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Dehydration (solid to solid)
Dehydration is defined as the application of heat under controlled conditions to remove, by evaporation, the water present in solid foods or by-products from agricultural raw material processing. The main purpose of dehydration is to extend the shelf-life of foods by reducing their water activity. Dehydration, however, affects the food texture, and colour, and causes the loss of volatile components, all of which has a detrimental effect on both the quality and the nutritional value of the food. The design and operation of dehydration equipment aims to minimise these changes by selecting appropriate drying conditions for individual foods. As far as the malting process is concerned, the drying step is essential and is required to create the desired colour and flavour.
Field of application
Some examples of dried foods are dried potatoes, starch derivatives, sugar, beet pulp, flour, pasta, beans, fruits, nuts, cereals, meal of oilseeds, tea leaves, vegetables and spices. The dehydration of wet germinated grain is applied in the production of malt (also called kilning).
Techniques, methods and equipment
For dehydration two different principles can be applied:
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 contact dryers
Surface drying by heat conduction through a heat transfer system (contact dryers)
The heating medium is not in contact with the wet food but separated from it by a heat transfer surface. The heat is transferred by conduction through the surface and by convection from the hot surface to the food product, for evaporating and removing water from the food. This has two main advantages compared to hot air dryers: less air volume is required and therefore thermal efficiency is higher, and the process can be carried out in the absence of oxygen
Methods and equipment
Fluidised bed dryers
Metal trays with mesh or perforated bases contain a bed of particulate foods up to 15 cm deep. Hot air is blown through the bed, causing the food to become suspended and then vigorously agitated (fluidised). The air thus acts as both the drying and the fluidising medium. Dryers may be batch or continuous by operated. Fluidised-bed dryers are compact and allow a good control over the drying conditions, relatively high thermal efficiencies and high drying rates. The dryer has very high rates of heat and mass transfer and consequently short drying times. Drying can take place with air temperatures below 100 °C, but may also be up to 170 °C or higher depending on the product/process. Fluid bed drying is often applied as a last drying step after spray drying (dairy industry).
Cabinet dryers (tray dryers)
These consist of an insulated cabinet fitted with a shallow mesh or perforated trays, each of which contains a thin layer of food. Hot air is circulated through the cabinet. A system of ducts and baffles is used to direct air over and/or through each tray in order to promote a uniform air distribution. Tray dryers are used for small-scale production. These have low capital and maintenance costs but are relatively difficult to control and produce variable product quality.
Conveyor dryers (belt dryers)
Continuous conveyor dryers are up to 20 m long and 3 m wide. Food is dried on a mesh belt. The airflow is initially directed upwards through the bed of food and then downward in later stages to prevent dried food from blowing out of the bed.
Pneumatic dryers, flash / ring dryers
In these dryers, powders or particulate foods are continuously dried in vertical or horizontal metal ducts. A cyclone separator or a bag filter is used to remove the dried product. The moist food is metered into the ducting and suspended in hot air.
Pneumatic dryers have relatively low capital costs, high drying rates and thermal efficiencies, and other close control over the drying conditions.
A slightly inclined rotating metal cylinder is fitted internally with flights to cause the food to cascade through a stream of hot air as it moves through the dryer. Airflow may be parallel or countercurrent. The agitation of 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 foods that tend to mat or stick together in belt or tray dryers. It is used on a large scale in the sugar industry for sugar and beet pulp drying. In the case of pulp, exhaust gases from the combustion plant are used as a heat source, and this reduces the fuel usage.
Thin layers of food are dried on trays, which are stacked on trucks programmed to move semicontinuously through an insulated tunnel in which hot air is circulated.
Steam bundle dryers
The heating medium (steam) is not in contact with the wet product; a heat transfer surface is used to transfer the heat to the product’s surface for drying. The steam passes through the dryer through cylindrical tubes/bundles which rotate, in order to avoid local overheating and to improve uniform drying. This dryer uses less air volume and subsequently emissions into the atmosphere are limited.
A special dryer design uses superheated steam produced via a heat exchanger. The dryer consists of a pressure vessel in which the water from the product is driven off, turned to steam and then used to dry more product. This system is used in the sugar industry, on a limited scale, for drying beet pulp. One advantage is the low energy consumption for drying.
Drying in kilns
Hot air, going from 40 ˚C in the beginning going up to about 85 ˚C for pale coloured malts, and up to 130 ˚C for more darker malts, is blown through a layer of green malt of about 50 to 150 cm thickness, without any fluidisation of the batch. The blowing lasts from 18 to 48h. At the end of the kilning process the dried malt is cooled down to about 25 to 35 ˚C and the malt culms are removed.
In the case of temperature-sensitive products, the external pressure must be lowered to avoid drying at a high temperature. One simple type of vacuum drying is vacuum roller drying. In this method, either one or two rollers are installed in a vacuum housing. The resulting vapour precipitates in a condensor located between the vacuum chamber and the pump. The product is removed by a screw conveyor.