GLASS TRANSITION TEMPERATURE (Tg) AND WATER ACTIVITY (aw) OF DEHYDRATED APPLE PRODUCTS

The onset glass transition temperature (Tgo) of Granny Smith apple products was evaluated as a function of drying conditions (air drying and freeze-drying) and moisture and water activity (aw) levels. Tgo was determined in the 0–0.75 aw range, and it was found to have a maximum value at aw= 0 (Tgo= 4.5 ± 1.6C) for freeze-dried apple and a minimum value at aw= 0.75 (Tgo= -81.4 ± 0.1C) for freeze-dried apple juice. A linear relationship between Tgo and aw was obtained in all cases. Theoretical Tgo values at aw= 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 Tg of solids values at aw= 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 Tg was detected.