Water Adsorption and Glass Transition of Spray-Dried Soy Sauce Powders Using Maltodextrins as Carrier

Moisture adsorption isotherm and glass transition temperature of various spray-dried soy sauce powders containing different types and concentrations of maltodextrins were studied and compared. Maltodextrins of dextrose equivalent (DE) = 5, DE = 10 and DE = 15, respectively, with concentrations of 20 or 40 % (w/v) were used as carrier agents. The equilibrium moisture content was reduced with increased maltodextrin concentration, whereas it was not apparently influenced by the value of maltodextrin DE. Both the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models could be applied to simulate the moisture adsorption behaviour of the soy sauce powders. The monolayer moisture content of the powders was determined by fitting experimental data to the BET/GAB models with a (w) up to 0.53, although both models could fit satisfactorily with the experimental data to a higher water activity level(.) The glass transition temperatures (T (g)) of the powders equilibrated under various water activities were determined using a differential scanning calorimeter. Increasing moisture adsorption of the soy sauce powders resulted in a T (g) reduction, and the experimental T (g) values fitted the Gordon-Taylor model well. The BET and Gordon-Taylor models were applied together to predict the critical moisture contents (i.e. 0.0464-0.0777 g water/g dry matter) and water activities (i.e. 0.032-0.241), above which the soy sauce powders become vulnerable to degradation and changes in their physicochemical properties.