Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
Generally, as an integral part of the process, the exhaust air is passed through cyclones and/or
filters to recover particulate materials or dust which are carried over in the exhaust air. The
recovered material is incorporated back into the product.
The principle of roller drying is that a thin film of material is applied to the smooth surface of a
continuously rotating, steam-heated metal drum. The film of the dried material is continuously
scraped off by a stationary knife located opposite the point of application of the liquid material.
The drier consists of a single drum or a pair of drums with or without satellite rollers. The
applied steam pressure in the drums can vary from 4 to 8 bar, depending on the product. Roller
drying is applied, e.g. for milk, starch and potato flakes.
Finally, in vacuum band and vacuum shelf driers, food slurry is spread or sprayed onto a steel
belt, which passes over two hollow drums within a vacuum chamber. The food is first dried by
the steam-heated drum, and then by the steam-heated coils or radiant heaters located over the
belt. The dried food is cooled by the second water-cooled drum and removed by a doctor blade.
The rapid drying and limited heat damage to the food makes this method suitable for heatsensitive
foods.
2.1.6.3 Dehydration (solid to solid) (F.3)
2.1.6.3.1 Objective
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 aw.
2.1.6.3.2 Field of application
Some examples of dried foods are dried potatoes, starch derivatives, sugar beet pulp, flour,
pasta, beans, fruit, nuts, cereals, meal of oilseeds, tea leaves, vegetables and spices. The
dehydration of wet germinated grain is applied in the production of malt which is also called
kilning. For the malting process, the drying step is essential and is required to create the desired
colour and flavour.
2.1.6.3.3 Description of techniques, methods and equipment
Dehydration affects food texture and colour, and causes the loss of volatile components, all of
which have 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. For dehydration, the two different principles
are, i.e. hot air drying and surface drying by heat conduction through a heat transfer system.
These are explained in detail in Section 2.1.6.2.
Various types of driers are used, i.e. fluidised, cabinet or tray, conveyor or belt, pneumatic, flash
and/or ring, rotary, tunnel, steam bundle, steam, kiln and vacuum driers.
FBDs are composed of metal trays with mesh or perforated bases that can 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. The air acts as both the drying and the
fluidising medium. They can be batch or continuously operated. FBDs are compact and allow a
good control over the drying conditions and relatively high thermal efficiencies and high drying
rates. These driers have 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. FBD is often applied as a last drying step after
spray drying (see Section 2.1.6.2), in the dairy industry.
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