Information on Coffee drying by Juarez De Sousa e Silva

1 INFORMATION ON COFFEE DRYING AND STORAGE by Juarez de Sousa e Silva¹ Aldemar Palonini Moreli ² Sergio Maurício Lopes Donzeles 3 Sammy Fernandes Soares 4 Douglas Gonzaga Vítor 5 Note: This article was written to help extension service technicians (public or cooperative) to correctly approach or assist small and medium coffee producers who have recently been adopting coffee drying in fixed layer dryers. Unfortunately, sellers of such systems give incorrect information and, among them, claim that the drying capacity, the low cost of the dryer and the unnecessary stirring of the coffee layer make drying more economical and less labor intensive. Before studying drying and storing of coffee, it is advisable for the grower to learn how to correctly operate all the steps of coffee production, including the ideal harvest. If the operations prior to coffee processing are carried out in accordance with good practices, the production of high-quality coffee will be reached only if all the processing steps are correctly done. The grower must carefully take care of all details to maintain the quality of the coffee during all processes, especially in the first three days after harvesting. The operators must work to reduce, as quickly as possible. the coffee moisture content to 18% (wb). Below this moisture content value, the coffee is relatively safe, and the grower can easily control one good final quality. As there are several possibilities for drying the coffee, the technician responsible for installing a project must guide for the best option. He must indicate to buy or build equipment that is guaranteed and that provides the best drying cost. As we'll see, the most used technology doesn't always mean the best option; the reader will conclude that the drying of coffee is comparatively more difficult to be carried out than that of other products. In addition to the high sugar content in the mucilage, and the high initial moisture content, usually above 62% (wb), the spoilage rate is high. If the objective is to produce high quality coffee, the grower must remember that he will only have the first three days to avoid big reduction in the quality obtained at harvest. As is known, the maximum quality is in the ripe fruit on the plant. In that way, whatever the preparation systems are used, the following aspects should be highlighted: a) Avoid unwanted fermentations during harvesting and preparation and during drying. b) Excessively high temperatures must be avoided. The coffee tolerates 40 °C for one day or two, 50 °C for hours and 60 °C for less than an hour, without damage. 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

2 c) Dry the fruits (natural coffee) or the beans (parchment coffee), avoiding the harmful effects of temperature, in the shortest time as possible, until the moisture content is 18% (wb). Below this value, coffee is less susceptible to rapid spoilage and, d) Try to obtain a product that presents uniformity in color, size, and density. To understand how drying takes place and how to control the process, both the producer and his employees must be trained to correctly perform all post-harvest operations. It is essential that they understand the relationship between the environment and the coffee that is in the process of drying or storage. They should be aware that incorrect handling of the air (drier or more humid) and of the drying system (terraces or mechanical dryers) can affect the coffe quality and, as happens in the nature, the conditions of the ambient air (dry or humid) that influence the drying process. In nature, after ripe, the fruits are spread out by birds or dried on the plant, falls to the ground, and germinates. Therefore, a good drying process must not affect the germination of the seeds. The drying speed will depend on the conditions of the air that passes through the product. For the same airflow, the drier the air, the faster the drying. Without control of the exposure time to the drying air, the coffee can dry out more than the necessary and suffer damage during processing. In addition, there will be unnecessary expenses with energy, labor, reduced weight and increased production costs. For a better understanding of the drying process, the extension technician must be prepared to answer, among others, some questions such as those that’s following: 1 - TO WHAT EXTENT (MOISTURE CONTENT) MUST THE COFFEE BE DRIED? First, we must differentiate what humid and soaked mean when talking about grains in general. When the coffee is harvested at the ideal point, it is a ripe fruit and has the maximum of “dry matter” and plenty of water (harvest moisture content) to be stored for long periods and, therefore, must be dried. If it passes through the coffee washer for a short period of time, it will contnue to be moist and soaked on the surface, and it will not gain moisture internally. The fruit will only start to dry (lose moisture) after it has lost the surface water that it gained from the coffee washer, and which was not part of the fruit when it was harvested. Unlike dry grains, watering the newly harvested ripe fruit does not cause problems. The drying or removal of excess moisture must be done in such a way that the product comes into EQUILIBRIUM (does not lose or gain moist) when exposed to the air in the environment where it will be stored. The drying must be done in such a way as to preserve the appearance, the organoleptics qualities for roasting, and the viability as seed. A coffee bean, from perfect fruit, which stops germinating due to drying, means that something has gone wrong during the drying process. Two values must be recognized to terminate the drying process, i.e., On-farm STORAGE moisture content and MARKETING moisture content. Therefore, it is necessary to adopt a “safe” storage unit and a good control of the environmental conditions. Be careful about not to miss the ideal drying point. To adjust excessive drying, will need time and money 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

3 (mainly labor and energy). Avoid gross mistakes like drying below 10% moisture content. In this value, it is possible to adjust the commercial moisture content, using the aeration technique in periods of high relative humidity (65 to 70%). Below 8% moisture content, the error may be irreversible; reputable roasters would not buy this coffee. 2 - HOW TO UNDERSTAND THE DRYING PROCESS AND TO AVOID MISTAKES? a) To fully understand how drying takes place, it is important to understand the relationships between the conditions of the drying air (temperature and relative humidity) and the coffee. As in nature, it is the ambient air conditions (dry or humid) that favor or difficult drying. b) Drying, as occurs in the plant, does not damage the seed germination. The Drying speed will depend on the drying air conditions. The drier and hotter the air that passes through the coffee fruits, the faster the drying will be and, depending on certain values, will damage the product and the equipment. To a limited extent, dry air is more important than hot air. 3 - DO YOU KNOW WHAT RELATIVE HUMIDITY IS? The air that naturally dries coffee fruits is the same air that we breathe in the field and is composed, roughly, of Nitrogen, Oxygen, Carbon dioxide and, also, WATER VAPOR. Moisture in the air is important because it moistens our nasal and lung mucous membranes and facilitates our breathing. For each temperature condition, the air may contain a maximum amount of water vapor. When this happens, we say that the air is saturated or with 100% Relative Humidity and, in the (SATURATED) condition, any small drop in temperature leads to condensation of moisture in the air. So, the water in the form of vapor passes to the liquid form and can wet the dried and stored product. Also, as SATURATED condition, air is not able to receive any moisture, it is not capable of drying any product. If the grower or his assistants have no well understanding what Relative Humidity of the air means, they will hardly understand drying. For instance, at 22°C, 1 kg of air can contain a maximum of 17 g of water vapor. If it contains only 8.5 g of water vapor, we say that the RELATIVE HUMIDITY is 50%. The relative humidity is equal to the amount of water vapor in the air, divided by the maximum amount of water vapor it could contain, under specified temperature, multiplied by 100. Thus, the relative humidity of our example would be: (8.5 / 17) x 100 = 50%. Roughly speaking, 1kg of air is approximately equal to 1m 3, of air, under normal atmospheric pressure conditions. If it is difficult to explain the theoretical meaning, the technician may asck the coffee grower to use the sense for the meaning, through sensitivity. As mentioned, a very dry air makes breathing difficult due to the drying of the nasal and pulmonary mucous membranes. On the other hand, a very humid air makes it difficult for to perspire (a lot of sweat) on the skin. One should explain that, generally, the ideal ambient air is one in which we feel comfortable and say the weather is pleasant. In this situation, the air temperature is around 22 °C and the relative humidity around 62%. In general, the relative humidity is lowest during the day and highest at night. It can also be said that during the day the air is drier and during the night it is more humid. The purchase of a thermo-hygrometer will help in decision-making during drying and storage of coffee, does not cost a lot and it's durable. If one take notes with 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

4 three readings a day, will have more information about the farm environment. Many already take notes to understand the rainfall regime. In an environment with a relative humidity of 62% and an average temperature of 22 °C, the coffee will dry to 12.5% (wb) moisture content (Equilibrium moisture). It will not dry below this value, no matter how long it is exposed to that air conditions. On the other hand, if the average conditions are 50% relative humidity and 22°C, the coffee will dry to 11% (wb) moisture contet. If the relative humidity is 40%, the coffee will dry to 9.5% (wb). This variation in the air drying capacity must be taken into account in the brazilian cerrado region, which has drier air than in the mountains of Minas Gerais State. On the other hand, if the average relative humidity is 80%, the coffee will only dry up to 16% (wb). This allways happens with the coffee dried on a suspended terrace in mountain regions. If any of the above stabilized conditions remain unchanged, the coffee will come into balance with the ambient air, or it will neither lose nor gain moisture. 4 - WHY DO SOME DRYERS DRY FASTER THAN THE IN-TERRACE DRYING? Whenever possible, the coffee grower and his direct assistants should be encouraged to participate in training programs. They should be guided to understand a little more about the drying process and the relationships between Relative Humidity and Drying Air Temperature and energy use. To make it easier, we must first make them to understand how the drying process takes place in terraces. We must convince them that the terrace should not be completely covered by the coffee fruits and that drying occurs, firstly, with the heating of the terrace surface by the sun's rays and that it is the natural ventilation that facilitates the removal of water vapor from the coffee. Remember that, only after heating, around nine o'clock in the morning, should the coffee be spread on the terrace, with a layer of approximately 4 cm thick. Then, with an appropriate tool, small windrows should be formed in the direction of the operator's shadow. These windrows must be repositioned as soon as the uncovered part of the terrace is heated again (Figure 1). The windrows formation and change operations must be carried out hourly, preferably.

Figure 1- Correct way to spread the coffee on the terrace drier or patio After the fifth day of drying, the coffee should be piled up, still heated, at three o'clock in the afternoon, and covered with a canvas over thermal insulation (jute bags) to avoid dampening with night dew and to maintain the windrow heated to distribute the internal moisture of the coffee. On the following day, at nine o'clock in the morning, the coffee must be 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

5 spread and stirred as explained above, and the operation must be repeated until the end of drying. The exclusive use of the terrace drying by many coffee growers is mainly due to the lack of concern with coffee quality, or the low purchasing power and technical level of the farm. Therefore, a sunny location with good ventilation is essential for the construction of the terrace dryer, which is very difficult in mountain regions. In most producing regions, drying on the terrace facilitates the development of microorganisms on the surface of the fruit, with increased respiration and product temperature, which are factors that accelerate the fermentation process. Despite of these risks, many small and medium producers intensively use the terraces as the only step for coffee drying. If the weather conditions are favorable and the operation of the terrace dryer is carried out within the technical recommendations, the natural coffee will be dry in 15 to 20 days and the parchment coffee in 10 to 15 days (Figure 2).

Figure 2 - Schematic of the drying process in a terrace drier Differently from terrace drying which is influenced by environmental conditions (insolation, ventilation, and rainfall) and with little operational interference, with mechanical dryers the operator must understand that every time he heats the air (through furnaces or burners), the heat will reduce the RELATIVE HUMIDITY of that air. Therefore, at the entrance of the dryer it will blow in warmer and drier air than the ambient air. In HOT AND DRY conditions, the air becomes more capable of removing water from the coffee, even in unfavorable environmental conditions, and substantially reducing the drying time from weeks to three to four days. The coffee drying process, in hot air dryers, is very similar to what happens with a hair dryer or clothes dryer, and, in the case of coffee and human hair, great care must be taken with the temperature of the air. Thus, the hotter is the air, the greater is the drying capacity and, observing the maximum temperature that the product can withstand, the shorter is the time needed to remove excess water to reach the moisture content for commercialization. The control of final moisture content or the drying time is critical. If excessive drying occurs, retweting to the ideal point is quite expensive. The grower must be warned that liguid water cannot be poured into the coffee to reverse excess drying. This work must be done with the injection of moist air that usually takes place at night and can take weeks with the fan turned on to reverse the moisture contente to the ideal point. 5 - WHAT IS THE REASON TO STIR THE COFFEE LAYER? 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

6 This question is, like the previous ones, very important for coffee, as well as for cocoa and other “grains”, such as rice. In reality, the only way to dry coffee without proper stirring is when it is on the plant. In this case, drying is characterized as natural in the field and has its disadvantages. One way to approach natural drying in the plant would be to place, properly spread out, a batch of coffee on a suspended screen under natural ventilation. In this case, the height of the layer should be equivalent to the diameter of the coffee fruit and drying would be classified as a thin layer. A lack of homogeneity in artificial drying, mainly, will be verified by a judicious grader. In others wourds, we can say that a deep layer of coffee (50 cm, for instance) dries as if it were composed of a superposition of several THIN LAYERS. Also, for a question of simplicity, it can be considered that a thin layer of coffee is one whose thickness corresponds to the diameter of the coffee fruit or the thickness of the parchment coffee. If the coffee fruit has a diameter of 1.0 cm and the grain on parchment, a thickness of 0.5 cm, a dryer type "Box or static layer", which receives a layer of 50 cm of the product, must be analyzed as if it were formed by 50 thin layers of fruit or, approximately, 100 thin layers of parchment coffee. In one "coffee deep layer", the drying air (heated or at room temperature) enters the bottom through the first thin layer that is on the support plate (perforated box plate) and is released to the environment (exhaustion) after going through the last thin layer (“50” layer for fruits or “100” layer for parchment grain, from the previous example). When drying in a "FIXED LAYER” or “BOX DRYER”, the air enters hot and dry and is exhausted cold (less hot) and very humid, on the upper surface of the coffee layer, as shown in Figure 3. Simplifying, when crossing the total layer of coffee or crossing each thin layer, the air will be cooled and humidified by the water released from the beans of the previous thin layer. Thus, after a drying time, in figure 3, each layer has a different moisture content which, in the figure, are represented by colors, in which light yellow represents drier and warmer grains and dark red represents the more humid and colder grains. This temperature and moisture gradient can only be noticed by an experienced operator or through measurements with appropriate equipment. If drying is prolonged for a longer time, the entire layer of coffee will be dry and homogeneous with moisture content equal to the equilibrium moisture corresponding to the drying air conditions (Figure 4). The figure illustrates what will happen to a layer of coffee that has not been stirred in the “BOX dryer type”.

1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

Figure 3 – Behavior of drying air in a fixed layer or "BOX dryer type".

Figure 4 - Representation of drying in a fixed layer dryer or "BOX dryer type", as a function of the drying time, without layer stirring. Figure 5, on the other hand, shows the dynamics of the drying front. In the figure, (T 0) represents the start time, when the dryer was turned on or the beginning of drying when coffee moisture is equal to (U 0). At time (T1) the drying front began to be formed and at time (T2) it was formed, and the drying front is highlighted, showing its height; in the times (T 3) and (T4) the drying front was moving up and leaving a layer of dry coffee below it, with moisture content equal to the equilibrium moisture cntent (U e). In the time (T5), the top of the drying front reaches the last sub-layer of coffee; in the time (T 6) the front starts to leave the last sublayers. In the time (T7) the base of the drying front completely leaves the layer of coffee and the whole of it will be dried to a final moisture (U e) with the drying air temperature.

Figure 5 - Dynamics of the drying front in the fixed layer or “Box type dryer”, without layer stirring. As explained so far, the coffee grower may think that it is not necessary to stir or mix, periodically, the coffee layer inside the drying chamber in a “BOX type drier”, he may think at the end of drying, all the coffee layers will be dry with the same moisture content. There's the big problem; all coffee layer has reached final moisture content (Ue) or equilibrium moisture content with very HOT and DRY drying air. As mentioned, natural air is composed of a mixture of gases, water vapor and a series of contaminants, that is known as "MOIST AIR". Te “DRY AIR" exists when water vapor and contaminants are removed from the natural air. Thus, the composition of "dry air"is relatively constant, despite small variations depending on geographic location and altitude. The average percentage composition is shown in Table 1. Table 1- Approximate chemical composition of the "dry air”

Components Nitrogen oxygen Argon Carbon dioxide

Formula N2 O2 Ar CO2

Number of moles in 100 moles 78 21 0,9 0,03

The knowledge of air humidity is very important in several sectors of human activity. The conservation of products such as fruits, vegetables, eggs and others, in refrigeration systems, depends of an adequate mixture (dry air / water vapor). The storage and handling of beans, including coffee, is also limited by atmospheric air conditions. The thermal comfort index of an atmosphere depends more on the amount of vapor present in the air than on the temperature itself. Thus, an air conditioner promotes greater humidity control and only small variations in the value of the ambient temperature. For all these reasons, the detailed study of the mixture of dry air (N2 + O2 + CO2 + others) and WATER VAPOR has become a discipline, called psychrometry, which studies the relationships of atmospheric behavior, especially with regard to the mixture of "DRY AIR" with "WATER vapor", called “MOIST AIR". 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

9 See an example: suppose that in one coffee farm in Minas Gerais State - Brazil, during coffee drying with a “BOX type drier”, the environmental averages are: 22 °C and 62% relative humidity. If this air were passed through a 50 cm layer of coffee, after a certain time (250 hours, for instance), all coffee layer would be dried at a moisture conten of 12.5% (wb). It turns out that a coffee with high moisture content, even taking 250 hours to dry, may have undergone some unwanted fermentation in the upper layers and impaired the final quality of the product. Thus, to speed up drying, the most viable solution would be to increase the drying temperature; an increase in airflow may also be an option, but less economical; increasing airflow is to increase electrical power due to the necessity of more powerful fans. Suppose the grower has decided to increase the drying air temperature to 40 °C. In this case, the relative humidity of the drying air went from 62% to 25%. Now, with air at (40 °C and 25% relative humidity), after a certain drying time, part of the coffee mass, until a sample in the last thin layers has dried at 12% (wb), will have reached a final moisture content of 7% (wb). If drying continues for longer, the coffee will be at 40 °C and 7% moisture content. This means, very dry and subject to breakage during processing, causing great financial loss, mainly due to weight loss and energy consumption. Reweting for the ideal moisture is expensive, time-consuming and harms the quality of the coffee. To solve the problem caused by inadequate coffee stirring, in static dryers (Box type driers), the designers try to dimension the maximum height of the coffee layer, equal to the height of the drying front (T 2 in Figure 5 or T3 in Figure 4), tolerating temperature gradients and moisture content for a particular product. Of commercial grains, the greatest tolerance is for common corn with three percentage points between the driest and wetter layer. Unfortunately, for quality coffee, the maximum tolerance is 0.5 percentage points (use a good moisture meter). If the variation in moisture between the beans increases, the coffee will have one of the most serious defects, which is the “bad roast”. In Figure 6a, it can be seen that the beans are inferior to the coffee shown in figure 6b, which is of good quality. In Figure 6a, you can clearly see the amount of beans over roasted and this is not tolerable in an important commercial roasting.

(a) (b) Figure 6 – (a) Uneven grain drying, (b) Homogeneous grain drying. According to explained, it is almost impossible to dry coffee and some types of beans without proper stirring of the layers if the drying air temperature is 5 oC above room temperature and the relative humidity is far below 50%. Therefore, the coffee must be stirred continuously in special dryers and, depending on the temperature and drying air flow, every 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

10 three hours in static dryers. To give a better idea of the subject, let's assume that the “Box or Static” dryer is operated with air at 40 °C, with ambient conditions (22 °C and 62% relative humidity). At 40 °C, the relative humidity of the drying air becomes 25%. With this air (40 °C temperature and 25% relative humidity), and after a relatively short time, the coffee, in the first thin layer, will have reached a final moisture of 7%. At this drying time, the thin upper layers will still be drying. If the operator stops the dryer because samples taken from the upper layers have reached 12% humidity, he will discharge the dryer with the coffee mass having an average moisture content of 9.5% (wb) under a moisture gradient of five percentage points (5 points of moiture content between the first and last thin layer). Suppose the operator, carefully, takes samples from all layers of the dryer and stops drying when the average moisture of the layers reaches 12% (wb). In this case, the upper layers may have a moisture content higher than 18% (wb). Drying is a physical process and as such, it cannot be modified without something going wrong. No honest grader will approve this coffee after careful analysis. At this point, it should be remembered that moisture meters do not measure the moisture of individual grains. It gives the average value of the sample. An uneven drying will only be noticed during processing and, especially, in the coffee roasting, which is done during the final classification. Depending on the temperature of the drying air, on the used air flow, the initial coffee moisture and, the height of the coffee layer inside the “Drying chamber”, the process takes place according to a front that moves from the bottom to the top layer. Again, dryer designers call this “drying front” (Figure 7) indicating that after several hours of drying (20 hours, for example), the drying front has formed, and it has already moved a few centimeters. Below the drying front, the entire product will be dry and in equilbrium with the drying air. After this, it no longer dries. suppose a drying time over 50 hours, for instance, the drying front will have crossed the entire grain mass and the entire product will be dry with the same moisture content or in the equilibrium moisture with the drying air. Therefore, to avoid overdrying the layer or to avoid large moisture gradients, it is necessary that the layer is not too deep or that it be continuously stirred or, at least, every three hours of drying.

Figure 7 – Development of the drying front in a Fixed Layer dryer. To dry coffee, with most traditional driers, the high initial moisture content of the product and honey production (mucilage), turns the stirring process impossible or very 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

11 difficult. So, it necessary that the coffee must be submitted to a pre-drying in cement terraces. More recently, combination drying (pre-dryer, dryer and in bin storage drying) has been studied and applied in specific locations. In this combination, there is a pre-drying in a terrace or dryers and the complementary drying in a silo with natural air. All these systems are detailed and find in: https://issuu.com/juarezufv/docs/cap_tulo_5_2013 . It should be remembered that, at the time of harvest, the coffee presents 100% of its “potential” quality and it is during the first three days that the grower must be very careful. If everything goes well within that period and good practices are maintained afterwards, high quality will be maintained until it reaches the consumer. On the other hand, coffee that, on the third or fourth day after harvesting, loses 15% of its potential quality, continues to deteriorate even if it reaches safe storage moisture content A safe moisture, depending on the drying system, is 18% (wb). At this moisture level, coffee can be dried more slowly with air heated to lower temperatures. Complementary drying can also be carried out during the storage period on the farm in drying silos. If the weather conditions are favorable and the operation of the drying terrace is carried out within the technical recommendations, the natural coffee will be dry in 15 to 20 days and the (parchment coffee will be dried in 10 and 15 days as shown in Figure 2. Therefore, the use of systems that can put the product in safe condition (18% moisture content) within three days or 50 hours of drying, must be installed. As poorly conducted drying can affect the product and is a physical process that involves the simultaneous transfer of heat and moisture between the coffee and the drying air, the extensionist must first provide information on fundamentals of hygrometry, grain moisture content, equilibrium moisture content, air flow, drying speed, classification, and coffee quality, so that the grower can take full advantage of the drying techniques to reduce production costs. Additionally, to obtain good quality coffee, it is necessary to use mechanical dryers to speed up the process. On the other hand, special care is also needed in controlling the temperature of the grain mass, especially from the moment the coffee starts to present a moisture content below 25% (wb). For moisture contents below 25%, depending on the used drying system, there is a tendency for the temperature of the grains to be equal the temperature of the drying air. This trend is caused by the difficulty in moisture migration from the innermost layers to the periphery of the grains. The maximum air temperature that coffee can withstand, in a conventional dryer, is 70°C, for a few minutes. Higher temperatures are detrimental to the product, as many beans that do not properly flow, inside the dryer, become too dry while another part reaches the ideal moisture content (11-12%bu) and, making roasting a difficultto-control process, as seen in Figure 6. In addition to speeding up the drying process and avoiding the dependence on weather conditions, mechanical drying at high temperatures is aimed to maintain product quality through better control of the process. In the Brazilian market, a wide variety of industrialized dryers is available, and the literature provides models that farmer, with the help of a local extensionist, can build on the farm itself. For the proper operation of most mechanical dryers, the coffee mass must not contain excess water, to facilitate fluidity inside the dryer or to obstruct the drier perforated plates; therefore, before dumping the coffee into the dryer, it must undergo pre-drying, which 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

12 is normally done in conventional terraces or, in pre-dryers, such as fixed layer or in hybrid terrace drier. Except for concurrent flows dryers, whose grain flow is parallel with the same direction to the drying air flow, and which can work with an air temperature close to 120°C, the drying air temperature must be prevented from exceeding 70 °C and that, under no circumstances, the temperature of the coffee mass may exceed 45 °C for periods longer than two hours. Unfortunately, to reduce the cost of labor with terraces drying and in feeding wood-fired furnaces, there has been a major setback about “BOX type drier”. Many coffee growers, inadvertently or influenced by unscrupulous sellers, start using the dryer in a stationary layer without the proper periodic stirring and using automatic feed furnaces to burn the coffee husk. To evaluate the performance of the called “BOX DRYER” in the coffee quality and drying conditions, in the region of Matas de Minas Gerais-Brazil, on-site verifications were carried out in 2016. Annotations, sample collections, and the drying process were verified in 21 properties in 8 municipalities. It was found that the "box dryers" had a basic structure like the dryer designed at DEA/UFV and that the differences were in the rectangular format for drying chambers and plenum and that the centrifugal fans were replaced by axial fans, in most cases. It was clear that the axial fans used had less capacity to pressurize the drying air than the centrifugal fans adopted in the original model and that the indirect fire furnaces burning wood and coffee husk had high consumption and INTOLERABLE SMOKE POLLUTION. Also, the noise level measured three meters away from a dryer was 93 Db cussed by the axial fan. During on farm evaluation, the following checks were noted: 1- The operation of the Dryers (generally very variable) each producer in its own way or guided by its neighbors operates the dryer. 2- The heights of the coffee layers varied between 60 and 100 cm, that is very different from what was recommended for the original dryer (maximum 40 cm) and with coffee stirring every three hours. 3- The stirring of the Coffee layer was not always adopted. Of the 21 properties visited: One did not practice and two were practiced daily (once in the morning). The others practiced stirring once during the drying process, which was carried out from the second day of drying. In the "dryers" with a layer of coffee around 100 cm, the surface was visibly moldy. 4- In most properties, the proportion of green fruits in the coffee harvested was very high; without going through pre-cleaning, it was put to dry. 5- Coffee washing was only verified in two cases and the production of parchment in only one farm. The pre-drying in a cement terrace was verified in five properties. Table 2 shows the main characteristics of the sampled products in different counties in the visited region. Table 2 - Physical and beverage characteristics of dried coffee in "box dryers" at ZMNMG. L = washed; R = from field; V = green; D = peeled; U = moisture. 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

13 Sample Number County 01 A. Caparaó 02 A. Caparaó 03 A. Caparaó 04 Araponga 05 Araponga 06 Araponga 07 Araponga 08 Carangola 09 Durandé 10 E. Feliz 11 E. Feliz 12 E. Feliz 13 E. Feliz 14 Fervedouro 15 Fervedouro 16 Fervedouro 17 Fervedouro 18 M. Soares 19 M. Soares 20 Simonésia 21 Simonésia

Coffee R R R R R R R R R R V D R R R R R R R L R

U % 12.4 11.3 11.8 14.2 13.8 13.0 14.0 13.8 12.3 12.2 11.2 11.8 13.7 15.2 14.4 13.6 11.2 10.8 14.5 10.6 11.5

Yield % 49 51 53 55 47 52 57 48 49 47 48 48 51 49 50 59

Eliminated % 38 16 15 45 30 32 32 29 24 40 90 52 58 15 15 16 32 22 22 20 22

Defects number 200 93 91 350 186 360 190 183 180 240 680 300 350 90 90 98 240 156 130 162 120

Aspect

Beverage

regular regular good regular regular irrigular regular unequal good good good regular -

Hard + 86 84 Hard/rioy Hard Hard/rioy Rio Hard Rio Hard/rioy Hard Hard/rioy Hard/rioy Rioy Hard Hard/rioy Hard/rioy Hard/rioy Hard Hard Hard/rioy

It was verified that: 1- half of the samples had moisture above 12.5% (wb) and should undergo additional drying. 2- The yield in green coffee beans was close to 50%. Considered normal. 3- All samples had more than 86 defects, which would not classify the coffee as “type 6” (reference for foreign market quotation). 4- Only two samples, at very high altitudes, provided only soft; the others were characterized as hard drink, hard/rioy and rio. 5- The high number of defects and the limited drink quality detected in dried coffee in box dryers, in the studied region, can be explained by: a- High proportions of unripe fruits. b- Excess impurities. c- Improper drying. d- Insufficient ventilation. e- High layer thickness and presence of MOLD; and f- Inappropriate or absent of coffee stirring. More recently, a study case was carried out with the “BOX dryer”, outside of the recommended specifications, when this type of dryer was disseminated in the early 1980s by UFV and by the extension system. At that time, a circular shape was recommended, maximum layer height 40 cm, air temperature at 50 oC with layer stirring each three hours of drying. The recommended circular shape, beside facilitating the distribution and homogenization of the air 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

14 flow, allows, when financially available, to adapt a mechanical stirring device as shown in Figure 8.

Figure 8 - Dryer in fixed layer adapted with a mechanical stirring device. Although many growers work with the “BOX type dryer” in non-recommended conditions or without any guidance, many are already purchasing mechanical stirrers adapted to the square format, using manual or automatic devices with screw to stir the coffee. To show that coffee layer stirring is necessary, technicians from UFV / EPAMIG / EMBRAPA-Café / IFES-VNI analyzed a rectangular "BOX type dryer" to dry fresh and wet parchment coffee placed in the drying chamber with only a 35 cm layer. The dryer was operated by the grower, as recommended by the supplier, and monitored by the team throughout the drying process. The verified characteristics were drying air temperature, air flow, coffee temperature and moisture content gradients, furnace, and ambient temperatures. The non-stirring of the coffee layer is the owner's routine.

For the evaluation, four (4) screened tubes (one tube in each corner) and 1 tube in the center of the “BOX dryer” were inserted 60 centimeters away from the walls. The tubes and the dryer were loaded with 35 cm layers of wet parchment coffee (Figures 9 and 10). Drying was completed, under the owner's methodology, with 11.5% moisture content on the upper surface of the coffee layer using a commercial electronic moisture meter. For each sample were measured the moisture content and density (Table 2).

Figures 9 - Details of a “box type dryer” in drying parchment coffee

Figure 10 - Sampling spots and sampler for moisture content, results according to layer depth and general average. The coffees contained in the screened cylinders were conveniently removed, at three depths, packaged, and sent for analysis. For each sample, the moisture content and test weight were determined (Table 3). Table 3 – Sampling points, moisture content and test weight (density) for parchment and processed coffee. Sample Spot

Distance from Surface (cm) 12.5 22.5 32.5 11.5 25.5 35,5 15 25 12.5 22.5 32.5 15.0 25.0 35.0

Parchment coffee Moisture Test weight content, % Kg/100L 9.96 37 6.47 37 7.10 35 7.41 37 7.58 36 7.54 35 7.68 36 7.78 36 9.22 37 8.93 36 7.97 35 8.30 36 8.27 35 8.38 35

Processed coffee Moisture Test weight content, % Kg/100L 8.64 64 6,72 63 7.64 62 7.64 65 8.30 64 8.35 64 7.60 65 6.46 64 8.12 64 9.01 64 8.88 63 9.09 65 9.02 64 8.91 64

As expected, in accordance with several works carried out with Fixed layer dryers, non-stirring causes serious problems, such as uneven drying and over drying, with losses in quality and energy. Due to the use of axial fans and coffee husk burning furnace, an excess of smoke was noted (figure 11) and a noise level of 93 Db at 3 m from the fan. Despite the large furnace temperature variation, the drying air temperatures never exceed 45oC and due to the use of a low fan power, one furnace attends alternatively two 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

16 drying chambers with each chamber occupied for twice the time necessary for continuous drying. On the other hand, in agreement with several works, the non-stirring causes problems, such as uneven grain moisture content and excessive drying, with losses in quality and energy. For the present case, there was a weight loss of 4% due to over drying of the lower layers. It was concluded that the stirring of the coffee layer and the control of air temperature are essential for coffee drying. Therefore, it is necessary to raise awareness among producers and publicize the possible consequences of the misuse of the "BOX dryer type”

Figure 11– Details of a "BOX-type dryer", drying coffee on parchment (a) and level of smoke emission (b).

a b Figure 12- Variation of ambient, furnace and plenum temperatures during an operating period (a) and drying curve (b) In addition to the mentioned problems, many farmers give up using firewood as fuel, claiming that the cost of labor to prepare firewood and feed the furnace is too high. To solve this problem, they invest in furnaces with automatic feeding, using coffee husk as fuel. Unfortunately, this practice, as shown in the evaluation of Iuna-ES experiment, the coffee ruck straw burning furnace has been used with great intensity in coffee plantations.

17 In addition to burning a high-value product to maintain soil quality, the furnaces are inefficient and cause great pollution both in the field and in the communities close to the coffee farms (Figure 11b), damaging the relationship between farmers and communities. There are also reports of car accidents caused by excess smoke generated by drying with air heated with coffee rusk burning (Figure 13). Fortunately, this activity has been banned at certain periods of day in some communities and drying unites closed by order of the Public Ministry and Environment Secretariats of some municipalities. In addition to these aspects, the burning of coffee husk should already have been completely banned from drying systems. Farmer must be informed that one ton (1T) of coffee rusk is equivalent, among other products, to 37 kg of potassium chloride.

Figure 13 – Condition of a highway (BR 101 - South of Bahia - Brazil) under smoke generated during drying with coffee husk. Problem’s mitigation 1- 1- With the high price of GLP, use furnaces that burn cultivated wood, rationally. 2- With coffee husk burning furnaces, use a smoke burner and replace furnace as soon as the first thermal damage appears (today, the Brazilian Coffee Research Consortium makes available models of furnaces without smoke emission. 3- Place noise controls on the axial fans or use centrifugal fans. 4 – If farmer insist on burning coffee husk, he must, at least, use continuous feeders with correct dosage. 5- Stir the coffee mass at least three times a day with the fan system turned off 6- Dry the coffee at an average moisture content of 13% b.u. For a maximum height of 40 cm and wait for slow cooling. 7 – In the case of parchment coffee, store in silos or bins with a ventilation system to homogenize the grain mass to the commercial moisture content.

CONSULTED BIBLIOGRAPHY COELHO, A. P. F; SILVA, J. S.; SOARES, S. F.; DONZELES, S. M. Secagem de café em pergaminho em secador tipo caixa: Estudo de Caso. In: 43° Congresso Brasileiro de Pesquisas Cafeeiras, 2017, Poços de Caldas. Anais. ISSN 2316-4115. 1: Professor Titular. DEA/UFV, Viçosa, MG – E-mail: juarez@ufv.br; 2: Ds em Agronomia, IFES/VNI, Venda Nova do Imigrante, ES - E-mail: aldemar.moreli@ifes.edu.br 3: Ds Engenharia Agrícola – EPAMIG – E-mail: slopes@epamig.br 4: Ds em Agronomia UFV, Embrapa-Café, DF - E-mail: sammy.soares@embrapa.br 5: Engenheiro Agrônomo, Bolsista CPC/EPAMIG, Viçosa, MG - E-mail: dougla.vitor@ufv.br

18 DONZELES, S. M. L. Desenvolvimento e avaliação de um sistema híbrido, solar e biomassa, para secagem de café (Coffea arábica L.). 2002. Tese (Doutorado) – Universidade Federal de Viçosa, Viçosa, MG. MACHADO, M. C. Viabilidade da Técnica de Imersão para Armazenagem Temporária dos Frutos de Café. Viçosa: UFV, 2005. 103 pg. Tese doutorado. SILVA, J. S. Secagem e armazenagem de produtos de agrícolas. Viçosa, MG: Editora Aprenda Fácil, 2008. 560 p.