Microwave vacuum drying of plain yogurt

A simulation model was developed to estimate temperature and moisture profiles of yogurt and to predict survival of lactic acid bacteria in yogurt during microwave vacuum drying.;Thermophysical properties of yogurt were modeled as functions of moisture content for bulk density, shrinkage, and specific heat, and as a function of both temperature and moisture content for thermal conductivity. Bulk density, shrinkage and specific heat showed linear relationships with moisture content of the product. Thermal conductivity showed a linear relationship with temperature but showed a logarithmic dependence on moisture content.;In the case of moisture sorption isotherms and their temperature dependence, G.A.B. model showed better fitness for yogurt products than Halsey equation. Concentrated yogurt and microwave vacuum dried concentrated yogurt powder showed a lower level of equilibrium moisture content than yogurt and freeze dried yogurt, respectively, which implied structural changes of yogurt during its preconcentration.;Effective moisture diffusivity of yogurt was estimated from the drying data of yogurt by two methods: method of the slopes of drying curve and regular regime technique.;Microwave energy absorption within the product was modeled as a function of its moisture content and radiation time. Power absorption showed a quadratic relationship with both moisture content and time. The simple model was successfully applied to calculate the internal heat generation during microwave vacuum drying of yogurt.;The combined effects of water activity and temperature were studied by determining the relationships of decimal reduction time with both water activity and temperature. The nonlinear model equations were useful for predicting survival of lactic acid bacteria during drying of yogurt under microwave field.;Finally, prediction of survival of lactic acid bacteria was performed by solving heat and mass transfer equations and by applying all the models described above. Predicted temperatures, moisture content, and survival of lactic acid bacteria agreed well with the experimental values. This type of simulation may be applicable for predicting quality changes and for optimizing processing conditions of other heat and mass transfer processes, if the kinetics for the quality changes are available. (Abstract shortened with permission of author.)...