MOISTURE ADSORPTION ISOTHERMS AND GLASS TRANSITION TEMPERATURE OF SPRAY-DRIED CUPUASSU PULP

The shelf life and stability of food powders are greatly influenced by water adsorption characteristics, furthermore, in these products, the glass transition temperature (T_g) must be considered as a critical parameter in storage studies, since the transition from glassy to rubbery state increases the molecular mobility of the matrix, which accelerates the rate of chemical reactions. Therefore, the aim of this study was to evaluate the adsorption isotherms and T_g of spray-dried cupuassu pulp produced with different wall materials: maltodextrin DE10 (MD10), gum arabic (GA), maltodextrin DE20 (MD20), and mixtures of MD:GA (MG) at ratios of 75:25, 50:50, and 25:75. The adsorption isotherm at 25 °C was determined by static gravimetric method, using saturated salt solutions, while T_g was determined by differential scanning calorimetry (DSC). The GAB model fitted the experimental adsorption isotherm data well. Cupuassu powder without wall material addition (FFCP) showed higher water adsorption, followed by those produced with GA, MD20, MG(50:50), MG(25:75), MG(75:25), and MD10. In the same way, the monolayer moisture content (X_m) values were 0.151, 0.098, 0.072, 0.068, 0.111, 0.059, and 0.054 g water/g dry matter, respectively. Regarding T_g, Gordon-Taylor model was able to predict the strong plasticizing effect of water on this property, with estimated values of its parameters T_gs and k, which varied from 62.6 to 77.8 °C and 4.04 to 5.61, respectively, for cupuassu pulp powder with wall materials. Whereas, for FFCP, these model parameters were 33.8 °C (T_gs) and 3.92 (k). The results showed that both the decrease in X_m and the increase in T_gs, relative to FFCP, were due to the incorporation of wall materials, which reduced the surface exposed to water molecules and increased the molecular weight, thereby contributing positively to powder stability. As conclusion, the addition of wall materials increased the T_gs of cupuassu pulp powder, decreasing its hygroscopicity and, consequently, increasing its storage stability (not susceptible to deteriorative changes such as collapse, stickiness and caking).