cherries are bright-red, glossy, and firm. The harvesting method chosen on any farm is a combination of the processing method, economic considerations and labour availability. Four basic harvesting systems are used: 1 - single-pass stripping, where all branches bearing fruit are harvested at once, thus collecting unripe, ripe and overripe cherries altogether; 2 - multi-pass stripping, where only branches bearing mainly ripe cherries are harvested, a method that relies mainly on the ability of the coffee picker (Figure 2); 3 - multi-pass selective picking (finger picking), where only ripe cherries are harvested (Figure 3) and 4 mechanical harvesting, where different types of machines are used to harvest fruit all at once (Figures 4a and 4b). In large areas of coffee Figure 2 - Harvest by stripping the cherries off the whole branch producing countries, coffee is harvested during the dry season. This generally causes a rapid passage from the ripe to the partially dry stage so that most of the production is not dried as ripe fruit (Sivetz & Foote, 1963). Ripe cherries are brightred, glossy, and firm. The harvesting method chosen on any farm is a conjunction of the requirements of the processing method, economic considerations and labour availability. Four basic harvesting systems are used: 1 - single-pass stripping, where all branches bearing fruit are harvested at once, thus collecting unripe, ripe and overripe cherries altogether; 2 - multi-pass stripping, where only branches bearing mainly ripe cherries Figure 3 – Selective picking of ripe cherries are harvested, a method that relies mainly on the ability of the coffee picker (Figure 2); 3 - multi-pass selective picking (finger picking), where only ripe cherries are harvested (Figure 3) and 4 - mechanical harvesting, where different types of machines are used to harvest fruit all at once (Figures 4a and 4b). 8


Introduction

In Brazil, coffee growers often start harvesting using the stripping method when more than 75% of the crop is perfectly ripe. Because of the maturation uniformity in almost all areas of the country, harvest by stripping is more efficient and costs less than selective picking. In the former method, the cherries are pulled off the tree and fall either on clean ground or on cloth covering Figure 4a – Portable stripping machine the ground under the tree (Figure 5). Harvesting by stripping yields a mixture of green, ripe (red and greenish cherries), overripe and dried fruits, leaves, branches, twigs, and, when the ground is not covered, large amounts of dirt and stones. In the later method, only slightly underripe, ripe, and slightly overripe cherries are removed from the tree, leaving unripe and green cherries to be harvested at a later time, which makes selective picking a costly Figure 4b – Combine harvest machine method. However, this method would be a better choice because it maximizes the amount of ripe cherries in harvested coffee. In the striping method, the accompanying dirt should be removed from the coffee, generally by wind winnowing, which is accomplished by throwing the mixture into the air so that the wind blows away the lighter dirt, while the heavier cherries fall back down for recovery (Figure 6a); after winnowing, coffee should be separated in its several fractions, i.e., unripe (green), slightly under-ripe (greenish), ripe (red), slightly overripe (brown) and overripe (black), or a mixture of these fractions, to be dried separately. If the coffee grower decides to harvest by selective picking, the operations described below will facilitate the process, and if the job is well done, the production of high quality coffee can be expected. 9


Hygienic Coffee Processing and Technologies

The set of operations after harvesting is called “coffee preparation” and can be executed through the dry and wet methods. In the dry method, part of the whole coffee fruits are allowed to partially dry on the tree, and the remaining moisture is removed either by a sun-drying terrace or in mechanical driers; coffee prepared in this way is called natural coffee. The dry method is a simple process, has a smaller initial cost and suits well for family coffee growing. Coffee prepared by the wet method is called washed coffee; in this method the skin, pulp and mucilage are removed, and the seed is left enclosed in its tougher skin called parchFigure 5 – Covering the ground to receive ment. However, when only the perithe harvested coffee fruits carp is removed in the wet method, the final product is called natural pulped cherry or “cereja descascado” (Silva & Berbert, 1999).

10


Coffee Preparation

2.Coffee Preparation

W

hen the coffee is not handpicked, it should be, as quickly as possible, transported to the separation processes before subsequent operations. Separation of the coarse foreign material can be achieved by sieving - manual winnowing (Figure 6a); coffee cleaning can also be completed with a tractor-powered machine (Figure 6b) or hand-powered machines as the one shown in Figure 6c.

Figure 6a – Coffee manual winnowing with a garden riddle

Figure 6b – Tractor-powered coffee cleaner

2.1. Separation and classification systems

Figure 6c – Hand-powered coffee cleaner

Even after the removal of unwanted matter (sticks, dirt, stones, and leaves) by winnowing, any mixture of harvested coffee in various stages of ripeness should undergo a preliminary selective separation process, which is generally based on the buoyancy and density properties of the cherries. The separation into mature and immature 11


Hygienic Coffee Processing and Technologies

cherries is performed in flowing-water based “hydraulic separators” or “coffee cherry washers” where, due to the principles of flotation, immature, partially dried and overripe cherries, beans damaged by insects, peaberry beans, and light impurities, such as leaves and sticks, float in water. Ripe coffee cherries and other denser material, such as stones, therefore sink. If coffee has been harvested by multi-pass stripping or multi-pass selective picking, it is desired, but not necessary, to wash the cherries to obtain natural high quality coffee. The different types of coffee fruit (floats and sinkers) must be dried and stored separately. Two types of hydraulic separators, which can be easily built on the farm are shown in Figures 7a to 7e (Silva et al, 2008). The first washer/separator, Figures 7a to 7e, is a very simple model and is ideal for small scale productions. It consists basically of two containers: • The lower container holds the volume of water used for separation and/or washing. It is made of steel sheet and set up on wheels (portable washer). Figure 7e illustrates the same type of washer in which the lower deposit is constructed of masonry materials or any other appropriate material; in this case, the structure is fixed to the ground (fixed washer/ separator). • The upper container is movable and made of a perforated metal foil or perforated metal sieve. Floating materials that are lighter than water accumulate on the surface, and are subsequently drawn off the top with a handheld sieve or with a perforated shovel; mature coffee cherries, which are a little heavier than water, remain in the container. Removal of mature coffee cherries (sinkers) is shown in Figures 7c

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Figure 7a – Portable hydraulic separator

Figure 7b – Floats removal


Coffee Preparation

and 7d. If there is not enough running water for continuous operation, the water in the container should be changed after each batch of 250 L of washed coffee cherries is taken off from the unit.

Figure 7c – Lifting the upper container

Another type of coffee washer that can be easily built on the farm is the traditional Brazilian coffee washer/separator, the so called “Lavador Maravilha”, whose basic details are shown in Figures 8a to 8f. The “Maravilha” coffee washer consists basically of a tank (masonry), two metallic or wooden canals (one for the floats and the other for the sinkers), and a trap at the bottom of the tank which allows periodic removal of stones or any other foreign material heavier than water. The coffee washer has a mechanism to deliver water under controlled pressure, so that mature coffee cherries, which are only slightly heavier than water, are channeled up to the appropriate canal (Figure 8b). One of the disadvantages of this type of coffee washer/separator is that it requires rather large amounts of water. Depending on its design characteristics and the amount of admixed impurities within harvested coffee, the equipment can use up to 10 L of clean water for each litre of coffee cherries. Depending on water availability, these washing systems can be built to clean up to 10,000 L of coffee per hour.

In case of water shortage periods, a system for partial or complete recirculation of wash water can be adapted to the equipment Figure 7d – Removing mature (Figure 8c). The complete system consists of cherries a reception tank (Figure 8d), two canals, a tank to separate stones and heavy materials from cherries that sink (Figure 8e); and a recycling tank (Figure 8f). In this latter tank, water is filtered on entry and is then conveyed to a series of flow-through sedimentation compartments; clean water is skimmed from the surface as it flows over the baffles. 13


Hygienic Coffee Processing and Technologies

Sludge residue located at the bottom of each tank is removed into a sludge collection pipe. An appropriate hydraulic pump is used to discharge the water flowing out of the last sedimentation tank after it has been used through successive washing cycles during the working day.

Figure 7e – Schematic diagram of a fixed hydraulic separator

Since the removal of dirt particles from water is not clear-cut, the effluent must be submitted to a subsequent treatment before it is discharged into a waterway. One must keep in mind that the largest portion of the water used in the equipment show in Figure 8c is intended to transport coffee cherries into the reception tank, and not to operate the equipment itself.

Efficient mechanical washers/separators, in which water is solely used to wash mechanically transported coffee, are available in the market (Figure 9). Besides minimal water consumption and smaller labour requirements, mechanical coffee washers require less space and can be moved or sold if coffee production is no longer profitable.

Figure 8a – Schematic diagram of a “Maravilha” hydraulic coffee separator

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Coffee Preparation

Following is a list of the main advantages of coffee washing and classification: • Maintain the potential of freshly harvested cherries to produce the best quality coffee. • Reduce mechanical problems during the subsequent operations of pulping, drying and husking. Figure 8b – Basic outline of the canals of the hydraulic coffee separator

• Allow the drying of homogeneous batches of coffee fruits.

• Secure the production of high quality coffee by avoiding contamination of good coffee fruits with cherries that bear or have been damaged by coffee borers.

Figure 8c – Hydraulic coffee separator

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Hygienic Coffee Processing and Technologies

Figure 8d – Reception tank

Figure 8e – Float and sinker canals, and the separation tank

Figure 8f – Decantation tanks to clean water in a coffee washer

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Coffee Preparation

2.2. Dry method for coffee processing After their separation into two grades (floats and sinkers), coffee cherries are either processed through the dry (or natural) method or through the wet (or washed) method. The dry method consists in reducing the moisture content of the cherries in the whole fruit form by spreading them on drying terraces or by using mechanical driers. It is simpler and requires less machinery than wet processing, and is the most traditional method for processing coffee in Brazil. In order to obtain high quality processed coffee, the following measures should be taken when the dry method is chosen: • if possible, start drying on the same day of harvesting; • spread the cherries as soon as possible in a drying terrace, controlling layer thickness (never heap) , and stirring regularly; • avoid soil contact; • avoid re-wetting; • cover with ventilation; • reduce cherries to 12% (wet basis) moisture as soon as possible; • clean and sanitize drying surfaces. Figure 9 – Commercial mechanical coffee washers/separator

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Hygienic Coffee Processing and Technologies

2.3. Wet method for coffee processing In preparing washed coffee, slightly under ripe, ripe, and slightly over ripe cherries are squeezed in a pulping machine which removes most of the skin and pulp, leaving a slippery layer of mucilage (Berbert & Silva, 1999). Since the mucilage cannot be readily removed with water, it must be removed by natural fermentation, fermentation with added enzymes or mechanically (Sivetz & Foote, 1963). The product is called washed coffee because the dispersible fermented mucilage is finally removed by washing with water. If freshly pulped cherries are dried without washing, i.e., with parchments still partially or completely covered with a layer of mucilage, a distinguished type of coffee may be obtained, the so called Brazilian “descascado”. In spite of requiring a larger initial investment, and compulsory treatment of residual water, coffee preparation through the wet method is more economical, more easily understood, and can be used with great success by small producers, as adopted in Colombia and other high quality coffee-producing countries. Small and medium-capacity pulpers (Figures 10a and 10b) are available in the Brazilian market, and may be employed in family-run coffee farms. The production of washed and Brazilian “descascado” coffees requires smaller drying terraces areas, smaller nominal capacity dryers, and shorter drying times. Also, the volumes of bins, warehouses or any space for storing palleted coffee can be reduced up to 50%. These advantages are due to the pericarp removal, seed uniformity, and the low water content of cherries, when compared with coffee prepared through the dry method. The following recommendations should be taken into consideration by washed coffee producers (ICO, 2002): • if possible, pulp on the same day as harvesting; • separate floaters; • control water quality; • sanitise the equipment; • completely separate parchment and pulp; • complete fermentation within local standards;

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Coffee Preparation

• if possible, rapidly remove excess moisture with artificial drying; • dry slowly to avoid cracking by excessive heat; • control layer thickness and stir at hourly intervals; • avoid re-wetting; • cover with ventilation, especially at night; • avoid soil contact; • use mats or drying tables when possible; • maintain mats/tables/terraces in clean and sanitised conditions; • achieve 12% moisture content as soon as possible without damage to the quality.

Figure 10a – Medium capacity coffee pulper

Figure 10b – Small capacity

coffee pulper

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Natural Coffee Drying

3. Natural Coffee Drying

T

he factors that must be taken into account when deciding which harvest method will be used to prepare and dry coffee are determined by economics, climate and rainfall distribution. In large portions of Brazil, a long dry and warm period usually coincides with the harvest season. This induces a very rapid passage from ripe to the partially dry stage; thus, making it impossible to harvest a big amount of fresh ripe fruit (Figure11). Cherries that are naturally dried to 45% moisture content or less cannot be pulped because of the hard skin. Consequently, it is necessary under these conditions to dry the cherries as whole fruits producing what was previously referred to as natural coffee.

Figure 11 – In Brazil, rapid passage from the ripe to dry stage is very common.

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Washed Coffee Drying

4. WASHED COFFEE DRYING

T

he drying of washed coffee is a drying method that may be considered by Brazilian producers who are contemplating opportunities to deliver a high quality product for local coffee processors or for exportation.

After draining off excess water, the coffee is dried to about 12% moisture content either in high temperature mechanical or fixed-bed dryers, or under the sun on well paved drying terraces. Carefully prepared and handled, washed coffee is generally clean in flavor and free from undesirable elements, but it lacks body or full flavored richness compared with well prepared natural coffee.

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PULPED COFFEE DRYING

5. PULPED COFFEE DRYING

S

omewhat different from the method used to prepare washed coffee is the processing method used to obtain peeled coffee. In this method, both pulping and partial mechanical removal of mucilage are accomplished in a single operation. After peeling, the coffee seeds are immediately spread on drying terraces to be sun dried. The incomplete or partial removal of mucilage increases the difficulty of drying for the first day or two because the seeds become very sticky and viscous, making the stirring operation very difficult and hard to handle. However, if favorable weather prevails, the parchment surface becomes dry and the coffee may be handled normally. Many Brazilian coffee growers already practice the process of “pulped or peeled coffee� to prepare floats (peeled floats). The process consists of maintaining the partially dry coffee cherries or floats for a 24 hours period in tanks with clean water to facilitate the mechanical removal of the dry pericarp. The procedures resemble the preparation of the peeled cherry (descascado).

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Coffee Quality

6. COFFEE QUALITY

G

ood control of coffee quality is related to the hygienic conditions in which the coffee was prepared, climate conditions, and the time it has been exposed to high temperatures, during drying. As an approximate guide, coffee will tolerate temperatures of 40°C for a day or two, 50°C for a few hours and 60°C for less than an hour without damage. If these temperature-time limits are exceeded, damage to coffee quality may be expected because overheating during drying produces sour or cooked flavors in the brewed coffee. Research performed out at the Federal University of Viçosa – Brazil showed that, compared to the sun drying procedure, the quality of the brew was not impaired when natural and washed coffees were dried with air temperature at 80°C in a 0.5 m deep fixed bed dryer with a 3h stirring interval, and an airflow rate of 12 m3 min-1 m-2. However, the real grain temperature was not measured (Lacerda Filho, 1986) In order to know the true coffee seed temperature, the dryer operator should put the sensitive element of the thermometer in the grain mass with the dryer ventilation system turned off. Otherwise, the indicated temperature will be of the drying air temperature at the location where the thermometer is inserted. Commercially speaking, uniform final moisture content is one of the most important parameters defining coffee quality. In cases where drying is carried too far in order to achieve the set final mean moisture content (bean moistures lower than 10% w.b.) can result in marked coffee quality losses. Upon storage, even if coffee beans re-establish their equilibrium moisture with the ambient air, the harm done to the coffee quality will not be totally corrected. To guarantee the high quality coffee, some general measures are recommended: • locate the processing plant in a dry area; • dispose pulp away from clean, dry coffee. Use correct pulp processing before using it as organic fertilizer in the field; • keep equipment and facilities clean, separating residual, partially processed material, accumulation of dust and discarded materials.

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Hygienic Coffee Processing and Technologies

7. COFFEE DRYING METHODS

D

ue to their high moisture content at harvest, around 65% w.b., natural ripe coffee cherries do not flow easily in handling equipment (gravity spouts, hoppers, bucket elevators, augers) and in conventional mechanical dryers. Conventional drum-type rotary dryers are an alternative but they are generally criticized for having air flow problems and low energy efficiency at the final of the drying process. As a result, the typical and commercial coffee drying process in Brazil consists of two different drying stages. In the first stage, freshly-harvested, whole coffee cherries are spread on paved terraces where they are allowed to dry under the sun until they reach 30 to 35% moisture content, whereas in the second stage the coffee is dried in hightemperature mechanical driers down to about 11% w.b Modifications in the drying and handling of the drum-type rotary dryer, accomplished at the Federal University of Viçosa, shows that the mentioned problem in the dryer may be easily solved by transforming the rotary dryer into an adequate equipment to work with wet natural or wet peeled coffee (Santos et al, 2006). If climate is such that dependable sunshine is available during the harvest season, the drying process may be conducted entirely in open air. In this case, the cherries are spread out on the drying terrace after harvesting, and dried to about 11% moisture content in a single operation. Another alternative is to dry (in one pass) the whole coffee cherries in a fixed-bed dryer or with the newest system for coffee drying, the “hybrid terrace” (Donzelles, 2002). Besides their relatively high cost, the main drawback in using conventional mechanical dryers is the fact that they were designed to dry other products, not coffee especially. Despite the recent efforts to change this situation, most of the commercially available dryers are still inefficient and have, thus far, been used in large scale operations. Since 1998, The CBP&D-Café - Consórcio Brasileiro de Pesquisa e Desenvolvimento do Café (Brazilian Research and Development of coffee Consorcium) is developing and transferring to coffee growers, drying and

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Coffee Drying Methods

storage technologies adapted for small coffee production, as will be shown further on. Some innovative grain drying methods, generally applied to cereals and oilseeds, have been introduced in coffee drying facilities as an effort to increase drying capacity and energy efficiency in conventional coffee drying installations, and at the same time to maintain high coffee quality. These methods include dryeration, reversed-direction-air-flow drying, concurrent flows, and counter-flow dryers. Several other high-temperature, continuousflow drying systems, and combination drying, traditionally used in the USA to dry cereal grains have been adapted to dry natural, washed, and pulped coffee. Irrespective of the drying method, the following aspects should be emphasized in coffee processing: • avoid undesirable fermentation; • avoid excessively high temperatures during drying; • dry the coffee within the shortest period of time, at moderate temperatures, to a moisture content of 18% w.b.; • work with product uniformity in size, coloration, and density.

7.1. Sun drying In Brazil, freshly harvested natural coffee has a wide range of moisture contents (65% - 25% w.b.). As previously mentioned, on arrival at the drying facility the coffee is subjected to water flotation to separate soft, ripe cherries from green and partially dried hard cherries. If the “wet way” processing method will not be used, these two groups of coffee are Figure 12 – Mechanical coffee spreader spread out in separate parts on the drying terrace using hand carts for water drainage. As the car is pulled along, a coffee layer of constant thickness is spread on the surface of the drying terrace (Figure 12). At the beginning of the drying process the thickness of the coffee layer is approximately 4 cm and may be increased up to 10 cm when the coffee approaches its final moisture content.

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Hygienic Coffee Processing and Technologies

At the beginning of the terrace drying operation, when the wet coffee is spread out on the drying terrace, the surface of the terrace becomes completely wet. If it is not immediately dried of the excess of water, the product will be highly susceptible to contamination, due to the high humidity beneath the coffee layer. To solve the problem, the terrace drying operator should open up the coffee layer, at least until the fifth drying day, in such a way to form small parallel coffee lanes as illustrate in Figures 13a, 13b and 14. The coffee lanes should be stirred continually in regular time intervals with the aid of a scraper pushed by hand (Figures 14 and 15a). The stirring intervals should not exceed 60 minutes.

Figure 13a – View of terrace drying after wet cherry coffee spreading

The terrace operator must leave terrace lanes exposed to be sun dried and heated for indirect drying of the coffee fruits at the next stirring. When opening the coffee lanes, the terrace operator must consider solar orientation (Figure 15c). Coffee lanes must follow the same direction of the operator’s shadow.

After roughly five days, when the coffee is already partially dry, at 3:00 PM, approximately it must be gathered into heaps or piles oriented along the highest slope of the terrace, and they should be covered with a plastic sheet or tarpaulins (Figure 15b). Therefore, the sun’s heat absorbed during the day is partially conserved during the night, guaranteeing uniform moisture Figure 13b – Real spreading and coffee stirring operation in cement floor redistribution in the mass of coffee seeds. In the following morning, at approximately 9:00 AM, the heaps or piles should be uncovered and removed from the place where it rested overnight in order to dry the used area. Soon afterwards, the product should be spread out on the drying terrace, repeating the operations performed in the previous days (Figures 13 and 14) until reaching the desired

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Coffee Drying Methods

moisture content for storage (12% b.u.) or until the point of 35% moisture content, which is ideal condition to start final mechanical drying until reaching a moisture content of 12%. Sun drying of washed coffee follows similar procedures used in Figure 14 – Scheme of the coffee pile formation the drying of natural coffee. The during cement terrace drying main difference is that washed, pulped coffee has very uniform initial moisture content and requires less drying time than natural or whole coffee fruit.

Figure 15a – Scrape shovel

Figure 15b – Coffee pile formation during terrace drying

Ripe green coffee should not be pulped, and must be dried separately from the natural colored coffee cherry or washed coffee. Drying ripe green coffee must be done at a slower rate than for natural or washed coffee. The terrace operator must choose one area of the drying terrace with less solar radiation, and spread the coffee in deeper layers or piles. Fast drying of ripe green coffee causes darkening of the silver skin which is a defect in coffee grading.

Figure 15c – Coffee cherries distribution in small piles system

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Hygienic Coffee Processing and Technologies

The drying terrace should be located in an open, well-drained, sunny, and ventilated area, on a level lower then the reception and preparations facilities, and at a higher level then the storage and hulling facilities. Solar drying terraces can be built in beaten soil or be paved with bricks, asphalt or concrete. The concrete floor produces better drying results and is more durable, easier to handle, and maintains better sanitation characteristics (Silva et al, 2005). An experiment conducted by Lacerda Filho (1986) shows the drying efficiencies and the coffee quality of drying terraces constructed with different materials (brick, concrete, asphalt, and soil). Table 1 shows the experimental results with natural coffee drying with 62% w.b. initial moisture content. All treatments were subjected to the same solar drying conditions. The product subjected to sun drying in compacted-soil terraces resulted in coffee of poor quality compared to that dried in brick, concrete, and asphalt terraces. Terrace type

Moisture content after 16 days (% w.b.)

Energy requirement (kJ kg-1)

Soil

18.0

17,870

Brick

14.2

16,600

Concrete

13.3

16,970

Asphalt

11.3

15,900

Table 1 – Terrace drying of natural coffee *

As previously mentioned the conventional drying terrace can be built on cement, bricks, asphalt or compacted soil. Besides easy contamination of the product, the compacted-soil drying terrace presents a poorer drying efficiency, and worse product visual presentation in relation to coffee dried in other terrace types. Ideally, drying should be performed on a cement terrace, which is more efficient, allows good cleaning and sanitation, is durable and presents smaller risks in reducing the coffee quality when compared with other terraces. Recently, in order to reduce costs of drying terraces and make them compatible with the investment capacity of family-owned coffee growers, some technicians, without a deeper study of the subject, are encouraging the construction of drying terraces with “asphalt mud”. However, they have proved to be inefficient as shown in the photographic sequence (Figures 16a, 16b,17a,17b, 18a and 18b) bellow, illustrates the problems found in a demonstrative “asphalt mud” drying terrace, used only in the 2003 harvesting season, in Viçosa - MG. * Source: Lacerda Filho, 1986

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Coffee Drying Methods

Figure 16a – Rectified area

Figure 16b – The applied layer is little resistant to the traffic

Figure 17a – Fragments that come unfastened can easily be mix with the product

Figure 17b – Plants can grow from cracks in the asphalt

Figure 18a – With little effort, the asphalt layer can be removed

18b – Cracks retain sources of contamination

29


Hygienic Coffee Processing and Technologies

The defenders of the technology argue that, “the characteristics of terrace with “asphalt mud” accelerate the drying process and reduce the investment costs in drying terraces”. Also, in the research done by Lacerda Filho (1986), problems of a practical nature were observed with asphalt paved small areas (adherence, resistance and uniformity of the surface, high porosity, vegetation emergence, etc.). Figure 19 shows a sample of parchment coffee collected in a recently built terrace paved with “asphalt mud”. It is important to point out that alternatives exist for the conventional terrace drying. The restriction of terrace drying is due to Figure 19 – Parchment coffee dried in recently built the low certainty level that sun drying presents “asphalt mud” terrace in producing a high quality coffee. Also, the drying terrace efficiency is highly dependent of the climatic conditions. In other words, is not always possible to produce a good coffee if the solar radiation conditions, wind, and the characteristic of the drying terrace are not favorable. If the use of terrace drying is compulsory or terrace drying is the only one possibility or the best alternative, a good terrace with a cement floor is recommended. Terrace borders and fixed spot separators cannot be built with sharp corners, and the use of movable separators (Figures 20, 21 and 22) is a good

Figure 20 – Movable barriers for terrace coffee separation

30


Coffee Drying Methods

idea to avoid the mixing of different types of coffees. For successful terrace drying, it is obligatory to do periodic maintenances such as the correction and repair of the drying surface (Figure 21), and improvement of the drainage system. For a good terrace drying system to produce hygienic and high quality coffee, it is of fundamental importance that the terrace is correctly managed, and that daily sanitation of the whole system is maintained.

Figure 21 – Maintenance of the terrace drying surface

Figure 22 – Protection walls

7.1.1. Summary of terrace drying procedures • Do not mix coffee fruit from different lots; • If possible, spread the natural or washed coffee on the same day it has been harvested, in fine layers (3 to 5 cm), and start the coffee lane formation (figure 15c); • In case there is a great percentage of green fruits, deeper layers must be used (about 10 cm of height) and stir the coffee layers more frequently (no more than 60 minutes); • Mix or stir the coffee at least eight times a day, observing the sun pathway to receive the maximum sun light. During the stirring process, the worker’s shadow should be in front or behind him (figure 15c); 31


Hygienic Coffee Processing and Technologies

• After the second drying day, during the afternoon (3:00 PM), make layers of 15 to 20 cm high. Spread the coffee early, at 9:00 AM, on the following day. This procedure will accelerate the drying and avoid rewetting; • Make deep coffee layers in the sense of the higher steepness of the drying terrace, in the case of rain. Those coffee layers should be changed as often as possible in order to increase the contact with the ambient air. When raining stops, the terrace operator must follow up by stirring the coffee layers until the terrace drying surface is dry; • Never make very deep layers with natural coffee before reaching 35% moisture content (at 35% moisture content, coffee won’t be sticky in the hand when held tight). Making deeper and deeper layers with coffee moisture content below 35% is a very important operation. The heat transfer between coffee beans provide homogeneous drying; • Make big coffee layers at about 3 p.m. and, if possible, leave them covered with plastic sheets until the following day; • Spread the coffee again at about 9 a.m., when air humidity is lower; • Maintain hourly stirring until 3 p.m., when it should be piled again; • Continue the process until final drying. For storage, collect the coffee when it is cold.

7.2 Drying terrace construction 7.2.1. Locating and building The area of the drying terrace should be calculated as a function of the average production per thousand trees, the number of coffee trees, and weather conditions in the farm. In the hypothesis of using the drying terrace only for natural coffee drying, the calculation of the area can be made according to equation 1: S = 0,0005 Q. T

32

(1)


Coffee Drying Methods

Where:

S = area of the drying terrace, m2 (production of 1,000 trees); Q = annual average production of natural coffee cherry, in liters/1,000 trees;

T = average drying time in the growing area, days. To accomplish only partial drying (from moisture at harvesting to approximately 35% b.u.) in order to use mechanical drying at the final, the drying terrace area can be reduced to 1/3 of the calculated value. Whenever possible, the drying terrace should be divided in blocks to facilitate the drying of different coffee types or according the harvesting days. To facilitate pluvial water drainage, the drying terrace should be built at a grade in the range of 0.5 to 1.0% with the drain plates located in the lower part of the terrace. The drains, measuring 0.4 x 0.25 m, are made in steel plates with 50% perforation, squared holes of 4 mm to impede the passage of the coffee fruits in case of heavy rain. In the case of adopting plates with circular perforations, use plates with holes of smaller dimensions (maximum diameter of 2.0 mm). The construction of protection walls is advised (Figure 22), measuring 0.25 m high and 0.15 m thick, around the drying terrace to avoid losses or mixtures of different coffee types. In the final phase of drying, when the coffee moisture content is around 35% w.b., the drying should take place in deeper convex piles (Figure 23) to maintain moisture equilibrium between the external and internal portions of the grain, and the grains mass. To achieve this goal, the coffee should be mixed daily and exposed to the sun for two or three hours before being covered again.

Figure 23 – Protected coffee pile

Inside of the drying terrace, circular or semicircular barriers or crowns can be built. The barriers are small walls with no more than 5 cm in height and 3m in diameter. The purpose of these barriers is to serve as places to pile up the coffee and avoid the flow of rain water under the coffee pile (Figure 24). 33


Hygienic Coffee Processing and Technologies

Figure 24 – Circular or semicircular barriers or Crowns

For the construction of a drying terrace, humid areas such as those in close proximity of dams, shaded places with trees or adjacent constructions, should be avoided. Table 2 shows the material needed for the construction of a 150 m2 terrace (10 m wide and 15 m long) paved with cement (1:4:8 with 8 cm thickness) and finished with 2 cm of mortar 1:3 (Cement/Sand). Discrimination

Unity

Quantity

Unity Price (US$)

Total Price (US$)

Participation (%)

Tractor service

h

2

70.00

140.00

5,76

Bricklayer

d

16

30.00

480.00

19,76

Servant

d

29

15.00

435.00

17,91

Cement

bag

58

10

580

23,88

Sand

m3

11

21.00

231.00

9,51

Broken stone

m3

11

35.00

385.00

15,85

2428,00

100,00

TOTAL Table 2 – Material required for a 150 m2 cement drying terrace.* * Cost/m2 = US$ 16.18 (Viçosa - MG price on 1/5/2011)

34


Coffee Drying Methods

7.3. Other sun drying methods 7.3.1. Drying with solar energy Despite being the primary source of energy, and presenting relative success when terrace drying is used, the application of direct solar energy for grain drying in deep layers is viable only if used for low temperature drying systems. The energy supply intermittence and the collector size to reach air temperatures usually applied in mechanical drying (10oC or more above ambient temperatures), will be not economically viable to reach the necessary levels of energy (120,000 to 300,000 kJ/h) for an average capacity mechanical dryer. However, if solar energy is used as complementary energy, the consumption of conventional fuel for small scale dryers will be substantially reduced. If the incidence of solar energy during harvesting is normal, and if working only during the day time to maintain a fixed-bed dryer (Figure 25) with a 6,000 L capacity and air temperature at 50oC, a 100m2 solar collector will be necessary in local with good solar radiation. The collector area will be almost the same area necessary to spread the 6,000 L of coffee cherries in terrace drying. However, if it is considered that the fixed-bed dryer requires an roof area bigger than 56m2, and if roof is converted into a solar collector coupled with one furnace to heat the air, during the day, it would save up to 100kg of firewood with continuous drying operation (the drying air is heated by the use of firewood burning during the night or in absence of solar radiation).

Figure 25 – Fixed-bed dryer

35


Hygienic Coffee Processing and Technologies

7.3.1.1. The solar energy collector In spite of their many similarities, a large variety of solar collector type can be found in specialized literature. In the next pages, there is a description of a solar collector that seems to be the most suitable one to convert the roof of the dryer into a solar collector to supply part of the energy demand for coffee drying. The roof must be design to receive the maximum solar radiation The solar collector roof should resemble that shown in Figure 26, constructed lengthwise in the east-west direction, and with the absorbing surface facing north for the southern hemisphere or facing south for the northern hemisphere. Another important point that should be noted refers to the inclination of the absorbing surface in relation to the horizontal level of the soil. A good inclination for the annual variation is in accordance with the local latitude where the dryer will be installed (Figure 27). A great advantage of the solar collector roof is the capacity in absorbing energy directly from the sun, in the form of direct radiation and diffuse radiation (radiation reflected by the earth and by the clouds). Depending on the adopted air flow and roof isolation, it is possible to increase the air temperature by up to 30oC with the solar collector roof. This means an increase of 10oC is considered a good value to obtain reasonable efficiency. Since the collector will be a substitute for the roof of the dryer, the total cost of the system will not be very high. A large portion of the material that would be used to build an ordinary roof for the dryer can also be used for building solar collector roof. Besides the mentioned factors, the solar collector roof is of relatively simple construction and low cost compared to other types of solar collectors.

Figure 26 – Details of the solar roof structure

36


Coffee Drying Methods

Figure 27 – Ideal inclination for a fixed solar collector (south hemisphere)

There are several models of plane collectors, but all of them have two basic characteristics: • a black plate to absorb the solar energy; and • circulating fluid (ambient air) used to remove the heat from the black plate and transfer it to a drying chamber. A solar collector cam be built with metal or with cement-asbestos tiles, painted in black. The tiles should form a channel with the roof structure, where the drying air should be forced to pass. The structure must have a kind of chicken wire metallic screen to support the transparent plastic cover. In addition to channeling the drying air, the transparent plastic covering is used to increase the collector efficiency and avoid heat losses from the black plate to the atmosphere (Silva et al, 1976). There are different ways of improving the collector efficiency. However, before improvements are implemented, the benefit of the additional investment should be analyzed. For example, is the isolation of the bottom of the collector a good idea? Usually, the most efficient collectors are also the most expensive ones. The desirable characteristics of a solar energy collector are: • absorb the maximum amount of the solar radiation; • minimum loss of heat into the atmosphere; and • easy transference of the absorbed heat to the circulating air.

37


Hygienic Coffee Processing and Technologies

Painting the roof in black will absorb more radiant energy than in any other color. A flat black surface can absorb up to 90% of the radiation that passes through the transparent covering. When the collector is not in operation or if the fan is turned off, the temperature can reach values above 70oC. So, it is advisable to cover the collector to avoid damages caused by these high temperatures. If possible, remove the transparent cover when the collector is not in use.

7.3.1.2. Other dryers using solar energy Even with the existence of several dryer types of dryer using solar energy for small coffee productions (Figure 28), two systems were built and tested at the Federal University of Viçosa (Correa et al, 1992).

Figure 28 – Suspended solar screen drying

The first - UFV J2 - is similar to a horizontal fixed bed dryer that has a solar roof (solar collector), a fan, a connection duct and a drying chamber (Figure 29).

The second - UFV JPC1 (Figure 27) - is a rotary solar dryer, which is an improvement of a suspended solar dryer or table dryer. The UFV JPC1 dryer consists of a box with borders made of wood and the front and bottom made of steel screen with 4 mm square mesh. The box has a central axis supported by two small, wood pillars to allow easy rotation. The product to be dried (coffee) constitutes the absorbing heat material in this dryer type. The natural ventilation removes the absorbed heat as latent heat in the evaporated water, as occurs in the traditional screen suspended terraces. In these dryers, the coffee will be simultaneously cleaned and dried. In the suspended terraces, as coffee is not in direct contact with cement or soil surfaces (with cleaning and disinfection

38

Figure 29 – General view of the fixed bed dryer “UFV-J2”


Coffee Drying Methods

problems), it has a smaller chance of being contaminated by undesirable microorganisms. For safe drying of high moisture coffee (62% w.b.), the solar roof dryer requires pre-drying to 30% w.b. (initial moisture content recommended for the UFV J2 solar dryer). Another disadvantage is the electric power spent to run the fan. For products that are relatively dry after harvesting, such as corn, rice or beans, initial terrace drying is not necessary.

Figure 30 – General view of the rotating solar dryers (UFV-JPC1)

7.4. Coffee drying in “hybrid terrace” The “Hybrid Terrace” is a conventional coffee drying terrace, adapted to use a ventilation system with the drying air heated by a furnace (Figure 31). It allows for coffee drying in total absence of solar radiation, during the night and cloudy or rainy days. In the drying terrace a ventilation duct (main duct) with six or more exits for the hot drying air is adapted in placed in the lengthwise direction, beneath the pavement. A metal aeration duct (perforated area of 23%) is also placed over the air exits, in the same direction as the main duct. The coffee cherries or peeled coffee are uniformly distributed over the aeration duct to be dried with hot air in the absence of solar radiation (Figures 32a to 32i).

Figure 31 – Superior view of the hybrid terrace dryer

For the 150 m2 drying terrace studied (7.000 L of coffee fruits capacity), the heated air is forced to pass through the coffee layer by a 5 hp centrifugal fan coupled with a furnace. In rainy periods, the coffee layer is covered by a removable polyethylene film, fixed in such way to permit the free flow of the exhaust air (Figures 33a and 33b).

39


Hygienic Coffee Processing and Technologies

40

Figure 32a – General view of the drying terrace before the aeration ducts are set up

Figure 32b – Details of the air exits for the aeration duct

Figure 32c – Entrance of drying air in the aeration

Figure 32d – Assembling the aeration duct systems

Figure 32e – Assembling the aeration duct systems

Figure 32f – Assembling the aeration duct systems


Coffee Drying Methods

Figures 32g and 32h – Details of coffee beans distribution

Figure 32i – Coffee layer in drying process and ready to be covered

Figure 33a – Covering of the coffee layer

Figure 33b – Ventilation turned on

41


Hygienic Coffee Processing and Technologies

7.4.1. Hybrid terrace drying operation Due to changes in the drying terrace procedures, the coffee drying can be done continuously, using solar energy during sunny days and heated air during the night or rainy days. Using the “hybrid terrace” procedure, fully wet coffee fruits from the hydraulic separator, can be dried till storage moisture content (12%w.b.) in less then 5 days. Figure 34 – Drying curve for natural coffee using terrace

drying with heated air system H1 (solar + biomass energy) From July to September, and H2 (only biomass energy) Donzeles (2002) tested, simultaneously, two hybrid drying terraces with 150m2 or 7.000 L capacity for coffee drying in two different ways and using two drying air temperatures (40oC and 60oC). For the first drying procedure (Hybrid Terrace 1), the drying was done as in the conventional cement terrace drying from 9:00 until to 3:00 PM, and from 4:00 PM until 8:00 AM on the next day, with forced air, heated by a wood burning furnace. For the second Figure 35 – Drying curve for pulped coffee using terrace drying procedure (Hybrid Terrace drying with heated air – H1 (solar + biomass energy) 2), the heated air system was and H2 (only biomass energy) run, continuously, 24 hours a day without the use of solar energy. For this procedure, the coffee layer, as shown in Figure 32 (i), was revolved at three hour drying interval to achieve homogeneous moisture content.

As a means of comparison, kinds of coffees with the same characteristics were dried in a conventional drying terrace from 9:00 AM until 3:00 PM (drying period for each drying day until final drying).

42


Coffee Drying Methods

According to Donzeles (2002), the following coffee drying treatments were studied: • T1 = Pulped coffee (Hybrid Terrace 2) air drying temperature at 60oC; • T2 = Pulped coffee (Hybrid Terrace 1) air drying temperature at 60oC; • T3 = Natural coffee (Hybrid Terrace 2) air drying temperature at 60oC; • T4 = Natural coffee (Hybrid Terrace 1) air drying temperature at 60oC; • T5 = Pulped 40oC;

coffee (Hybrid Terrace 2) air drying temperature at

• T6 = Pulped coffee (Hybrid Terrace 1) air drying temperature at 40oC; • T7 = Natural coffee (Hybrid Terrace 2) air drying temperature at 40oC; • T8 = Natural coffee (Hybrid Terrace 1) air drying temperature at 40oC; • T9 = Pulped coffee (conventional terrace) comparison test for (T1 and T5); • T10 = Pulped coffee (conventional terrace) comparison test for (T2 and T6); • T11 = Natural coffee (conventional terrace) comparison test for (T3 and T7); • T12 = Natural coffee (conventional terrace) comparison test for (T4 and T8). As a function of the average drying temperature, the results show that the necessary times to dry the coffee from treatments T1, T2, T3 and T4 were smaller than the drying times for treatments T5, T6, T7 and T8, independent of the drying system and coffee type. 43


Hygienic Coffee Processing and Technologies

When comparing the necessary times to dry the coffee, Hybrid terraces took 4.6 and 5.6 less time than the drying times for natural coffee and pulped coffee, respectively, in conventional drying terraces (Figures 34, 35 and 36). From the results (Table 3), the dried coffees were considered of excellent qualities if compared with the traditional coffees types in the same Figure 36 – Drying curves for pulped and for producing area “terrace drying natural coffee using terrace drying with heated air H1 (solar + biomass energy) in Zona da Mata-MG”. The cup quality did not differ with the use of the drying systems (Hybrid Terraces 1 or 2) for the same coffee type. However, the same is not true when the drying results from the Hybrid Terrace system are compared with the drying results from conventional drying terrace. They showed less final quality in all tests performed. Treatment

Cup quality

Type

Aspect

HT2 (pulped) 60 C

Only soft

4/5

Good

HT2 (pulped) 400C

Only soft

4/5

Good

ConvTD* (pulped)

Hard

6/7

Good

0

HT1 (pulped) 60 C

Only soft

4/5

Good

HT1 (pulped) 400C

Only soft

4/5

Good

ConvTD (pulped)

Hard

6/7

Good

HT2 (natural) 60 C

Hard

5

Good

HT2(natural) 400C

Hard

5

Good

0

0

ConvTD(natural)

Hard/Rioy

6/7

Good

HT1(natural) 600C

Hard

5

Good

HT1(natural) 40 C

Hard

5

Good

ConvTD(natural)

Hard/Rioy

6/7

Good

0

*Conventional cement terrace drying Table 3 – Cup quality and coffee beans characteristics after drying tests

44


Coffee Drying Methods

7.5. Table dryer and suspended terrace drier The portable and suspended terrace dryer, according to Dafert (1896), was invented by Geronymo L. C. Souza, in 1888. It consists, concisely, of several rectangular boxes in wire mesh, forming mats of 3.0 x 1.5 m, mounted on 0.8 m high wood pillars (Figure 37). As shown in the figure, the suspended and fixed terrace dryer marketed in Brazil (Figure 38) has similar basic concepts and doesn’t differ much from the suspended terrace idealized by Geronymo Souza. Research works performed by Vilela (1997), Hardoim (2001) and Roberto (2008) indicate that the total drying time in this dryer type is longer than the drying time with the conventional cement terrace.

Figure 37 – Basic outline of the “suspended terrace” or portable terrace dryer

Figure 38 - Group of suspended terraces marketed

7.6. Movable terrace dryer The rights to a movable terrace dryer were registered in November of 1889 by Corrêia da Silva, consisting of several meshed trays to retain the coffee beans (Dafert, 1896). Trays with appropriate dimensions are mounted over rails, and the system is housed under a fixed covering for protection against rain. The dryer manager must pull the trays to expose the coffee layers to solar radiation for drying (Figure 39).

Figure 39 – Basic outline of a movable suspended terrace dryer

45


Hygienic Coffee Processing and Technologies

8. HIGH TEMPERATURE COFFEE DRYING 8.1. Natural convection dryer

A

ir, moved by natural convection, is an alternative to solving drying problems in small coffee farms with no electric power supply. This type of dryer does not need fans, can be easily built with materials found in the farm, and needs little specialized construction labor. Figure 40 shows one dryer with natural convection using one heat-exchanger to transfer the received heat from the combustion gases to the drying air entering lengthwise through openings in the lower part of the dryer walls. The movement of the air that crosses the coffee layer is due to the pressure gradients produced by the temperature difference between the drying air and the ambient air.

Figure 40 – Longitudinal cut of a natural convection dryer

The dryer with natural convection has the following characteristics: • no fans required; • low initial cost; • non-specialized construction labor; • low thermal efficiency as compared with forced convection dryers; • uneven temperature and air distributions for inadequate plenum chamber project; and

46


High Temperature Coffee Drying

• risks of smoke contamination in case of perforations or leaks in the heat-exchanger. However, when well dimensioned and adequately built, the problems above are minimized.

8.2. Flex dryer The Flex dryer is a combination of the traditional fixed-bed dryer with the natural convection dryer and a solar dryer (UFV-J2). The drying air can be heated with energy from the combustion of firewood, charcoal or gas, with solar energy or even a mixture of solar energy and combustion energy. Because it is capable of using different sources of energy, it was named “Flex dryer” (Figures 41 to 43). Figure 41 – Flex dryer with a solar roof

Basically, the Flex dryer as the natural convection dryer, is composed of a common furnace, a heat exchanger and a chimney. To complete the dryer system, a fan was added to force the convection of the hot air to overcome the resistance offered by deeper coffee layers. In this case, the dryer works as if it was a traditional fixed bed dryer with a furnace for indirect heating of the drying air.

It is important to observe that in the absence of electricity, which is very frequent in rural electric supply, Figure 42 – General view of flex dryer drying will not be interrupted and will have continuous operation with natural convection. In this case, the height of the coffee layer should be reduced or more frequent stirring is needed. Besides forcing the convection in the dryer “plenum chamber”, the other function of the fan is to force the ambient air to pass by the channels in the “collector roof” and be pre-heated by solar energy.

47


Hygienic Coffee Processing and Technologies

The use of the complementary solar energy, reduces consumption of other fuels causing no pollution, and the “collector roof” is just a little more expensive than a common roof

Figure 43 – Details of Flex dryer components

8.3. Fixed-bed drying For small scale operations, drying of coffee in fixed-beds has become one of the most widely used techniques in the “Zona da Mata, MG – Brazil”. Stripped coffee with a wide range of initial moisture contents is placed in the dryer and heated air is forces to pass through the coffee layer using a powerdriven fan. Loading and unloading are generally done by hand. Traditionally, as in cross flow dryers, the air temperature must be kept at moderate levels (below 50°C) in order to minimize over-drying in those layers closer to the air inlet of the dryer mainly at the final drying. Generally, drying is stopped when the average moisture content of the whole grain layer has reached the required level for safe storage. By this time, the grain close to the exhaust air side of the dryer may be still under dried. Two or three hours of manual coffee stirring is a common practice adopted to break up the temperature and moisture content gradients to obtain a more uniform dried product during fixed-bed drying. If the fan is not turned off during stirring operation, the dryer operator will be exposed to harsh conditions of the exhaust drying air. So far, very little research on coffee-stirring devices has been done to determine the extent of mechanical damage to the product. However, the introduction of

48


High Temperature Coffee Drying

stirring devices increases both the fixed and operating costs of the fixed-bed dryer. Silva and Lacerda Filho (1984) designed and built a fixed-bed dryer to dry natural, washed and pulped coffee (Figures 25 and 44a). The dryer has been widely adopted by small and medium scale coffee farmers and has been used to dry other agricultural products such as beans and corn. The dryer can be built with a drying chamber up to 5.0 m in diameter and 0.6 m high. The plenum chamber with the same height, results in a total dryer height of 1.2 m. The dryer walls are consisted of a 0.15m think brick layer plastered on both surfaces. Originally, the generation of heat for the fixed-bed dryer takes place in a one stage downdraft direct combustion furnace using firewood as a fuel source. The original furnace (Figure 44b) is equipped with a cyclone chamber where the ash is trapped and where natural air is mixed with the combustion gases to obtain the desired drying air temperature. The cyclone chamber is made with bricks and consists of a 1.2 m high cylinder with a 1.0 m diameter. The mixed gas is drawn through the same backward-curved centrifugal fan that is used to force the drying air through the coffee layer. However, any kind of heat source can be used to heat the drying air for the fixed-bed dryer. The charcoal burner (Figure 44c) developed by Lopes & Silva (Silva et al, 2000) is now widely used for air heating in coffee dryers.

Figure 44a – General view of a fixed bed dryer (model UFV)

Figure 44b – Smokeless direct wood fired burner

The fan must be selected based on Figure 44c – Smokeless direct charcoal previous calculations to provide the amount of fired burner excess air needed for a complete combustion and a clear burning (smokeless). For coffee drying, not only particulate emissions must be below acceptable levels, but also the dried product must be free of odor or any harmful contaminants.

49


Hygienic Coffee Processing and Technologies

Drying tests have consistently shown that the fixed-bed dryer (Figure 44a) has potential to reduce the moisture content of a layer of natural coffee (0.4 m high) from 62% to 12% w.b. in 50 h, provided that the drying air temperature is kept at 55째C, and coffee bed stirred at 3 hour intervals. The amount of energy (6,600 kJ kg-1 water removed) consumed during the drying of a 0.4 m deep natural coffee layer is approximately 65% greater than the amount (4,100 kJ.kg-1) consumed during the drying of the same layer of washed coffee. These values were obtained for coffee originally at 52% w.b and subsequently dried to about 14% w.b. with an airflow rate of 12 m3 min-1 m-2, drying air temperature at 60째C, and a stirring interval of 3 h. Dryer output increases from 9.8 (natural coffee) to 18.7 kg h-1 m-2 (washed coffee) for these same drying conditions.

8.4. Pneumatic coffee dryer The fixed-bed dryer, a model used for coffee drying, requires stirring for product homogenization during the drying process. When manually executed, the stirring operation requires great physical force, mainly at the beginning, when the product still maintains high moisture contents. During manual stirring, the fixed-bed dryer presents problems such as: use of unnecessary labor, energy loss, and increasing in the drying time. For dryers with mechanical stirring, the increasing of the grain mass flow rate in the drying chamber generally produce coffee with better final quality. However, increasing the grain mass speed will increase the specific energy consumption. Grain that passes by the drying chamber with higher speed will lose less moisture per unit of time. Besides the mentioned drawbacks, systems for coffee load/unload and stirring may cause difficulties in the project installation, in dryer maintenance, and increase initial cost. A simple transport system with relatively low cost is the pneumatic transporter. Used in storage facilities, this transporter had its origin in the pressure equipment used to load and unload ships with grain. The pneumatic transporters move the grains by the use of air at high-speeds, through a hermetic piping system. With the pneumatic system, the product can be transported in any direction, including curved pathways. Another interesting aspect of pneumatic transport is its use in fixed installations which can be built without significant structural changes. One fixed bed dryer coupled with a low cost pneumatic transport system produced a new coffee dryer (Figure 45) that allows the movement of the

50


High Temperature Coffee Drying

product (loading, unloading and stirring) and the flow of drying air, using a single centrifugal fan powered by a 2 hp electric motor (Sampaio et al, 2007). With these characteristics, the small dryer can delivery up to 60 kg of dry parchment coffee per hour. The coffee drying system was built with metallic foils and masonry bricks, and consists of the following parts. (1) Charge hopper: A cylindrical hopper, 1.09m in height of and 0.6m in diameter contains coffee beans to feed the dryer during the drying process. The wall of the conical base was inclined at 35°, forming a lower section with a height of 0.13 in height and of 0.6 m in diameter, and 0.32 m3³ in volume. (2) ) Conveying system: The conveying system consists of a metallic piping for grain stirring and air flow conveying to the drying chamber. The controls for drying, stirring and unloading of the dryer were manually operated by the located registers along the piping. The conveying system, due to available facilities, was built of square section, and it is 6.7 m, in length, 0.01 m²2 in transverse sectional area, and 0.067 m³3 in volume. The main characteristics of the pneumatic system are shown in Table 4. (3) Feed hopper: Besides receiving the flow of beans from the pre-drying stage, the feed hopper was used to distribute the grain in the drying chamber and was provided with a rubber pipe shock absorber for reduction of the grains impact. Its volume was 0.30 m3³. (4) Drying chamber: The drying chamber was a special structure composed of two pyramids constructed with perforated foils (23% perforation) and connected at their bases. The structure allowed the air flow, in direct contact with the coffee beans, to dry in a reverse flow. With a basal area of 1.10 m²2 and height of 0.48 m, the pyramids can hold 0.12 m³3 of coffee beans. To maintain evenly distributed Figure 45 – General view of the pneumatic coffee dryer system coffee in the drying chamber, two special devices were built. With pyramidal forms, the mixing devices were constructed on the external part of the upper pyramid and, internally, in the lower pyramid. The devices for grain

51


Hygienic Coffee Processing and Technologies

mixing were composed of parallel bars (Figure 45). In addition to allowing for good distribution of the product in the pyramids, it established repose angles predetermined and they allowed for gravity to move the product. With the coffee beans evenly distributed, the drying chamber took the form of a hollow octahedron, with 5 cm hick walls, forming two plenum chambers. (5) Dryer unloading: the dryer is unloaded trough unloading valves by blocking the air flow from the plenum chamber and the flow of beans from the feed hopper. Worked manually, the transportation system can be used as a stirring device during drying or as an unloading device once drying is completed. Characteristics

Air

Air+Product

Total pressure, mm.c.a

71.2

72.4

Static pressure, mm.c.a

32.0

40.0

Air velocity, m.s-1

25.3

23.0

Air flow rate, m .min

15.2

13.8

Air flow density, m3.min-1.m-2

1.52

1.38

Product velocity, m.h-1

-

21.6

Transport capacity, kg.h-1

-

318,92

Fan power, cv

0,51

-

3

-1

Table 4 – Main characteristics of the pneumatic system (figure 45)

8.5 Counter flow coffee drying To introduce counter flows system for coffee drying, a prototype dryer (Figure 46) was developed, built, evaluated, and adapted to dry coffee under Brazilian farm conditions. For the dryer analysis (Silva, 1991), natural coffee fruits with 30% moisture content, pre-dried in a fixed bed dryer were used. The performance of the prototype for drying of coffee to 12% w.b. moisture content under an airflow rate of 18.5 m3 min-1 m-2 is 52

Figure 46 – Basic elements of a counter flow dryer (model UFV)


High Temperature Coffee Drying

summarized in Table 5. Despite using considerably higher drying-air temperatures as compared to those normally used in conventional coffee dryers, no quality deterioration was observed in the coffee after drying and roasting. Regarding the cup quality, the product presented better results when compared with the same coffee dried in cement drying terrace. Air temperature (°C)

Drying time (h)

Energy (kJ kg-1)

Yield (kg h-1)

60

21.5

8,300

50.2

80

14.2

7,550

76.1

100

10.2

6,440

105.9

Table 5 – Counter flows drying of natural coffee: Effect of drying air temperature on total drying time, specific energy requirement, and yield (dry coffee).

8.6. Concurrent flow coffee drying With the same objectives as the counter flow dryer, an intermittent concurrent flow dryer (Figure 47), presenting similar architecture as the counter flow dryer shown in Figure 46, was designed, built, and evaluated. The main characteristics of the prototype Figure 47 – Basic elements of a concurrent are: high thermal efficiency, low initial flow dryer (model UFV) investment, simple operation and maintenance. With this dryer, it was possible to dry natural coffee from an initial moisture content of 25% to 11% w.b. in 7.5, 6.0, and 5.0 h for drying air temperatures of 80, 100, and 120°C, with specific energy requirements of 5,700, 4,870, and 4,760 kJ kg-1 of evaporated water, respectively (Lacerda Filho, 1986). The dryer was built with an effective height of 4.0 m, capable of holding approximately 2,300 kg of natural coffee at 25% w.b. moisture content, and operating with an airflow rate of 27 m3 min-1 m-2. Inside the dryer, grain flows by gravity from a 1.5 m deep holding tank, enters into a 0.7 m deep drying chamber, and then a tempering section of 1.8 m high. The unloading auger, located at the bottom of the dryer, maintains the coffee mass velocity at 3.5 m h-1 to guarantee a retention time of 0.2 h in the drying section. 53


Hygienic Coffee Processing and Technologies

The unloading device removes the partially dried coffee from the tempering zone and transports it back to the holding section at the top of the dryer. It must be remembered that due to the evaporative cooling effect at the inlet of the drying chamber, the coffee cherries never reach the drying-air temperature. Even though utilized temperatures were greater than those used for conventional high temperature coffee drying, quality deterioration in the coffee brew was not observed. In an effort to reduce specific Figure 48 – Details of a dryer with mixed energy requirements and improve the flows efficiency during the drying of natural coffee (Pinto, 1993) proposed a new dryer design, as shown in (Figure 48 and 49). The design combines the counter flow and concurrent flow drying methods, in separate stages. The inlet drying air is located halfway down the coffee column. Drying in the first stage is accomplished using the counter flow drying concept, and then the coffee flows directly to the second stage, where it is dried with concurrent air flow. The dryer is fitted with a 60° gravity hopper which delivers the coffee to a central point below the tempering section. From this point the product is transported to the bucket elevator pit. With each drying section being 1.10 m deep, the dryer is capable of holding 4,500 kg of natural coffee with 30% moisture content. With a flow rate of 1.44 m.h-1, the coffee mass takes approximately 0.8 h to pass through each drying section in each drying pass.

Figure 49 – Farm dryer with mixed flows

54

Table 6 shows the results of performance evaluation conducted with the experimental counterflows-concurrent flows dryer (natural coffee) using drying-air temperatures at 80, 100 and 120°C, and an airflow rate of 20 m3 min-1 m-2. The production rate (kg. h-1) is based on a 30% to 12% moisture content reduction. A comparison of the results presented in Table 6 shows that total drying time and the specific


High Temperature Coffee Drying

energy requirement decreases 44% and 6.4%, respectively, while the drying capacity increases 80%, when temperature is increased from 80 to 120°C. Because of the small differences between the specific energy consumptions (Table 6), three drying tests with pulped coffee were performed. The drying air was heated by an indirect heating furnace to 75oC. From the moisture contents of 32% to 13%, the energy consumption was 10.3 MJ.kg-1 (Silva et al., 2001). Air temperature (°C) Drying time (h) 80

22.5

Energy (kJ kg-1)

Throughput (kg h-1)

6,070

200

100

15.7

5,660

290

120

12.6

5,680

360

Table 6 – Effect of drying air temperature on total drying time, energy requirement, and production rate (dry coffee) with intermittent two-stage mixed flows dryer

One very interesting variation recently developed at the Federal University of Viçosa (Melo. 2008), is the concurrent flow dryer shown in Figure 50. Using a pneumatic transport system similar to one shown in Figure 45, the new dryer has a static capacity to dry 2.500 L of parchment coffee, and was constructed in modules to form a easily assembled KIT. The KIT is composed of module 1 (main pit), module 2 (distribution pits), module 3 (drying chamber), Module 4 (pyramidal roof with vent), module 5 (pneumatic system), module 6 (reception pit) and a centrifugal fan. Besides the low price and high energetic efficiency, one of the big advantages of the new dryer is the capability to dry any quantity of coffee from 50 L till full capacity (2.500 L) of the studied prototype.

Figure 50 – Concurrent flow dryer using a pneumatic transporter (UFV/CBP&D -Café)

* Source (Pinto, 1993)

55


Hygienic Coffee Processing and Technologies

8.7. Dryeration The dryeration process which involves high temperature drying, product resting or tempering and final aeration is one of the innovative drying methods that have been adapted to coffee drying (Figure 51). When using the dryeration process the moisture content of the coffee exiting the high temperature dryer is approximately 2.5 % higher than the desired level for safe storage. The hot coffee is conveyed to a holding silo where it is tempered, with no airflow, for no less than 6 h. Then the coffee may be cooled or transferred to another silo to be cooled with low airflow. During the aeration or cooling, some drying takes place and the remaining moisture is removed (less than 2.5%). Dryeration provides three advantages over traditional coffee drying methods: • increased dryer capacity; • reduced energy requirement; and • production of better coffee quality in terms of moisture content. The effect of drying temperature and tempering on the moisture difference throughout a fixed bed of natural coffee, and energy requirement during the dryeration process has been determined (Cordeiro, 1982). Table 7 summarizes the mean values of the final moisture content difference throughout a 0.4 m deep coffee bed for three drying-air temperatures (50, 60, and 70°C), airflow rate (15 m3 min-1 m-2), and three tempering periods (0.0, 6.0, and 12.0h). The values presented in Table 7 Figure 51 – Working scheme of the coffee were obtained during drying of during dryeration processing natural coffee with 28% initial moisture content. High temperature drying was interrupted when the moisture content of the coffee fruits reached 13%. The 2% of moisture was removed during aeration or cooling.

56


High Temperature Coffee Drying

From the values presented in Table 7, it can be concluded that drying air with a temperature of 50°C and 12 h of tempering was the best dryeration treatment for natural coffee with final moisture gradient of 1.7% between the upper and lower layers; almost 50% less than the difference obtained with the conventional fixed-bed drying with no tempering. An airflow rate of 15 m3 min-1 m-2 significantly exceeded the values normally encountered in grain dryeration processes. However, if coffee storage silos are not equipped with perforated floors, the fixed-bed dryer can also be used as a tempering silo during the night periods. Tempetarue (0C)

Tempering period (h) 0.0

6.0

12.0

50

3.2

2.3

1.7

60

3.6

3.0

2.8

70

3.8

3.0

2.9

Table 7 – Values of final moisture content difference (first layer and top layer) obtained during the dryeration of a 0.4 m deep coffee layer (fixed-bed of natural coffee drying).

8.8 Combination drying (High temperature plus ambient-air drying) Combination drying is one of the latest concepts in coffee drying technology. It is a drying method in which any high temperature process is combined with solar drying, low temperature drying or natural air drying in an attempt to provide a high quality product as an alternative to conventional coffee drying. Any initial safe drying method can be used to reduce coffee moisture contents from approximately 60% w.b. to 25% w.b. or less so that natural air drying can be successfully used during the storage period to reduce the coffee moisture content to safe limits of 12% w.b. Unlike the dryeration process, where coffee is tempered by a separate silo for several hours before being slowly cooled, in combination drying the partially dried coffee is transferred directly to the storage silo where the excess * Source: Cordeiro (1982).

57


Hygienic Coffee Processing and Technologies

moisture content is removed using ambient air or low temperature drying (Guimarães, 1995). In this process, coffee is partially dried to a specified moisture level above the commercial value, before being dried with a recommended airflow rate during the storage period (Table 8). Product

Moisture (%w.b.)

Air flow rate (m3 min-1m-3)

Natural coffee

Below 20

2.5

Pulped coffee

Below 25

2.5

Table 8 – Recommended airflow rates and initial moisture content for natural air drying of coffee during storage

Combination drying provides three advantages over any conventional drying method (terrace drying or high-temperature drying): (a) increased drying capacity; (b) reduced fuel requirement; and (c) better coffee quality by the homogeneous final moisture content throughout the deep coffee layer. As compared with the dryeration process, the main advantage of combination drying is the elimination of the extra handling step associated with the tempering silo. The main disadvantage of combination drying is the requirement of a relatively high initial level of investment and management in silos with aeration systems.

8.8.1. New combination System (Pre-dryer, dryer and Silo-dryer) Figure 52 shows the basic scheme of a combination model recently adapted at the Federal University of Viçosa for coffee drying (Melo, 2008). The system is composed of one pre-dryer (hybrid dryer), one pneumatic concurrent flow dryer and silos with ventilation system for final Figure 52 – Basic scheme of the UFV combination drying, all combined. However, drying model. any kind of high temperature coffee dryer can be used in this combination. There is no need for drying terrace and it is indicated way for parchment coffee. As for drawbacks, beside the necessity of more silo volume, if 58


High Temperature Coffee Drying

compared with parchment coffee, the drying of natural coffee in silo is an electrical energy consuming process. As it is shown in Figure 53, in the UFV combination project only one silo-dryer was built. However, the farmer can opt for two or more silos including silos of different sizes for complete storage. The commercial qualities of the coffee dried in the new system (Figure 53), for one full silo, are shown in Table 9 (Melo, 2008).

Figure 53 – Real combination model installed in UFV processing area

Layer

Moisture content %b.u**

Screen %

Sorting %

Type

Benefit %

Cup quality

1 (superior)

11.8

67*

8

5

71

soft

2

11.8

62

8

5

73

soft

3

12.0

60

11

5

73

soft

4

11.8

68

10

5

68

soft

5

11.6

64

12

5

73

soft

6

11.5

64

10

5

72

soft only

7

11.2

63

16

5

73

soft only

8

11.7

79

10

5

73

soft

9 (inferior)

11.4

59

12

5

70

soft only

Composed***

11.5

67

12

5

71

soft

Table 9 – Results of coffee classification, after final drying in the silo-dryer

* Screen 17 above. ** Commercial moisture meter. *** Composed sample, taken during silo unloading.

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Hygienic Coffee Processing and Technologies

8.8.2. Combination drying procedures After appropriate hydraulic separation, the coffee cherries must be pulped and washed to have part of the mucilage removed. Afterwards, the washed coffee is pre-dried by any safe drying method that works appropriately for coffee with high moisture contents. The new drum type dryer (Figure 54) can be used to dry wet cherries or pulped coffee. This technology can easily be adapted in a commercial drum type rotary dryer (Santos et al, 2006).

Figure 54 – Small drum type dryer (600 L capacity) for wet coffee drying

In any high temperature dryer, the drying air for parchment coffee should be heated indirectly in the case of firewood burning or directly when using gas or charcoal as source of heat energy. In the terrace drying or high temperature drying phases, the washed coffee should be dried until its moisture content reaches values indicated in Table 8 and, soon afterwards, be transferred for the complementary drying in silos with natural or slightly heated air; the average value of relative humidity for coffee drying in silo must stay around 65% (Silva et al, 2008). The usual procedure to dry natural coffee using the combination method is as follows: ripe cherries are separated by water flotation and screening, and then dried in either two or three separate stages. Where three stages are used, the coffee is first dried on paved terraces until reaching approximately 35% w.b. Then, the partially dried fruits are moved to a high temperature dryer where coffee moisture content is reduced to less than 20% w.b. In the third stage the coffee is silo-dried to approximately 12% w.b. moisture content, using natural air. When using two drying stages, the sun drying procedure is eliminated and the moisture content after washing is reduced to approximately 20% w.b. using the high temperature dryer alone or a in combination as shown in Figure 53. Figure 55 shows a silo used in combination drying for small-scale operations. The silo, with 2.0 m diameter chamber, can hold approximately 2,520 kg of coffee with a moisture content of 22% w.b. The silo wall consists of one brick layer 0.12 m thick covered with impermeable mortar on both surfaces and painted with a reflecting paint.

60


High Temperature Coffee Drying

The silo is equipped with a perforated floor for drying-air distribution. The surface of the coffee bed is generally open to the atmosphere but shielded from rain by a roof erected over the silos. Silos must be built on a waterproof concrete floor. A centrifugal fan driven by an electric motor is used to force air upward through the coffee bed. The fan should be turned on as soon as the first load of coffee is delivered into the silo, and is turned off during some periods of adverse drying weather or when the average moisture content of the coffee in the upper surface reaches the target moisture content (12%). For natural air drying systems, the total drying time depends on the initial moisture content and climatic conditions. Attempts to use low temperature drying methods for natural coffee with high initial moisture contents have so far been unsuccessful (Guimarães.1995). For example, experimental drying tests have shown that for high initial moisture contents, 27 to 30% w.b., excessive heating develops within the coffee bed after approximately 10 days of fair weather conditions (0.0124 kg kg-1 humidity ratio and temperature of 18°C), and a careful inspection will generally indicate fungi invasion. In cases where drying is not interrupted, it is observed that Figure 55 – Small silo with aeration system for the ambient air generally is not cool drying and storage enough to serve as a barrier to fungi development, and a strong musty odor will develop. The results obtained thus far, suggest that drying highmoisture natural coffee in the low temperature stage of combination drying is a very hazardous procedure. On the other hand, drying tests using high moisture pulped coffee have been successfully performed (Melo, 2008). It takes approximately 1,000 h to dry coffee, initially at 37% w.b., to average moisture content of 12% w.b. With an average moisture reduction rate approximately 4 times greater than that obtained with natural coffee, low temperature drying of pulped coffee is considerably more efficient.

61


Hygienic Coffee Processing and Technologies

8.8.3. Recommendations To correctly operate the combination drying of coffee the following procedures are necessary: • install a fan with air flow (m3.min-1.m-3 of grains) appropriate for coffee with initial moisture content, according to the values presented in Table 8; • despite the fact that the silo drying is technically viable for natural coffee, the farmer should be informed that energy consumption (electricity) for coffee husk drying, and the silo capacity will be underused and; • if there is an option to produce natural coffee, it is better to perform an economic feasibility study of the system.

8.8.4. Combination drying management The combined drying system, composed of seven silos is an excellent system to dry coffee and constitutes the most appropriate option when a new installation has to be built (Figure 56). After drying is completed, the product can be sold or maintained in the silos (Silva et al, 2008). Independent of the partial drying procedure, it is important to avoid, during all processing phases, any undesirable fermentation in order to produce high quality coffee with natural flavor. To achieve good results the operator should be familiarized with the process and be attentive to the fan operation during the silo drying phase.

Figure 56 – General view of the seven silos installation

62


High Temperature Coffee Drying

The combination drying should be processed in the following way: • The pulped or washed coffee should be sent for pre-drying as soon as possible to have its moisture content reduced to a preset value, according to farm environmental conditions. The moisture must be set according the average temperature and relative humidity during the harvest season (Silva et al, 2008); • • Besides the fixed-bed dryer or drum type dryers, any type of terrace of proven quality can be used for this operation when climatic conditions are favorable. During pre-drying, when high temperature dryers are used, the system must be operated using a smokeless furnace to avoid product contamination (never let coffee temperatures be greater than 40oC). • After pre-drying, the coffee should be transferred to a drying silo, and soon afterwards the fan must be turned on until the top layer in the silo reaches a moisture content of 17% w.b. Below this value, the fan will stay on only during periods with ambient relative humidity below 70% (usually during the day time). The ideal would be an automatic controller coupled to the ventilation system, working automatically for preset temperature/relative humidity. In spite of this, it is advised daily inspections to verify the absence of fungi growth in the layer being dried. A new layer will be added only when the previous one is already dried or in equilibrium with the ambient conditions. • The fan must be turned off when the product moisture content in the top layer reaches a value close to 12% w.b. or if the drying air is no longer removing moisture from the product. The total drying time will depend on the depth of the coffee layer in the silo, the added amount of coffee, the air flow rates, the climatic conditions, and the initial moisture content of each added coffee layer. • For most coffee farms in Brazil, the equilibrium moisture content is around 12% w.b • When the fan is turned off for the real storage phase, the operator must close the fan air entrance to avoid rewetting and deteriorations of the product. • After drying, daily inspection of temperature and moisture content should be done.

63


Hygienic Coffee Processing and Technologies

• In the case of grain mass heating or increasing moisture content, the cause must be verified. The fan should be turned on until the whole mass of coffee returns to its normal condition. Although combination drying can be accomplished with a reduced number of silos, resulting in a smaller installation cost, the adoption of a system composed of seven silos is highly recommended. This system is nothing more than the use of seven silos (metallic or masonry). With this number, the silos will be loaded weekly with a preset coffee layer, calculated according its initial moisture content and air flow rate (Figure 57). By the end of the harvest season each silo will be fully loaded when the last coffee layer is added. In the silo #1 all added coffee layers are already in equilibrium with the ambient conditions and the fan #1 can be turned off. The silo #6 will be dried in one more week. This indicates the end of the drying process and all fans are turned off. Figure 57 shows that silo number seven should always be empty to account for eventual problems during the drying period. In conclusion, combination drying of natural and pulped coffee offers a reliable alternative to farmers who wish to deliver a high quality coffee. Success depends on the design, proper sizing and good operation of the drying ventilation system. Under appropriate weather conditions, it is feasible to dry natural coffee from approximately 20% moisture content down to 13% using appropriate airflow rates. The drying of natural coffee is not recommended under poor weather conditions. On the other hand, low temperature drying is very effective to dry fairly high moisture pulped coffee.

64

Figure 57 – Silo charge system for eight weeks harvesting


Coffee Storage

9. Coffee Storage

A

fter drying, the dry natural coffee or the dry washed coffee should be stored in appropriate places to avoid quality deteriorations.

The dry coffee fruits are usually conditioned in jute bags with a capacity of 30 kg or bulk stored in appropriate deposits or silos. In bag storage, the bags are piled according to product origin (Figure 58). The storage facility should be clean, sheltered from the sun and rain, and be well ventilated. The advantage of the jute bag is its resistance and capacity to have the opening closed after sampling. Due to the great volume to be stored and the high cost of the conventional storage operation, the dry coffee cherries can be bulk stored in silos. Despite the dry peel protection, the possibility of physical and chemical modifications exists, mainly if the silos are not provided with adequate ventilation and humidity protection. In general, the hulled or green coffee is traditionally stored in 60 kg jute bags instead of in bulk storage. With some disadvantages, bag storage allows lots segregation, an important aspect in product evaluation, to fit commercial standards or origin. Besides easy access, air circulation around the pile, easy inspection, and sampling, the spaces between piles and walls are important factors to be considered when bag storage is used. In general, bag storage presents small or no control of the environmental conditions. In spite of this, it is possible to maintain the product stored for relatively long periods without great deterioration risks Figure 58 – Details of coffee bag storage

9.1. Green coffee bag storage Despite technological advances, in recent years, almost all green coffee is stored in bags and piled in warehouses. A 60kg coffee bag is one unit that can be adapted for handling and commercialization on a small scale. This kind 65


Hygienic Coffee Processing and Technologies

of storage has advantages and disadvantages in comparison with silo coffee storage systems. Any system should be economically compared before adoption. The advantages of the bagging method are: • uniform conditions to manipulate different quantities and types of coffee, simultaneously; • product individualization contained in the same lot; • eradication of potential localized deterioration within the lot; and • less initial investment. Disadvantages: • the high cost of bags, which must be frequently replaced; • high handling cost, large labor demand; and • need for greater space per stored weight. Increasing the disadvantages, usually presented by bag storage, the whitening process and the specific mass reduction will change coffee quality. Due to damage during storage, the product can be rejected or suffer a price reduction. Some construction aspects which influence warehouse use should be considered when evaluating a bag storage system. It is necessary: • sufficient number of doors, strategically installed, to facilitate the loading and unloading operations; • doors should be installed linearly, similar alignment on opposite walls; • minimum wall height of 5.0 m; • the walls must have smooth surfaces without sharp corners, mainly in relation to the floor; • the doors and lateral walls, close to the floor and roof, must be rodent and bird proof;

66


Coffee Storage

• lateral ventilation must be provided and be adjustable and screened; • technically installed, air vents provide good air circulation; • use of some transparent yellow roof tiles to improve natural illumination; • the floor must be impermeable and at least 40 cm above soil level; • for each external door, shelter must be installed for loading and unloading during rainy days; • for maximum capacity, the floor area must be calculated according to the pile size and the work lanes (Figure 59); and • system installation for fire prevention and control.

Figure 59 – Warehouse division for blocks and lot segregation

9.2. Warehouse floor The characteristics of the material employed in the construction of warehouse floor should be chosen based on technical and economical aspects involved, and directly linked with the coffee quality preservation. Cement is the primary material used, however, in some unfavorable conditions, wood can be the best option but the floor must be built as a suspended structure in relation to the soil (Figure 60). a) Wood floors: provides a cover surface with good heat isolation characteristics, it can prevent great temperature oscillations inside the warehouse. The main disadvantages are: 67


Hygienic Coffee Processing and Technologies

• wood has relative cost higher than concrete floor; • it is water permeable; and • may present reduced durability. b) Cement floors: currently, is the floor type most used. In relation to wood floors, cement floors provide higher durability and lower cost. It has poor heat insulation and lacks water impermeability. To be water proof, it Figure 60 – Detail of an on farm storage must be constructed with facility with suspended floor appropriate techniques and special products. The use of wood bases or pallets to protect the piles on the concrete floor is compulsory in Brazil.

9.3. Bulk Storage Although uncommon on the producer level, bulk storage of hulled coffee is a procedure that has been adopted by innovative producers and companies with a large market of coffee with uniform characteristics.

Figure 61 – Metal silos for green coffee storage with aeration system

68

For good bulk storage over long periods it is necessary to adapt a silo with an aeration system (Figure 61). The complete in-silo storage system must have an efficient thermometric device to allow for maintenance of the coffee bean mass at ideal temperatures and moisture contents.


Coffee Storage

An objection to coffee bulk storage is the difficulty to accomplish necessary inventories. Any small variation in the apparent density can cause great errors in the stock evaluation, a problem not faced when the coffee is stored in bags. The importance of necessary inventories of the stored product is the high coffee price. The main advantages of bulk storage are the possibilities of using mechanization with substantial reduction in the necessary labor in comparison with traditional bag storage.

9.4. Good practices The storage conditions have great influence in the final quality of the product. Therefore it is recommended: • To avoid cross contamination, never put parchment coffee, dry cherries or green coffee together. Separate finished products from debris; • Store coffee in leak-proof warehouses and away from the walls. Use warehouses with heat insulated roofs, if possible; • Use clean, dry and odor-free silos only for coffee storage, avoid possible cross contamination; • In humid areas, keep coffee in warehouses for short periods to avoid moisture absorption; • Store coffee with moisture content below critical levels (12%); • Make sure walls, floors and roofs are waterproof; • Use proper roof construction to minimize heat transfer; • Avoid product contact with walls; • Adequately sanitize deposits, silos, and warehouses; • Before harvest, a complete cleaning must be done in silos, warehouses, and deposits; • Similarly, all machines, equipment and utensils used in the postharvest coffee processing must pass through a hygienic treatment process; 69


Hygienic Coffee Processing and Technologies

• The removal of all grain residues, dusts, debris and foreign materials of the previous crops is a practice that should never be forgotten; • High-pressure hydraulic cleaners, compressed air and vacuum cleaners are proven to help in cleaning procedures; • Storage areas should be kept free from cleaning residues to avoid the presence of insects, birds and rodents; • In general, debris and residues should never be gathered in the proximities of the storage facilities and must be burned or buried; • A complete inspection should be made of all roofs, gutters, ducts and galleries in order to eliminate eventual leaks and bad drainage of any kind of water; • The walls and floors should be kept dry and the roof should be free of leaks or possible condensation; • Storage areas should be free of rodents, bats and birds and periodically treated and disinfected with appropriate products; • Broken pallets, damaged materials or any other material, no longer in use, must be removed from the storage areas; • Application of insecticides is one component of good hygiene practices. Insecticides aim to eliminate any existing insects in the environment and create a barrier against the invaders. Therefore after general cleaning, extermination with residual insecticides approved by the Department of Agriculture must be done; • Products with infestations must be separated and removed for insect elimination; • External areas must be paved and clean, free from junk, scraps and material out of use; • In the external area of the storage facilities, special attention should be given to eventual vegetation that can be used as shelter or feeding sources for insects, rodents or other plagues. Grass, when present, must be trimmed to avoid plague proliferation;

70


Coffee Storage

• External areas should be well illuminated with light bulbs installed in distant locations to reduce the attraction of insects near doorways; and • The sidewalks or working areas close to the walls should be kept free and clean to prevent infestations. Any general hygiene practices that guarantee high quality products are recommended: • Keep equipment clean. Mud and debris should be removed from equipment and utensils should be cleaned daily; • Check for defects that prevent adequate cleaning or accumulate dirt. Equipment should be replaced or fixed when necessary; • Harvesting containers, equipment, and utensils should be cleaned daily after each delivery. If kept outdoors, it should be cleaned before being used again; • Drying terraces should be cleaned before drying of each coffee lot; • All areas near dryers and furnaces must be kept clean to facilitate easy access and prevent infestation sources. Such procedures prevent accidents; corrosion of metallic parts and avoids contamination of the products being dried; • Keep the areas around storage facilities in good hygienic conditions (free from garbage, residues and non-used material); • Inspect regularly all the facilities to ensure there is neither plague population or domestic animal contamination; • Establish an effective plague control program; and • Keep records of the inspection dates, inspection reports and corrective actions.

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Hygienic Coffee Processing and Technologies

10. Silo Construction on coffee farm 10.1. Selection of the Area

T

he place to install a silo should be of easy access and preferably close to coffee processing units. It should fit the operational sequence: preparation, drying, natural coffee storage, hulling and green coffee storage (green coffee should be stored in the commercialization warehouses). The ideal is to construct or maintain the silo under a covered area, allowing for loading and unloading operations independent of the weather conditions (Figure 53). Besides making handling easier, the cover will give better protection to the silo structure and stored product.

Figure 62 – Silo for drying and storage (UFV model).

The type of silo presented in this work (Figure 62) has a 5,500 L of storage capacity. However the farmer can build bigger silos or several smaller silos located side by side. To fit the farm storage need, the silo dimensions should correspond to the lowest cost per stored ton. However, despite being more expensive, several small silos provide a better handling option and, in the case of coffee processing, it is easier to dry and keep the product based on differentiated classes. Silo construction begins by providing or distinguishing the silo base with an external diameter of 2.20m as shown in Figure 63. Mortar

72

Figure 63 – Silo base configuration.


Silo Construction on coffee farm

for base construction should have the proportion 1:3:3 or 1 part of cement, 3 parts of sand and 3 parts of stone (size zero). After construction, the base should have the dimensions shown in Figures 63 and 64. Before wall construction, the perforated or false floor should be placed over the base to form the “plenum” chamber. This chamber allows uniform distribution of Figure 64 – Base with fan and the airflow through the grain mass. The false floor false floor can be made in different ways and with different materials but must maintain a perforated area of at least 20%. The most common perforated floor is made of a metallic plate with round and alternating 1.5mm diameter holes distanced at 5.0mm. These characteristics provide a perforated area of roughly 20%. The perforated plate should be supported by a circular iron base (mesh of 25x25cm) as illustrated in Figure 65.

Figure 65 – Details of the false floor with 20% perforated area.

10.2. Silo with screen frame In this silo model, the wall skeleton is basically made of a wire screen frame (used for fence). The skeleton provides structure and gives resistance to the silo wall. The recommended screen is made with n. 14 wire and 50mm mesh. The screen should have the dimensions of 1.80 x 6.30 m (11.4 m2), as indicated in Figure 66. Separately, it is necessary to prepare 5 pieces of smooth wire, 6.4 m in length and 8 wire rods 6.35mm in diameter (1/4’’) and 1.8m in length. The smooth wires, in a subsequent stage, will be used to make the rings that will be distributed around the circumference of the screen, giving the fence a cylindrical form. 73


Hygienic Coffee Processing and Technologies

With the open screen (Figure 67), the 8 iron rods should be woven into the mesh and distributed around the circumference at intervals of approximately 0.80 m. The first iron rod should be placed at 0.40m around the screen extremities. The purpose of these iron rods is to sustain the frame in the vertical direction. After the iron rods are placed, as shown in the figures, make the screen “junction” or union at the extremities.

Figure 66 – Specification and dimensions of the wall screen

Figure 67 – Disposition of the irons for screen vertical sustention

Figure 68 – Details of the screen “seam” or silo frame wall

Figure 69 – General view of the wall metallic frame

74

To accomplish the screen junction, the last wire must be removed to approximate the two extremities so that the mesh vertexes are alternate when the removed wire is inserted again (Figure 68). At the finished “junction”, the wire extremities must be jointed with those of the screen. Starting from this point, the frame is ready to receive the five wires that, horizontally, will be put between screen mesh in such a way to give a cylindrical format to the screen frame. These wires should be distributed so that two wires will be placed in each extremity and the others spaced from each other by 0.50m, approximately.


Silo Construction on coffee farm

After fastening, no sharp wire points should be left inside of the cylindrical skeleton. This is important because, a plastic sheet will be placed inside the mesh to provide impermeability and protection to the stored product.

Figure 70 – General view of the silo skeleton

To conclude the silo frame formation, a longitudinal notch is cut along a plastic tube (3/4’’) measuring 6.3m in length. The tube must be adapted and fastened with wire to the outer extremity of the frame (Figure 69). Finally, cylinder screen is placed on the silo base (Figure 70).

10.3. Internal plastic sheet The internal plastic sheet is necessary for the wall construction and serves as protection for the stored product, most importantly during the first time that the silo is used. When using the plastic sheet, the silo becomes impermeable and the product will not be in direct contact with the environment. The plastic sheet should be new or free of perforation. For safety, two 6m x 6m plastic sheet layers should be used. To prepare the plastic sheets, following steps must be followed:

the

• Carefully, open a plastic sheet in a clean location. Take care to avoid perforating it; • Join the two sides of the plastic sheet, with adhesive tapes in order to form a tube (keep a 10cm overlap as shown in Figure 71);

Figure 71 – Internal plastic tube formation

• Repeat the operation with the Second plastic sheet; and • Place one tube inside the other to form a two layer single tube. Put the formed tube inside the screen structure.

75


Hygienic Coffee Processing and Technologies

10.4. Silo loading Any kind of grain can be stored in the silo. However, two points should be observed (Silva et al, 2008): • Product moisture content: whatever the grain, before being stored it should undergo a previous pre-drying stage to remove excess moisture content (Besides storage, a silo with a ventilation system serves as a natural air dryer); and • Foreign materials: straws, peels, leaves, stones, dust, etc are deterioration agents, cause reduction in the airflow and in the drying efficiency. Therefore, the product should be cleaned and free from contamination for successful drying and storage. The necessary cares for a perfect plastic sheet must be stressed again. It should fit well in the frame with the excess sheet coiled at the top of the frame (Figure 72a and 72b). The excess plastic sheet will provide a good covering to protect and facilitate the silo wall finishing (Figure 73). During loading the grain mass must form a cone at the top of the silo to facilitate wall plastering.

Figure 72a – Internal plastic protection

76

Figure 72b – Silo loaded

Figure 73 – closed and ready for wall construction


Silo Construction on coffee farm

10.5. Silo external finishing As shown, the presented silo model will be covered externally after being completely packed. This is done because the plastic sheet and the stored product pressure over the wall will serve as a barrier for the mortar application and to, finally, form the silo wall. The mortar, with the proportion 1:6:2 (cement: sand: clay), is applied with the plastic sheets protection, so it is not perforated by any external agent and, finally, gives complete protection to the stored product. Figure 74a – Mortar application

The screen wall is plastered similar to the stucco technique (Figure 74a and 74) and will have a thickness of approximately 2.5cm when finished and should receive a white external coat of paint. By the next harvest season, when the silo is emptied, the plastic sheet should be removed and put back again after the application of the internal protection as was done for the external side. After this procedure the silo will be finished. If the silo is not built under a fixed shelter, coverage should be provided, as illustrated in Figure 75.

Because the ventilation system is more expensive in comparison with the total silo cost, it is advised to construct larger diameter silos. A fan that Figure 74b – Silo ready to supplies at least 2.5m3.min-1 of air per cubic meter receive the painting layer of grains must be adapted to complete the system. A single fan with larger air capacity to supply ambient air to more than one silo can also be adapted.

Figure 75 – Silo removable covers

77


SILO WITH MASONRY WALLS

11. Silo with Masonry Walls

I

f it is decided not to build the silo wall using the previous mentioned technology, the traditional masonry technology may be used insted.

Figure 76 – Metallic compass for circular construction

To obtain brick walls with perfect circumference and vertical rise, a special compass as shown in Figure 76 can be used. The walls should be built as shown in Figure 77a and 77b. For each 0.50m of wall a small cement ring should be made (Figure 77a and 76b). As can be seen in the illustrations, there is no need for special construction of framing for the cement ring. Two concentric layers with bricks are laid to form a circular channel where the concrete mortar with a 5/16” iron ring (wall reinforcement) will be placed. Another option is to make the wall reinforcement using perforated bricks as shown in Figure 78. At the end, the silo must be plastered and painted like it was done for the silo in previous technique.

Figure 77a – Circular wall construction and details of the compass use

Figure 77b – Circular wall construction with perforated bricks

Figure 78 – Detail of brick cut to help cement ring formation

78


Silo Isolation and Fumigation

12. Silo Isolation and Fumigation

I

n case preventive storage treatment is necessary, the product must be prepared to be fumigated by the use of aluminum phosphide tablets. For this operation, a ž� PVC pipe can be used, with lateral holes that do not allow grains to pass and free to the formed gas (Figure 79). The pipe should be closed on the interior end with a conic plug to facilitate the pipe introduction in the grains mass. Through the exterior end, introduce the correct number of aluminum phosphide tablets recommended by the manufacturer, and insert, as soon as possible, an appropriate plug to close the exterior end of the pipe. After this operation, the internal plastic sheets must be tied up as if it was a sack. The silo closing will be finished with the placement of an additional clear plastic sheet in the top of the silo. The top plastic sheet that will cover part of the silo wall must be well tied to avoid gas escaping. Observation: aluminum phosphide is also lethal for the operator. Therefore, for correct and safe application, follow the cautions procedure and dosages recommended by the manufacturer, which is presented on the product label.

Figure 79 – PVC tube for aluminum phosphide application

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Hygienic Coffee Processing and Technologies

13. Hulling and Classification

G

reen coffee processing is a post-harvest operation used to transform, by husk elimination and grains separation, the dry cherries (natural or parchment coffee) in hulled coffee or green coffee. Coffee processing should be performed as closely as possible to the commercialization of the product, in order to maintain the original product qualities.

Figure 80 – Stones and metals separator

Depending on previous drying and storage conditions (due to coffee physical or biological changes) it is convenient to submit the product to a rapid heating for moisture content equalization to an ideal level for hulling. When using high temperature drying, care should be taken to not hull a hot product. Natural cooling avoids the incidence of broken beans. A coffee processing unit at the farm level should have the following equipment: • Vibratory cleaner: it is formed by a group of sieves with different types of holes, to separate the coffee from light foreign materials (big and small). This machine should be located between the entrance pit and the stones and metals separator machine;

• Stones and metals separator machine: usually coupled with a ventilation system, the machine is used to separate the heaviest foreign materials, including heavy hulled coffee from light ones and the husks. The system has a magnetic device that retrieves metallic materials (Figure 80); Figure 81 – Coffee huller

80


Hulling and Classification

Figure 82 – Polisher

Figure 83 – Density separator

Figure 84 – Classifying machine

• Coffee huller: coupled with a ventilation system, the huller consists of a group of regulated rotary metallic razors and a fixed razor. The machine removes the peel and the parchment (Figure 81). Husks are removed by the ventilation system, and the coffee bean goes down to a pan, where the clean coffee is separated from non-hulled coffee. The clean coffee passes to the polisher (Figure 82) and, the non hulled coffee returns to the huller; • Classifying machine: is a system used to separate coffee beans by size, format and density. It is constituted by a group of sieves with different sizes and types of holes. The system has a regulated air column that separates the light foreign material or poorly formed coffees beans (Figure83). More sophisticated coffee processing units have reprocessing machines, such as density separators or vibratory tables (Figure 84) and the electronic sorter (Figure 85). These machines are used to improve coffee bean type, according to the market demand and other equipments like scales, packing machines and transporters should also exist in an ideal coffee processing unit. In Brazil, most small coffee growers who do not have the financial resources to own processing machines or without the possibility of group service, usually pay for a traveling processor (Figure 86). However, small machines for family coffer producers are available (Figure 87).

Figure 85 – General view of the electronic sorter

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Hygienic Coffee Processing and Technologies

During coffee quality improvement some good practices are recommended: • Keep beans, parchment and husks completely separate; • Separately transport green coffee, parchment coffee, and natural coffee; • Avoid re-wetting; • When appropriate, clean the transportation system.

Figure 86 – Traveling processor

Figure 87 – Commercial hulling machines for small coffee production

82


Coffe Moisture Content Determination

14. Coffee Moisture Content Determination

C

offee moisture content denotes the quantity of water per unit of mass of either a wet or dry coffee bean. This can be expressed by the relationship between the quantities of water and the dry matter (dry basis – eq. 2) or between quantities of water and the total grain mass (wet basis – eq. 3). For drying, storage, and processing purposes, moisture content has been considered one of the most important coffee quality characteristics. Moisture content is the most importance factor to prevent coffee deterioration during storage. If coffee moisture content and respiration are kept at low levels, the development of microorganisms is minimized. So, it is necessary to know the coffee moisture content from the moment of harvest until the final process and commercialization. Coffee beans above the ideal moisture content represent losses for the buyer since he is paying for the excess of water. For the coffee producer, an excess of moisture content means extra expenses in energy for drying, deterioration of the equipments and, in some cases, losses in coffee quality. Table 10 shows the ideal moisture contents for harvesting and storage of some grains. In Table 11 are the discount percentages or premium, for the moisture contents over or under the ideal moisture for commercialization. The moisture content can be represented in percentage wet base (w.b.) or dry base (d.b.). The first is used for commercialization and the second in investigations or specific calculations. Table 12 shows the value in percentage for premium for low moisture under the ideals for commercialization. MC(d.b.) = (mass of water / mass of dry matter)x100

(2)

MC(w.b) = (mass of water / total grain mass)x100

(3)

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Hygienic Coffee Processing and Technologies

Product

Harvest

Ideal

Safe storage

maximum

optimum

After drying

1 year

5 years

Coffee

62

62

12

11

10

Corn

23

20 - 22

11

11

9 - 10

Rice

21

17 - 19

11

11 - 12

9 - 11

Soybean

-

-

-

11 - 12

9 - 10

Sorghum

26

23 - 26

9

11 - 12

9 - 10

Wheat

23

15 - 17

8

12 - 13

9 - 11

Table 10 – Moisture content for mechanical harvesting and safe storage, % wet base

Commercial moisture content

Real product moisture content (% w.b.)

(% w.b.) 10

11

12

13

14

15

16

17

18

10

0

1.1

2.2

3.3

4.4

5.6

6.7

7.8

8.9

11

-1.1*

0

1.1

2.3

3.4

4.5

5.6

6.7

7.7

12

-2.3

-1.1

0

1.1

2.3

3.4

4.5

5.7

6.8

13

-3.5

-2.3

-1.1

0

1.1

2.3

3.4

4.5

5.8

14

-4.7

-3.5

-2.3

-1.1

0

1.1

2.3

3.5

4.7

Table 11 – Discount and Premium (%), for grains marketed out of the standard moisture content (*) The negative values mean that premiums should be paid for products commercialized with lower moisture content.

84


Coffe Moisture Content Determination

w.b. (%)

d.b.

w.b. (%)

d.b.

w.b. (%)

d.b.

8

0.087

15

0.176

22

0.282

9

0.099

16

0.190

23

0.299

10

0.111

17

0.200

24

0.316

11

0.123

18

0.220

25

0.333

12

0.136

19

0.234

26

0.351

13

0.150

20

0.250

27

0.370

14

0.163

21

0.265

28

0.389

Table 12 – Moisture content conversion (%) wet bases in dry base (decimal)

14.1. Moisture content determination methods There are two methods for grain moisture content determination: • Direct or basic oven method (Figure 88); distillation or Brown-Duvel (Figures 89 and 90); and infrared radiation; • Indirect (electric and electronic methods (Figures 91 and 92) – calibrated with the oven method or another official direct method); One very simple method recently used in Brazil is the EDABO or DEWOB (direct evaporation of the water in oil bath) which is a variation of the official Brawn-Duvel distillation method. The DEWOB method is illustrated by figures 93 and 94.

Figure 88 – Equipment for the standard method of moisture determination

85


Hygienic Coffee Processing and Technologies

Figure 89 – Brown-Duvel (laboratory)

Figure 91 – Indirect electrical meter

Figure 90 – Brown-Duvel (commercial)

Figure 92 – Indirect electronic meter

Figure 93 – Outline of the EDABO method

86


Coffe Moisture Content Determination

Figure 94 – Basic elements of the EDABO method

14.2. How to use the (EDABO) DEWOB The examples below will illustrate how the method works. Example 1 - Determine the moisture content of a lot of grain using the DEWOB method. Solution: 1 - Take a representative sampling (after homogenization); 2 - Weigh 100g of the product (in a scale for 500 g, accurately in ± 0.5 g) and place the sample (100g) in a recipient with approximately 10 cm diameter and 20 cm height, resistant to high temperatures. With a perforated cover (thin type) having a bigger hole (1cm diameter) to insert thermometer graduated till 250oC; 3 - Add vegetable oil (soybeans oil or other vegetable oil) sufficient to cover the 100 g of the product (sample); 4 - Weigh the recipient + product + oil + thermometer and register the initial weight (Wi); 5 – Heat up the system (recipient + product + oil + thermometer), during +- 15 minutes, until reaching the temperature shown in Table 13. Then remove the source of heat and wait for the bubbles to cease. Weigh the system again (recipient + product + oil + thermometer) and call it (Wf); 6 - Subtract (Wf) from (Wi) and register the moisture content directly in % wet bases. Example: If Wi = 458,9 g; = 13,5 g, or 13,5% w.b.

Wf = 445,4 g;

the difference Wi - Wf;

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Hygienic Coffee Processing and Technologies

Product

Temperature (0c)

Product

Temperature (0c)

Rice

200

Corn

195

Hulled rice

195

Soybean

135

Natural coffee

200

Sorghum

195

Green coffee

190

Wheat

190

Beans

175

Table 13 – Temperature for moisture content using the (EDABO) DEWOB method

88


Important Properties and Notation

15. Important COFFEE Properties and Notation 

grain temperature, °C 

relative humidity, 0 

bulk density, kg m-3 cp

specific heat of coffee, kJ kg-1 °C-1

hfg

enthalpy of vaporization of water in coffee, kJ kg-1

M

moisture content of coffee, decimal dry basis

MR

moisture ratio, (M - Me) / (M0 - Me)

T

air temperature, °C

Tabs

absolute air temperature, K

t

time to dry to MR with drying air temperature Tabs,

  

1.0

h

Suffixes e

equilibrium value

eq equivalent 0

initial value

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Hygienic Coffee Processing and Technologies

Appendix - Physical properties a. Thin layer drying equation. The following equation describes the drying rate of a thin layer of natural coffee at drying air temperatures ranging from 40°C to 80°C [22]. MR = exp [-105.756 teq 0.60564 e (-2751.51 / Tabs) ]

(4)

b. Equilibrium moisture content. One of the main problems in previous attempts for setting up mathematical models to simulate coffee drying was the lack of reliable equations for its equilibrium moisture content. Rossi and Roa [23] presented the following empirical relationship for the desorption isotherms for natural coffee: Me = (15272 - 32478 2 + 333413) exp [(-0.029458 -0.0016309 - 0.013695 2 + 0.0132050 3) Tabs]

(5)

The resulting S shape or sigmoid curve obtained from Eq. (5) is too steep for relative humidity above 60% and it does not fit well with most of the experimental data found in literature. It is believed that many failures to validate coffee drying were due to the use of this equation to predict equilibrium moisture content. On the other hand, better overall accuracy is attained using the following empirical equilibrium moisture content equation proposed by Arteaga (1986). Me = 1.1282 [-ln(1 - e ) / (Te + 40.535)]0.5405

(6)

c. Enthalpy of vaporization. The absence of an accurate equilibrium moisture content equation for coffee has also led to inaccuracies in the development of equations for the enthalpy of vaporization of moisture in coffee beans. Berbert & Queiroz (1991), Developed the following equation based on the method of correlating vapor pressure and latent heat data presented by Othmer (1940). It was developed using experimental data from Arteaga (1986) in the 0.15 decimal d.b. to 0.25 decimal d.b. regions. hfg = (2501 + 1.775  ) [1 + 1.872 exp(-20.601 M)]

(7)

d. Specific heat. The following equation developed by Villa & Roa (1976)] represents the specific heat of coffee at constant pressure. cp =

90

1.674 + 2.510 [M / (1 + M)]

(8)


Important Properties and Notation

e. Bulk density. The effect of moisture content on bulk density of natural coffee is given by the following equation developed by Silva (1991). = (396.48 + 224 M) / (1 + M)

(9)

In a study conducted by Castro (1991), the estimated bulk density of washed coffee as a function of moisture content was defined as in the following equation. ď ˛ = 371.78 + 255.17 M

(10)

f. Volume reduction. Grain volume shrinkage is generally assumed to be negligible during simulation studies of the drying processes. This is based on the assumption that the decrease in bed height is not substantial for most cereal grains in continuous-flow dryers [12]. However, the assumption of negligible volume shrinkage for natural coffee during drying can impose serious limitations in simulation accuracy. Kinch (1967) reported a pronounced reduction in the volume of parchment coffee during drying. Volume reduction of 23.5% was observed when drying from 55% w.b. to a final moisture content of 12% w.b. No correction for bed height variation due to volume reduction are applied to the coffee simulation models because to date no real comprehensive study describing the phenomenon for natural coffee has been undertaken. g. Pressure drop. The pressure drop in experimental drying tests performed with natural coffee were always less than 7.5 mm of water column per meter of coffee depth, whereas for pulped coffee this value was 10.0 mm H2O per meter depth (GuimarĂŁes, 1995). It is realized that a pressure drop of 7.5 mm of water column per meter depth of clean coffee is smaller than those values normally found in practice where foreign material will occupy some of the free space between the cherries, hence compacting the bed and increasing the pressure drop. Afonso (1994) made a series of measurements of pressure drop through beds of coffee. For a bed depth of 1.75 m and airflow rates of 10.2 and 9.9 m3 min-1 m-2, the pressure drops were 5.1 and 5.4 mm of water column per meter depth, for 23% and 14% w.b. moisture coffee, respectively.

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Hygienic Coffee Processing and Technologies

16.References 1. AFONSO, A. D. L. Gradiente de pressão estática em camadas de frutos de café (Coffea arabica L.) com diferentes teores de umidade. [Static pressure drop in fixed beds of coffee (Coffea arabica L.) as affected by coffee moisture content]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1994.68 pp. 2. ARTEAGA, M. S. Modelación del proceso de secado. [Modelling the drying process]. Seminario de secado solar, 2, 1986, Cusco, Peru. Lima: Instituto General de Investigación, 1986. p. 51-5623. 3. BERBERT, P. A. Secagem de café (Coffea arabica L.), em camada fixa, com inversão de sentido de fluxo de ar. [Drying of coffee (Coffea arabica L.) in a fixed bed with airflow reversal]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1991. 83 pp. 4. BERBERT, P. A.; Queiroz, D. M. Método para determinação de uma equação para a entalpia de vaporização da água contida em grãos de café. [A method to determine an equation for the enthalpy of vaporisation of moisture in coffee]. Engenharia Rural, 1991. 2(1):1-17. 5. BERBERT, P. A.; Silva, J. S. Coffee Drying. In: CIGR - The International Commission of. (Org.). CIGR - HANDBOOK of Agricultural Engineering. 1 ed. St Joseph Michigan: ASAE, 1999, v. IV, p. 457-474. 6. CASTRO, L. H. Efeito do despolpamento, em secador de fixo sob alta temperatura, no consumo de energia e na qualidade do (Coffea arabica L.). [Energy requirement during the drying of natural washed coffee (Coffea arabica L.) in a high-temperature fixed-bed dryer]. Thesis. Universidade Federal de Viçosa, MG, Brazil, 1991. 61 pp.

leito café and MSc

7. CLOUD, H. A.; Morey, R. V. Dryeration and in-storage cooling for corn drying. St. Paul, MN. Agricultural Extension Service Report M-162, University of Minnesota, 1980. 8pp. 8 . CORDEIRO, J. A. B. Influência da temperatura e tempo de repouso na secagem de café (Coffea arabica L.) em camada fixa. [Influence of

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temperature and tempering time on the drying of coffee (Coffea arabica L.) in fixed beds]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1982. 60 pp. 9. CORREA, P. C.; Silva, J. S.; Camps-Michelena, M. Secado de café con energía solar. In: XXIV Conferencia internacional de mecanización agraria, 1992, Zaragoza. Anais do XXIV Conferencia internacional de mecanización agraria, 1992. 10. DALPALQUALE, V. A. Secagem de grãos em altas temperaturas. [High temperature drying of grains]. Viçosa, MG, Brazil. Centro Nacional de Treinamento em Armazenagem. [National Centre for Storage Training], 1984.29 pp. 11. DONZELES, S.M.L. Desenvolvimento e avaliação de um sistema híbrido, solar e biomassa, para a secagem de café (coffea arábica L.). Viçosa-MG: UFV, 2002, 122p. Dissertação (Doutorado em Engenharia Agrícola) – Universidade Federal de Viçosa, 2002. 12. FAO - Food and Agriculture Organization of the United Nations. Good hygiene practices along the coffee chain. A training resource for coffee producing countries, CD-ROM, 2004. 13. FOSTER, G. H. Dryeration - A corn drying process. USDA Agric. Marketing Service Bulletin AMS-532. 12. Brooker, D. B.; Bakker-Arkema, F. W.; Hall, C. W. 1974. Drying Cereal Grains. Westport, Connecticut: The AVI Publ. Co. Inc., 1964. 265 pp. 14. GOMES, R.A.R. Avaliação do desempenho de uma fornalha a lenha de fluxo descendente e com sistema de aquecimento direto. [Performance assessment of a downdraft direct-fired furnace]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1988. 56 pp. 15. GUIMARÃES, A. C. Secagem de café (Coffea arabica L.) combinando sistemas em altas e baixas temperaturas. [Combination drying of coffee (Coffea arabica L.). MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1995. 64 pp. 16. HARDOIM, P.C. Secagem de café cereja, bóia e cereja desmucilado em terreiros de concreto, de lama asfáltica, de chão batido e de leito suspenso em Lavras. In: 27º. Congresso Brasileiro de Pesquisas Cafeeiras, 27, Uberaba, 2001. Anais.


Hygienic Coffee Processing and Technologies

17. KINCH, D. M. Design criteria for mechanical drying of coffee beans. Transactions of the American Society of Agricultural Engineers, 1967. 10(1): 40-42. 18. LACERDA Filho, A.F. Avaliação de diferentes sistemas de secagem e suas influências na qualidade do café (Coffea arabica L.). [Assessment of several different drying systems and their effect on coffee quality (Coffea arabica L.)]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1986.136 pp. 19. LOEWER, O. J.; Bridges, T. C.; Bucklin, R. A. On-farm drying and storage systems. St. Joseph, MI. American Society of Agricultural Engineers, 1994. 560 pp. 20. MELO, F. A. O. Desenvolvimento e avaliação de um secador de fluxos concorrentes, com carga, revolvimento e descarga pneumáticos. Viçosa – MG: Universidade Federal de Viçosa. 2008. 122p. (Tese de doutorado em Engenharia Agrícola). 21. NAVARRO, S.; Calderon, M. Aeration of grain in subtropical climates. Rome: FAO Agricultural Services Bulletin 52, 1982. 119 pp. 22. OIC - Enhancement of coffee quality through prevention of mould formation – 2002. Disponivel em: http://www.ico.org/documents/pscb36. pdf. retrieved on: 04/10/2011 Seria 2010. 23. OIC – Organização internacional do comércio. Guide for the prevention of mould formation in coffee, 2006. 26p. data.

24. OTHMER, D. F. Correlating vapor pressure and latent heat Industrial and Engineering Chemistry 32, 1940. 841-856p.

25. OSÓRIO, A. G. S. Projeto e construção de um secador intermitente de fluxo concorrente e sua avaliação na secagem de café. [Design and construction of a recirculating-batch single-stage concurrent-flow dryer for natural coffee]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1982. 57 pp. 26. PAULSEN, M. R.; Thompson, T. L. Effects of reversing airflow in a crossflow grain dryer. Transactions of the American Society of Agricultural Engineers, 16(3): 1973. 541-544p.

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27. PINTO, F. A. C. Projeto de um secador de fluxos contracorrentes/concorrentes e análise de seu desempenho na secagem de café (Coffea arabica L.). [Design and performance evaluation of a counterflowconcurrent flow dryer for natural coffee (Coffea arabica L.). MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1993. 72 pp. 28. ROBERTO, C.D. Aplicação dos princípios do sistema de análise de perigo e pontos críticos de controle na avaliação da segurança do café no processamento pós-colheita. Viçosa – MG: Universidade Federal de Viçosa. 2008. 132p. (Tese de doutorado em Engenharia Agrícola). 29. SAMPAIO, Cristiane Pires; Nogueira, Roberta Martins; Roberto, Consuelo Domenici; SILVA, J. S. Development of a dryer with air flow reversal and a pneumatic system for grain movement. Biosystems Engineering, v. 98, p. 33-38, 2007. 30. SANTOS, R.R.; Lacerda Filho, A. F.; Silva, J. S. & Melo E. C. Modificações técnicas e operacional de um secador rotativo para a secagem de café (coffea arábica L.). Revista Brasileira de Armazenamento, Viçosa. 2006. N. 9, p. 1-11. 31. SILVA, J. S. (Ed.) Secagem e armazenagem de produtos Agrícolas. Viçosa, MG, Aprenda Fácil Editora, 2008. 560 p. 32. SILVA, J. S.; Berbert, P. A. Colheita, Secagem e Armazenagem do Café. Viçosa-MG: Aprenda Fácil Editora, 1999. 146p. 33. SILVA, J. S.; Giudice, P. M.; Sediyama, G. C.; Hara, T. Determinação das Dimensões dos Coletores Planos de Energia Solar. EXPERIENTIAE, v. 21, n. 12, p. 249-265, 1976. 34. SILVA, J. S.; Lacerda Filho, A. F.; Ruffato, S.; Berbert, P. A. Secagem e Armazenagem de Produtos Agrícolas IN: Secagem e Armazenagem de Produtos Agrícolas. Viçosa, MG, Aprenda Fácil Editora, 2008. 560 p. 35. SILVA, J. S.; Lopes, Roberto Precci ; Machado, Marise Cotta . Fornalha a Carvão Vegetal para Secagem de Produtos Agrícolas. Associação dos Engenheiros Agrícolas de Minas Gerais, 2000 (Boletim Técnico). 36. SILVA, J. S.; Lacerda Filho, A. F. Construção de secador para produtos agrícolas. Viçosa, MG, Brazil: Conselho de Extensão da Universidade Federal de Viçosa. Informe técnico no. 41, 1984. 17 pp. 95


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37. SILVA, J. S.; Pinto, F. A. C.; Melo, E. C.. Projeto, construção e avaliação de um secador de fluxos (concorrentes/contracorrentes) para secagem de café.. In: II Simpósio de Pesquisa dos Cafés do Brasil, 2001, Vitoria. II Simpósio de Pesquisa dos Cafés do Brasil - Anais. Brasília : EMBRAPA, 2001. v. 1. p. 965-980. 38. SILVA, J. S.; Nogueira, R.M.; Magalhães, E. A. Secagem em Silo – Uma Opção para o Café. Revista Engenharia na Agricultura – Boletim Técnico No 9. Viçosa. 2008. 31p. 39. SILVA, J. S.; Nogueira, R.M.; Roberto, C.D. Tecnologia de secagem e armazenagem para a agricultura familiar. Viçosa – MG. 2005. 138p. 40. SILVA, L. C. Desenvolvimento e avaliação de um secador de café (Coffea arabica L.) intermitente de fluxos contra-correntes. [Design and performance assessment of an intermittent counterflow dryer for coffee (Coffea arabica L.)]. MSc Thesis. Universidade Federal de Viçosa, MG, Brazil, 1991. 74 pp. 41. SIVETZ, M.; Desrosier, N. W. Coffee technology. Connecticut: The AVI Publishing Company, 1979. 716 pp.

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42. SIVETZ, M.; Foote, H. E. Coffee processing technology. Vol. I: Fruit - green, roast, and soluble coffee. Westport, Connecticut: The AVI Publishing Company, Inc., 1963. 598 pp. 43. VILLA, L.G.; Roa, G. 1978. Simulação matemática de secagem de café e cacau. [Mathematical simulation of the drying of coffee and cocoa beans]. Campinas, Departamento de Engenharia Agrícola, 10 pp. 44. VILELA, E. R. Secagem de Café em terreiro e silo com energia solar.. Dissertação (Mestrado). Unicamp, Campinas, SP. 1977.

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Embrapa Coffee, the Embrapa’s branch in coffee´s agribusiness, in addition to developing researches and technological innovations is responsible for the management of the Coffee Research Consortium. The Coffee Research Consortium, established in Brazil in 1997, is a strategic arrangement where ten of the leading Brazilian Research and Development Institutes working on coffee united with the goal of sharing scientific and technological knowledge, thereby compounding efforts towards the development and transfer of new technologies that help coffee growers in sustainable and quality production. By opting for the development of research lines and new technologies together with other institutions in the market, the consortium demonstrates the social values at the base of its actions. The Coffee Research Consortium works for Brazil and for Brazilians. Its main objective is the delivery of high quality solutions for the development of Brazilian coffee production, both socially and economically, to raise the quality of products offered and, as a consequence, the profits of our producers.

Hygienic Coffee Processing and Technologies  

This book was especially written to help the technicians that works with the service of rural extension and coffee grower that want to know...

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