Glass transition temperature (T-g) and water activity (a(w)) of dehydrated apple products

Abstract

The onset glass transition temperature (T-go) of Granny Smith apple products was evaluated as a function of drying conditions (air drying and freeze-drying) and moisture and water activity (a(w)) levels. T-go was determined in the 0-0.75 a(w) range, and it was found to have a maximum value at a(w) = 0 (T-go = 4.5 +/- 1.6C) for freeze-dried apple and a minimum value at a(w) = 0.75 (T-go = -81.4 rt 0.1C) for freeze-dried apple juice. A linear relationship between T-go and a(w) was obtained in all cases. Theoretical T-go values at a(w) = 0 were calculated using these equations, and were within the range of 3.0C (freeze-dried apple) and 10.5C (apple dehydrated at 30C). When Gordon and Taylor's linear model was applied to the experimental data of all the apple products, K values within the range 2.4 (freeze-dried apple juice) and 3.6 (apple dehydrated at 60C), and the theoretical T-g of solids values at a(w) = 0 between the range -16.3C (freeze-dried apple juice) and -1.9C (apple dehydrated at 60C) were obtained. No effect of the type of drying on the value of T-g was detected.