Drying is a separation process and can be carried out using a number of techniques. Removal of water can be by mechanical expression, by evaporation at atmospheric or reduced pressures, and by freezing the water and subliming the ice.
Sorption isotherms provide the base relationship between moisture content and equilibrium relative humidity that underlies much of drying technology. In fact critical processing points may be marked on the isotherm to denote packaging, storage and atmospheric conditions (Gal, 1983). Sorption data are normally given at temperatures close to ambient, in part because of the practical control of humidity through saturated salts. Sorption isotherms have been extensively studied and described by equations such that the constants characterise the various food components.
Freeze-drying confers maximum quality after rehydration although it is the most expensive of the drying methods. It preserves the approximate shape and size of the sample.
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