Effect of Air-Drying Conditions on Physico-chemical Properties of Osmotically Pre-treated Pomegranate Seeds

The drying of pomegranate seeds was investigated at 40 °C, 50 °C and 60 °C with air velocity of 2 m/s. Prior to drying, seeds were osmodehydrated in 55 °Brix sucrose solution for 20 min at 50 °C. The drying kinetics and the effects of osmotic dehydration (OD) and air-drying temperature on antioxidant capacity, total phenolics, colour and texture were determined. Analysis of variance revealed that OD and air-drying temperature have a significant influence on the quality of seeds. Both anthocyanin and total phenolic contents decreased when air-drying temperature increased. The radical diphenylpicril-hydrazyl activity showed the lowest antioxidant activity at 60 °C. Both chromatic parameters (L*, C* and h°) and browning index were affected by drying temperatures, which contributed to the discolouring of seeds. The final product has 22%, 20% and 16% of moisture; 0.630, 0.478 and 0.414 of aw; 151, 141 and 134 mg gallic acid equivalent/100 g fresh matter (FM) of total phenolics; 40, 24, 20 mg/100 g FM of anthocyanins and 46%, 39% and 31% of antioxidant activity, for drying temperatures of 40 °C, 50 °C and 60 °C, respectively. In view of these results, the temperature of 40 °C is recommended as it has the lowest impact on the quality parameters of the seeds. Differential scanning calorimetry data provided complementary information on the mobility changes of water during drying. Glass transition temperature (Tg′) depends on moisture content and as consequence, on drying conditions. In fact, Tg′ of seeds dried at 60 °C (Tg′ = −21 °C) was higher than those dried at 50 °C (Tg′ = −28 °C) or 40 °C (Tg′ = −31 °C) and osmodehydrated seeds (Tg′ = −34 °C). During OD and drying process, the texture of seeds changed. The thickness of seeds shrank by 55% at 60 °C.