Effect of Pretreatments and Air Temperatures on Drying Characteristics and Color Changes of Ziziphus Fruits

Abstract

The aim of this study was to investigate the effect of pretreatments and hot air temperature on drying kinetics and color changes of Ziziphus fruits dried in an oven drier. The average weight of Ziziphus spina-christi (ZS) fruits was 4.12g, while that of Ziziphus mauritiana (ZM) fruits was 41.02g. Drying curves of ziziphus fruits were affected by the applied pretreatments and hot air temperature. No constant drying rates were observed which indicate that moisture removal was carried out principally by moisture diffusion from pulp to fruit outer surface. Consecutive dipping in solutions of sodium hydroxide, citric acid and sodium metabisulfite followed by drying at 90°C shortened the drying time to an average of 6.2h for ZS fruits and 5.27h for ZM slices. Page-model was better than Newtonian-model for mathematical representation of drying curves for ziziphus fruits. Drying process caused reduction in lightness (L-values), increase in redness (a-values), decrease in yellowness (b-values) and increase in browning index (BI-values). Color saturation (chroma) of the dried fruits was higher than that of fresh fruits, while Hue-angles were radically changed from yellow region towards orange-red region upon drying.