Dehydration is defined as the application of heat under controlled conditions to remove, by evaporation, water contained in solid foods or by-products of unprocessed agricultural products. The main purpose of dehydration is to reduce the shelf life of foods through their water activity
Dehydration affects the tissue structure and color and causes the loss of volatile components. All of this has an adverse effect on both the quality and nutritional value of the food. The design and operation of dehydration plants aims to minimize these changes by selecting suitable drying conditions for individual food products. As far as the malt process is concerned, the drying step is essential and is required to obtain the desired color and taste.
Field of Application
Some examples of dried food products are dried potatoes, starch derivatives, sugar, beet pulp, flour, pasta, beans, fruit, nuts, grain products, oilseed meal, tea leaves, vegetables and herbs. The dehydration of wet germinated grain is used in the production of malt.
Description of techniques, methods and installation
Two different principles can be used for dehydration:
Hot air drying
Hot air is used as a heating medium and is in direct or indirect contact with the wet food. The hot air is blown over or through the wet food.
Surface drying by heat conduction through a heat transfer system (contact driers)
The heating medium does not come into contact with the wet food, but is separated from it by a heat transfer surface. The heat is displaced by conduction through the surface and by convection from the hot surface to the food product to evaporate and remove water from the food. This has two major advantages over hot air dryers: less air content is needed and therefore the thermal efficiency is higher and the process can be carried out in the absence of oxygen.
Methods of installation
a) Fluidized bed dryers
Metal trays with a mesh or perforated base have a bed of fine foods up to 15 cm deep. Hot air is blown through the bed which causes the food to be suspended and then stirred vigorously (fluidised). The air functions like the dryer and the fluidizing medium. Dryers can be run in batch and continuously. Fluidized bed dryers are compact and allow good control over the drying conditions, relatively high thermal competences and high drying speeds. The dryer has very high speeds of heat and mass transport and as a result short drying times. Drying can be carried out at air temperatures below 100 ° C, but the temperature can also rise to 170 ° C or higher depending on the product / process. Fluidized bed drying is often used as a final drying step after spray drying (dairy industry).
b) Cup dryers
These consist of an insulated cabinet equipped with a shallow mesh or perforated boxes each containing a thin food layer. Hot air is circulated through the cabinet. A system of pipes and bulkheads is used to guide the air over and / or through each tank in order to promote uniform air distribution. Cup dryers are used for small-scale production. These have low power and maintenance costs, but are rather difficult to control and produce with variable product quality.
c) Transport dryer
Continuous transport driers are up to 20 meters long and 3 meters wide. Food is dried on a mesh belt. The air flow is first led up through the food bed and down in later stages to ensure that dried food is blown out of the bed.
d) Pneumatic dryers, flash or ring dryers
In these dryers, powders and fine foods are continuously dried in vertical or horizontal metal tubes. A cyclone separator or a bag filter is used to remove the dried products. The moist food is measured in the tube system (metered into) and suspended in the hot air. Pneumatic dryers have relative power costs, high drying speeds and thermal competences and other control of the drying conditions.
e) Rotary dryers
A slightly descending rotating metal cylinder is equipped with stairs inside to ensure that the food falls through a stream of hot air as it moves through the dryer. The air flow can run parallel or counter-current. Stirring the food and the large area of food exposed to the air produce high drying rates and a uniformly dried product. The method is especially suitable for food products that tend to stick together or stick together in chain or baking dryers. It is widely used in the sugar industry for the drying of sugar and beet pulp. At pulp, depleted gases from the combustion plant are used as a heat source and this reduces fuel consumption.
f) Tunnel dryers
Thin layers of food are dried in bins that are stacked on trucks, programmed to go through an insulated tunnel half-continuous through which hot air circulates.
g) Steam bundle dryers
The heating medium (steam) does not come into contact with the wet product; a heat transfer surface is used to direct the heat to the surface of the product to dry it. The steam passes through the dryer through cylindrical tubes / bundles. These beams rotate to prevent local overheating and to improve even drying. This dryer uses less air, which therefore limits emissions into the atmosphere.
h) Steam drying
A special drying design uses superheated steam produced via a heat exchanger. The dryer consists of a pressure vessel in which the water is driven out of the product, turned into steam and then used to dry more product. This system is used on a limited scale in the sugar industry for the drying of beet pulp. An advantage is the low energy consumption during drying.
i) Drying in kilns
Hot air, which initially rises from 40 ° C to about 85 ° C for pale-colored malts and up to 130 ° C for darker malts, is blown through a layer of green malt of about 50-150 cm thickness without any fluidization of the batch. The blowing takes 18-48 hours. At the end of the drying process, the dry malt is cooled to about 25-34 ° C and the malted apples are removed.
j) Vacuum drying
In the case of temperature-sensitive products, the external pressure must be reduced to avoid drying at a high temperature. A simple type of vacuum dryer is vacuum roller dryer. With this method one or two rollers are installed in a vacuum housing. The resulting vapor settles in a condenser that sits between the vacuum chamber and the pump. The product is removed by a screw conveyor.