Evaporation kinetics and quality attributes of grape juice concentrate as affected by microwave and vacuum processing

In this study, grape juice was concentrated through a microwave vacuum-assisted technique. The concentrate was prepared at different powers (30, 50, 80 W) and vacuum pressures (35, 50, 70 kPa) for different time intervals to optimize the processing conditions. The highest evaporation rate (5.32 g/100 g min) and the lowest moisture ratio (0.18) were observed at a higher level of microwave power (80 W) and vacuum pressure (70 kPa). Moreover, different mechanistic models were used to predict the values of the moisture ratio. Among them, the Midilli (R² = 0.999, SSE = 1.970 × 10⁻⁸, RPD = 0.012%) model best fits the experimental values. The effective diffusivity (1.69 × 10⁻¹⁴ m²/s), activation energy (1.56 W/kg), and extraction yield (69.30%) possessed maximum values at 80 W and 70 KPa. While concentrate had exhibited higher antioxidant activities (77.2%) than fresh juice (25.28%) at 30 W. Overall results indicated that the microwave-vacuum technique had significantly reduced the processing time and energy requirement.