An approach for estimation of critical points of water sorption isotherms of foods

The problem of optimal storage conditions of dry foods with sigma-shaped Water Sorption Isotherm is investigated. It is assumed that the optimal value of water activity, A(o), corresponds to the critical points of the isotherm where its local slope (the first derivative) has a minimum value. Two approaches were tested. In the first one the experimental data were fitted by the modified GAB/BET model and the optimal point was calculated by differentiation of the resulting equation. In regression the second approach estimates of local slope of the isotherm between adjacent experimental points was calculated. The data were fitted by the proposed empirical model for the first derivative of the sigma-shaped isotherm. Then, the minimum value of the first derivative was computed. Comparative analysis of both methods based on experimental moisture sorption data for pistachio nuts at 10, 20 and 30 degrees C revealed that the relative error in the optimal point by the second method is less than that by the first one.