Modeling Hygroscopicity, Antioxidant Properties and Color Changes of a Dehydrated Green Tea : Apple Product

A fortified apple product was obtained by freeze-drying a heat stabilized apple puree formulated with a green tea water extract. A single serving (50 g) of this product provides 130 mg of epigallocatechin gallate, which is an amount comparable to that found in a cup of tea. The addition of tea extract increased 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity by 4.6-fold with respect to an unfortified apple product that served as control. Quality of dehydrated products is critically dependent upon storage humidity. Hence, the aim of this study was to evaluate the hygroscopicity, antioxidant properties, and color of the fortified and control products at the time of production and during storage in different water activity environments (aw = 0.109 - 0.736) in the dark at 30°C. The adsorption isotherm of the product fitted the Guggenheim-Anderson-de-Boer equation and was not affected by tea addition. Estimated monolayer water content was 0.106 g water/g dry solids(confidence limits at 95%: 0.093 and 0.120). During storage the colorimetric parameters L* (lightness) and b* (yellowness) decreased, whereas a* (redness) increased. The pseudo-zero order rate constants for changes in color were minimal at the monolayer water activity and increased with increasing water activity. The degradation of flavanols, dihydrochalcones, chlorogenic acid and ascorbic acid followed pseudo-first order kinetics, with the slowest rates at the lowest water activity. Ascorbic acid was the most unstable; the pseudo-first order rate constants for its degradation changed from 0.0049 +/- 0.0007 to 0.54 +/- 0.04 days-1 with increasing water activity from 0.109 to 0.736. Kinetic equations were used to devise mathematical models to predict quality changes of the fortified product during storage as a function of water activity. A water activity range of 0.109-0.327 was found to be optimal to maintain the antioxidant properties and color of the product.