Somaye Mirzaee, Gholam Reza Askari, Zahra Emam-Djomeh, Farhad Garavand
In this study, black grape juice containing different concentrations of soluble solids (15, 20, 30 and 40°Brix) obtained by microwave or conventional heating at different operational pressures (12, 38.5 and 100 kPa) was investigated for bioactive compounds, quality attributes and rheological behaviour. The more concentrated the juice, the higher the anthocyanin and phenolic compound degradation, which in turn resulted in decreased antioxidant potential. All Hunter colour parameters (L*, a* and b*) decreased with juice concentration. Microwave heating showed better performance compared to conventional thermal heating in terms of colour retention, anthocyanin and total phenolic contents, and the antioxidant activity of the juice concentrate. Samples processed at lower operational pressures showed a slighter decrease in quality attributes regardless of the heating method used. The results indicate that the use of a higher microwave power (600 W instead of 450 W) at an evaporation pressure of 38.5 kPa resulted in better preservation of quality characteristics against thermal destruction. The juice concentrates obeyed Newton’s law and the heating method did not influence rheological behaviour.