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Quality Changes in Food Materials as Influenced by Drying Processes

Drying and dewatering plays a major role in food manufacturing or food processing activities worldwide. Often one of the last operations in the food processing, it controls to a large extent the quality of the final product. Drying is applied to a wide variety of food products, from cereals to finished goods, from raw materials to byproducts. The processes used are numerous, according to the type and quantity of product to dry, the amount of water to eliminate, the final desired quality or functionality of the dried product (Tab. 1.1). Drying and dewatering impact the mechanical, sensory and nutritional properties of food products, and can be used to create new functionalities (Bonazzi and Bimbenet, 2003, 2008). Drying is one of the main techniques for preserving agricultural and food products; it takes place in the processing of many products, as the main operation or as a consequence of other processing steps. Heat and mass transfer phenomena which are typical of drying also appear during other processes, as in cooking, baking, roasting, smoking, refrigeration, freezing, during storage, and during pneumatic transportation. The main objective of drying is to decrease the water activity (aw) of various perishable materials to values <0.5, in order to enable their storage at ambient temperature.The importanceofaw incontrolling theshelf-lifeof foodsbysuppressing the growth of micro-organisms, by reducing the rates of chemical reactions, and by inhibiting enzymatic deterioration is well established. The respective relationships have been summarized by Labuza et al. (1970) in a diagram similar to that of Fig. 1.1. Water activity is more important to the stability of a food than the total amount of water present, and it makes it possible to develop generalized rules or limits for the stability of foods. For most foods, the critical point below which no micro-organism can grow is in the 0.6–0.7 water activity range. A food product is most stable at its monolayer moisture content, which varies with the chemical composition and structure. Water activity is also useful for predicting the final moisture content at equilibrium versus drying conditions (temperature and relative humidity), and for