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Drying (liquid to solid)

Objective

Drying is defined as the application of heat under controlled conditions, to remove the water present in liquid foods by evaporation to yield solid products. It differs from evaporation, which yields concentrated liquid products. The main purpose of drying is to extend the shelf-life of foods by reducing their water activity.

Field of application

Typical applications for drying techniques include dairy products (milk, whey, creamers), coffee, coffee surrogates, tea, flavours, powdered drinks, and processed cereal based foods.

Techniques, methods and equipment

Two different principles can be applied for drying:

a) Hot air drying

Hot air is used as the heating medium and is in direct or indirect contact with the liquid product. The heat transferred from the hot air to the product causes evaporation of the water content.

b) Contact dryers

I.e. surface drying by heat conduction through a heat transfer system.
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 may be carried out in the absence of oxygen.

Spray dryers

In spray drying, the material to be dried is suspended in air, i.e. the liquid is converted into a fog-like mist (atomised), providing a large surface area. The atomised liquid is exposed to a flow 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 of 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 rapidly to a temperature of about 95°C (outlet temperature of the air). The product temperature will be 20 to 30°C below the air outlet temperature. Heating the drying air can be accomplished by steam or by direct gas-fired air heaters or by indirect heaters fired by gas, liquid or solid fuels. Spray drying is applied on a large scale in the dairy industry and for drying coffee. Generally, as an integral part of the process, the exhaust air is passed through cyclones and/or filters to recover particulate materials (dust) which are carried over in the exhaust air. The recovered material is incorporated back in the product.
The composition of the atomiser is such that it contains a number of lubrication contamination control points (LCCPs). Therefor it is necessary to use food grade lubrication for atomisers.

Roller drying

The principle of the roller drying process (or drum 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 dryer 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 for example, for milk, starch and potato flakes.

Vacuum band/vacuum shelf dryers

Food slurry is spread or sprayed onto a steel belt (or "band"), which passes over two hollow drums within a vacuum chamber. The food is first dried by the steam-heated drum, and then by steam-heated coils or radiant heaters located over the band. 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 heat sensitive foods.

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