Drying Rate

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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 characteristics : vitamins, proteins and - Rehydration properties : rehydration rate, rehydration capacity. During the last decades, much attention is paid on the quality of dehydrated foods. The specific drying method as well as the physical-chemical changes that occur during drying seems to affect the quality of dehydrated products. More specifically, drying method and process conditions affect significantly the drying constant, colour, texture, density and porosity and sorption characteristics of materials. The increasing need for producing efficiently high quality and convenient products at a competitive cost has led to the employment of several drying methods in practice. Conventional air-drying is the most frequently used dehydration operation in food and conventional chemical industry. Dried products are characterized by low porosity and high apparent density. Significant colour changes occur during air -drying, and most frequently the dried product has low sorption capacity. Microwave drying is an alternative drying method that has recently been used in food industry. Applying microwave energy under vacuum combines advantages of both vacuum drying and microwave drying as far as improved energy efficiency and product quality are concerned. Vacuum dried materials are characterised by higher poros ity, depending on level of vacuum, and less deterioration of colour and volatile aroma. Paper 31 – PAGE 1/18

Osmotic dehydration minimises the heat effects on colour and flavour, prevents enzymatic browning and thus limits the use of sulphur dioxide, increasing in this way the retention of nutrients during subsequent convective drying. Osmotic dehydration greatly affects apparent density and porosity. Freeze drying is one of the most sophisticated dehydration methods. It provides dried products of porous structure and little or no shrinkage, superior taste and aroma retention, better rehydration properties, compared to products of alternative drying processes....
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