Drying (from liquid to solid)
Drying is defined as the application of heat under controlled conditions to remove the water present in liquid foods by evaporation to obtain solid products. It differs from evaporation that produces concentrated liquid products. The main purpose of drying is to increase the shelf life of foods by reducing their water activity.
Field of Application
Drying techniques are used in, among other things, dairy products (milk, whey, cream), coffee, coffee substitutes, tea, flavorings, powder drinks and processed cereal-based food products.
Techniques, methods and installation
Two different principles can be applied for drying:
- a) Hot air drying*
Hot air is used as the heating medium and comes into direct or indirect contact with the liquid product. The heat that is moved from the hot air to the product causes evaporation of the water content.
b) Surface drying by heat conduction through a heat transfer system (i.e. contact driers)
The heating medium does not come into contact with the wet food, but it is separated by a heat transfer surface. The heat is transported through conduction through the surface and through convection of the heat surface to the food product for the evaporation and removal of water from the food. This has two major advantages compared to the hot air dryers: less air quantity is required and therefore the thermal competence is higher and the process can be carried out without oxygen.
Spray dryers
In spray dryers, the material to be dried is suspended in the air, i.e. the liquid is turned into a fog-like mist (atomized) giving a large surface area. The atomized liquid is exposed to a stream of hot air in a drying chamber. The moisture evaporates quickly and the solids are recovered as a powder consisting of fine, hollow spherical particles. Air inlet temperatures up to about 250 ° C or even higher (depending on the type of product) are used, but due to evaporation the air temperature drops very quickly to a temperature of about 95 ° C (exhaust air temperature). The product temperature will be 20-30 ° C below the air outlet temperature. The drying air can be heated using steam, direct gas-burning air heaters or indirect air heaters that burn gas, liquid or solids. Spray drying is widely used in the dairy industry and for drying coffee. Generally exhausted air, as an integral part of the process, passes through cyclones and / or filters to regain fine material (dust) that is transferred into the depleted air. The recovered material is then reintroduced into the product.
Roll drying
The principle of the rolling-drying process is that a thin layer of material is placed on the flat surface of a continuously rotating and steam-heated metal drum. The thin layer of dried material is continuously scraped off by a stationary knife opposite the point where the liquid material is applied. The dryer consists of a single drum or a number of drums with or without 'satellite' rollers. The steam pressure in the drums can vary, depending on the product, from 4 to 8 bar. Roll drying is used, for example, in milk, starch and potato flakes.
Vacuum belt and vacuum board dryers
Food suspension is placed on a steel chain (or belt) or sprayed. This chain is about two hollow drums inside a vacuum chamber. The food is first dried by the steam-heated drum and then by steam-heated coils or radiant heaters along the belt. The dried food is cooled by the second water-cooled drum and removed by an ink knife. The rapid drying and the limited heat damage to the product indicates that this method is suitable for heat-sensitive products.
