Impact of sugar reduction on the glass transition temperature and sorption isotherm of freeze-dried tomato powder

The objective of this work was to improve the drying of tomato juices after identifying the main parameters that are responsible for sticking during drying. Glass transition temperature (T_g) which is one of the key parameter, depends on the juice composition, mainly sugar. In order to reduce sugar, centrifugation was applied to the juices. T_g was determined by differential scanning calorimetry (DSC) for freeze-dried tomato juice and tomato pellet conditioned at various water activity (a_w) (0.1–0.5). The glass transition curve showed that T_g decreased with increasing of the moisture. Comparison of powders obtained by juice and pellet from centrifugation shows that the T_g of freeze-dried pellets is higher than that of juice at equal a_w values. Adsorption isotherms of freeze-dried tomato juice and pellet were determined at 25 °C and a relative humidity (RH) of 0–98% using a dynamic technique. The data obtained were fitted to GAB model that best fitted the experimental data. The equilibrium moisture content (X) value of the freeze-dried pellets are lower and had the same value of 8% dry mass. X is lower because the pellet powders are less hygroscopic than tomato juice powders because of the reduction of soluble compounds by centrifugation, notably sugars. Reducing the product sugar content by centrifugation enables to raise T_g without using carriers and obtain less hygroscopic powders, which is advantageous for product storage.