Pulsed vacuum drying of Thompson seedless grape: Effects of berry ripeness on physicochemical properties and drying characteristic

The effects of ripeness on the physicochemical properties and pulsed vacuum drying characteristics of Thompson seedless grapes have been explored. Grape berries were sorted based on their density by floating them in sodium chloride solutions of various concentrations, corresponding to densities of 1081, 1088, 1094, 1100, 1107, and 1115 kg/m3. Class samples were selected for determination of soluble solids, pH and titrable acid, total phenols, T1 and T2 weighted magnetic resonance imaging (MRI), and drying kinetics. The resulting dried raisins were characterised by browning index (BI) and texture. Results revealed that with increasing grape ripeness, the total phenol concentration, soluble solid content and pH all increased significantly while the titratable acidity content decreased (p < 0.05). The drying rate and moisture effective diffusivity dramatically also increased with increasing ripeness. MRI analysis indicated that with increasing ripeness the free water content of grape increases significantly and the bound water distribution decreases, which explains why the samples with higher ripeness dried at faster rates. The ripest berries, of density greater than 1100 kg/m3, were found to be better suited for raisin processing taking account of drying time, the browning index (BI) and the texture profile of the raisins. The findings indicate that classification of grapes according to ripeness prior to drying is necessary for more effective and high-quality raisin processing.