Phytochemical stability in dried apple and green tea functional products as related to moisture properties

In this study, apple products made with and without added green tea extract were freeze-dried and stored for up to 45 days at 30 °C in low and intermediate moisture environments (water activity, aw, 0.11, 0.22, 0.32, 0.57, and 0.75). Kinetic models were developed for the changes in color and decreases in contents of selected green tea and apple monomeric and polymeric flavanols and ascorbic acid. Moisture isotherms were developed for each product. At various moisture levels, the glass transition temperature (Tg) was measured by DSC and water mobility by 1H NMR. Chemical changes were related to Tg, aw, and water mobility in the products.Phytochemical degradation occurred more rapidly at higher moisture contents, except for caffeine which was stable. In the product containing apple with green tea, the content of monomeric flavan-3-ols decreased by 34% and 39% after 45 days of storage at aw of 0.56 and 0.75, respectively. Phytochemical degradation correlated with increasing aw, Tg, and water mobility. This study showed that, in general, storage at aw 0.75 most affected phytochemical stability and color.