A study on hot-air drying of pomegranateKinetics of dehydration, rehydration and effects on bioactive compounds

Pomegranate arils were dehydrated at 55, 65, 75 °C using hot-air technique, and the impacts of drying on color, texture, rehydration ratio, shrinkage of arils were evaluated as well as bioactive compounds like total phenolics, total monomeric anthocyanins, total tannins and radical scavenging activity. Sigmoid model gave the best results at all studied temperatures for drying kinetics. Effective moisture diffusivities of arils were 3.5689 × 10−11 m2 s−1 (at 55 °C), 9.3950 × 10−11 m2 s−1 (at 65 °C), 1.9330 × 10−10 m2 s−1 (at 75 °C), and activation energy was 80.33 kJ mol−1. Averaged convective mass transfer coefficients and moisture extraction rates were also calculated, and their highest values were observed at 75 °C. Rehydration was only conducted at 25 °C, and two-term exponential decay model was the most suitable equation for describing rehydration phenomenon. Thermal operation caused important changes in L*, b*, hardness and shrinkage (p < 0.05). Total phenolics, total monomeric anthocyanins and total tannins of dried arils were changed between 5512.37–6895.80 mg gallic acid equivalent, 163.87–324.58 mg cyanidin-3-glucoside equivalent and 1024.99–2467.77 mg per kg dry matter, respectively. The radical scavenging activity was decreased from the initial value of 77.42% to circa 24.79% by drying. Because high temperature had harmful effect on fruit, temperature of 65 °C may be advisable for dehydration of pomegranate arils.