Drying Rate

31) QUALITY OF DRIED FOODS AND DETERIORATIVE REACTIONS DURING DRYING
Consumer demand has increased for processed products that keep more of their original characteristics. In industrial terms, this requires the development of operations that minimize the adverse effects of processing. The effect of food processing on finished product quality ultimately determines the usefulness and commercial viability of that unit process operation. In the particular case of food drying this indicates that loss of volatiles and flavours, changes in colour and texture, and a decrease in nutritional value. Furthermore, residual enzyme activity and microbial activity in dried foods are essential parameters that effect product quality and shelf life. The quality of dried foods is dependent in part on changes occurring during processing and storage. Some of these changes involve modification of the physical structure. These modifications affect texture, rehydrability and appearance. Other changes are due to chemical reactions, but these are also affected by physical structure, primarily due to effects on diffusivities of reactants and of reaction products. The most commonly examined properties of dried products can be classified into two major categories, engineering and quality properties. The engineering properties of the dried products involve effective moisture diffusivity, effective thermal conductivity, drying kinetics, specific heat, and equilibrium moisture content. In addition there are properties related to product quality. These properties are necessary for the determination and the characterization of the quality of dried products can be grouped into: - Thermal properties :state of product; glassy, crystalline, rubbery, - Structural properties :density, porosity, pore size, specific volume, - Textural properties :compression test, stress relaxation test, tensile test, - Optical properties :colour, appearance, - Sensory properties :aroma, taste, flavour, - Nutritional

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