CLEANING AND DRYING OF HAZELNUT

The product has to be preferably cleaned and dried as soon as possible after harvesting to avoid rising humidity which could compromise the quality of the hazelnuts. The best solution is to clean and dry the nuts within the day of collection (24 hours), and where this is not feasible, within the shortest possible time, especially if harvested after rains or with high moisture contents (>12%).

Cleaning hazelnut 3.1.

Most of the harvesting machines do not allow to accurately separate fruits from soil particles and stones of the same size as the hazelnuts; also, leaves, empty hazelnuts and small pieces of wood are difficult to separate from the hazelnut during harvest operations.

The cleaning and drying process generally starts from a hopper when the product, arriving from the field, is unloaded (Fig. 3.2). The product then pass through a separator fan with a rotary sieve (or other aspiration systems) to remove the lighter particles and then through a swilling tank with water to remove the stones.

In order to be sure that a clean product gets inside the drying silos, further manual sorting can be done, where operators can remove materials that have “escaped” the cleaning system.

3.1.1. Dry cleaning system

The dry cleaning machine uses air flow generated by a separator fan with a rotary sieve, useful for separating impurities of different dimensions and weight with respect to the caliber of the fruit, taking advantage of their different specific weight (Fig. 3.3.). Cleaning machines can be electrically or mechanically powered (with their own motor or driven by the tractor’s PTO) and can be operated by a single operator. Different models are available on the market that use simple separator fans or more complex aspiration systems. The work capacity of these machines depends on the operating systems (from 600-1200 kg / h up to 4000-4500 kg/h). The more complex cleaners require a first passage inside a sieve in which the coarser impurities (soil and large residues) are eliminated, a second passage through ventilation that eliminates the lighter parts (leaves, empty fruits, twigs) and a final step that eliminates the smaller portions. There are also on the market other systems cleaning the products well by aspiration and the use of this or other technologies needs to be evaluated based on parameters like system capacity (total quantity of crop to be processed per day), quantity of foreign bodies harvested and delivered to the facility and others.

3.1.2. Water cleaning system

The system is needed solely to sort out the stones from the hazelnuts in a really quick passage. The operation is automatic and easily managed through control panels: only the adjustment of few elements requires a manual regulation. The water cleaning system takes place in a swilling tank (Fig. 3.4) with an auger, equipped with a recirculation pump. The product introduced into the tank quickly floats and is pushed by a stream of water (caused by the immersion pump) towards a grid, where the water rushes into the recovery tank while fruits roll towards the exit, while heavier bodies, such as stones, go to the bottom and are expelled by an auger.

The washing tank is combined with water outflow systems which can consist of an inclined perforated auger, for the simplest systems, or a water outflow sieve consisting of a single rotary mesh screen, for processing systems of larger dimensions. The tank can also be combined with a perforated sorting belt, equipped with a fan that allows the flow of water from the tank and visual inspection of the product before it arrives in the dryer (Fig. 3.5). When water washing is used, fruits that come out of the washing should be promptly dried. The advantage of cleaning with water is the speed of the operation (fi. 9 tons / hour max of working capacity) and the excellent effectiveness compared to traditional mechanical systems.

Drying hazelnut 3.2.

The hazelnut has variable moisture upon harvesting which depends on the climatic conditions of the previous days; it generally requires a more or less long drying period to reach the commercial humidity values required for subsequent storage.

The drying process is influenced by different factors:

• Temperature of hot air

• Thermal differential (temperature difference between hazelnuts and hot air)

• Initial hazelnut moisture content

• Extension of the surface of contact

• Degree of air exchange

• Material used in the construction of dryer (influence of heater)

Traditional drying involves leaving the hazelnuts in a single layer exposed to the sun in the courtyard (Fig. 3.6), with frequent mixing of the mass. This technique, still used in different parts of the world and in some small companies, takes at least 2-3 weeks depending on climatic conditions and management of the operation; it is not recommended because drying too slow exposes the hazelnuts to acidification and rancidity phenomena which reduce the quality and shelf life of the product and increase the possibility of aflatoxins’ development.

Some useful suggestions for proper natural drying are:

• Don’t keep hazelnuts in a pile or inside a jute bag or in mass after harvest

• Place the hazelnuts immediately in a single layer

• Dry hazelnuts in clean area avoiding the direct contact with the soil/ground

• Allow air flow through hazelnut layer

• Monitor the moisture of kernel each day to reach the final goal (6% of kernel humidity).

The use of specific dryers is the safest method to ensure the quality and safety of our crop. A correct drying increases the shelf-life of the product by improving chemical and microbiological stability and avoiding formation of aflatoxins for at least one year.

The drying silos continuously mixes the product using special augers (Fig. 3.7) and the nuts are dried by hot air produced by burners and fed through a ventilation system. During this phase, which depends on the initial moisture content of the hazelnuts, the operator can sample the product inside and deliver it to the laboratory to check the moisture content reached by the product itself. Once 6% of seed moisture has been reached, the product has completed its cycle and the entire lot can be sampled again for the randiman calculation (see 2.2.3) and subsequent packaging in big bags.

• Air humidity and Climate conditions in general Suggestions on how to manage correct artificial drying (indications are intended per single cycle):

• Start the air ventilation and auger when the dryer is half full: this helps to start removing the excess of external water without waiting for the complete loading of the dryer

• Insert in the dryer hazelnuts with same kernel humidity (homogeneity of kernel humidity of the lot is essential)

• Set the drying temperature lower than 40°C

• Do not apply more than 6-8 hours of drying (burner + fan)

• To estimate the correct drying duration, take into account the starting kernel humidity.

• The drying duration is empirically calculated considering that it takes about 1 hour of drying to lower the humidity by 1%

• After drying, set a ventilation cycle (using only fan and auger)

• The hot dried hazelnuts are unloaded once they reach about 7% of humidity; in the following days, they will automatically and naturally reach about 6% of internal moisture content.

Models of dryers with different characteristics of capacity and type of power are available on the market. The capacities of the dryers range from small models (2-6 cubic meters) up to dryers of 30 cubic meters (for hazelnuts

1 mc = 0.50 t; from 1 to 15 t). The small-sized dryers (2-6 mc), suitable for small companies, are available in wheeled or carried by tractor models, diesel/gas fuel or electrically powered (including model that requires 220 Volts power supply and consumption lower than 3 KW, in order to be used with normal domestic meters). The larger models are fixed and have capacities up to 30 cubic meters; they can be arranged in series (Fig. 3.8) until reaching the capacity required by the processing line (shelling systems).

• Hopper for product unload

• Bucket elevators

• Separator fan (dry cleaning)

• Swilling tank (water cleaning)

• Water drainage sieve

• Perforated belt with ventilator

• Load and unload augers

• Dryers (artificial drying)

• Conveyor belt.