Abstract
Although hot air convective drying of foods is carried out at high temperatures especially for spray drying of liquid foods, low temperature drying is needed for heat-sensitive foods. For example, pasta and pasta-like foods are dried at low temperatures in order to avoid formation of cracks on the surface of the product. Heat-sensitive biomaterials such as proteins and microorganisms might be dehydrated at low temperatures as well.
In this study we investigated the water diffusion and the water desorption behavior for liquid foods and pasta-like foods for temperatures between 283K and 323K. The isothermal drying experiments were carried out in order to obtain isothermal regular regime drying curves, from which the apparent water diffusion coefficients as a function of water content were determined. The desorption isotherms were determined from the equilibrium water contents at constant relative humidities (water activities). The equilibrium water content at low water activities (monolayer coverage) increased with a decrease in temperature especially for sucrose whereas the values did not change significantly with temperature for high molecular weight sugars and pasta-like foods. Enzymes became very stable at low water content regions (less than 0.5). Similarly thermal stability of yeasts increased when the water content was below 0.5.
