The effect of three drying methods on biophysical and biochemical properties of raisin

Raisin is the main product of grape in Iran that obtains through drying process. Regarding to mass production of grape in this country, optimization of the raisin preparation methods in order to increase world market demand is required. In this research, effects of three different methods of drying including of sun drying, shad drying and alkaline emulsion drying (potassium carbonate + ethyl oleate) were investigated on biophysical and biochemical properties' of Sultana grape's raisin including raisins yield, drying rate, dry weight, and rachis weight and also antioxidant capacity (based on DPPH, FRAR methods), total flavonoids, total phenols, total protein, soluble sugars (fructose, glucose, sucrose, raffinose), organic acids (ascorbic acid, malic acid, tartaric acid) and some nutrients under a completely randomized design with three replicates in Iranian Grape and Raisin Institute of Malayer University. In early Sep. of 2015, fruits were harvested manually based on TSS index of 21 (°Brix) and then were laid on drying net racks until they reached 15% moisture. Based on results, significant differences were found among three drying methods regarding to both biophysical and biochemical properties'. The highest drying period was related to shade drying methods while the lowest was found in alkaline emulsion drying method. Furthermore, the highest and lowest grape to raisin drying rate was belonged to alkaline emulsion and shade drying methods, respectively. Raisin yield was highest in alkaline emulsion drying method and lowest in sun drying method. The highest total phenol, total flavonoid and measured antioxidant capacity by DPPH and FRAR was found to be higher in alkaline emulsion drying method compared to other methods. Moreover, soluble sugars (sucrose, glucose, fructose and raffinose) and organic acids (tartaric, malic but not ascorbic acid) was higher in alkaline emulsion drying method than other methods. In conclusion, owing to faster grape to raisin drying rate observed in alkaline emulsion drying method, the oxidation of phenolic compounds and flavonoids of these raisins was lower compared to both sun-and shade drying methods which showed higher nutritional value and antioxidant capacity than other methods.