Influence of freezing pretreatments on kinetics of convective air-drying and quality of grapes

The effects of freezing pretreatments on drying kinetics and qualitative properties (shrinkage, rehydration, color, texture, total phenol content, antioxidant activity) of hot air dried grapes were studied. Results showed that the use of a freezing pretreatment before grape drying reduced the shrinkage (from 84 +/- 1 to 72 +/- 1%), color changes, firmness (from 1.7 +/- 0.1 to 0.9 +/- 0.1 N), drying time and increased rehydration, total phenol content (from 9.1 to 10.7 mg/g), antioxidant activity, effective moisture diffusion coefficient and activation energy ((from 27 +/- 3 (control samples) to 36 +/- 3 (samples treated using cryogenic freezing) and 37 +/- 3 kJ/mol (samples treated using conventional freezing)). Whereas, increasing the drying temperature increased the shrinkage and firmness. The Page model with the highest correlation coefficient and lowest root mean square error and chi-square was selected as the best model. Freezing cracks were observed in both conventional and cryogenic freezing samples using scanning electron microscopy.