Drying of food products - Devices and applications

Drying operations play an important role in the food industry. They often represent the last operation in a product's manufacturing process, with a strong influence on final quality. Drying involves a wide variety of products, from cereals to finished products, from raw materials to by-products. Processes are numerous and depend on the type and quantity of product to be dried and water to be evaporated, the desired final quality, or the desired functionality of the dried product.

The properties of food are linked to the complexity of its chemical composition (sugars, proteins, lipids, minerals, vitamins, flavors, etc.), which evolves during the drying process and must be controlled. Changes in water content and water activity must also be taken into account, as well as many other factors with positive or negative effects:

• environmental temperature and humidity conditions;

• changes in shape, structure, porosity, mechanical properties ;

• bonding and crystallization phenomena associated with the glass transition ;

• chemical reactions, their nature and speed in relation to pH and temperature, etc.

Many of these parameters are difficult to determine and/or measure.

This dossier follows on from the article Drying food products. Principles [F 3 000], which reviews the physical principles of drying and details the calculations and balances that can be envisaged on a dryer. This second dossier describes the specific features of the main dryers used in the food industry and the impact of the process on product quality.

The final section is devoted to a number of case studies, and a description of the most widespread techniques, or those under development.