This study was conducted to decrease Saccharomyces cerevisiae cell count and to maximize levels of quality characteristics in red grape juice during ultrasound process using response surface methodology (RSM). The effects of three process parameters (considered as independent variables), namely temperature (25–50C), time (20–40 min) and frequency (0–135 Hz), on the yeast inactivation, total anthocyanin content (TAC), total color difference (TCD), antioxidant activity (AA), polymeric color content (PCC), pH, total titratable acidity (TTA) and total soluble solids (TSS) (considered as dependent variables) were studied based on RSM. The optimum operating conditions for the maximum yeast inactivation and the optimal juice quality characteristics were found to be frequency of 135.0 kHz, temperature of 30.9C and time of 40.0 min. In these conditions, the yeast inactivation, TAC, PCC, AA, TCD, pH, TTA and TSS were 1 cfu/mL, 808.0 mg/g, 67.85%, 0.94 mmol/L, 1.64, 3.47, 977.4 mg citric acid/100 mL and 20.04, respectively.
In recent years, emerging nonthermal methods, such as ultrasound (US), for inactivating microorganisms have been developed in food industries to manufacture natural products. Red grape juice, due to the presence of anthocyanins and flavonoids, has many health-promoting effects. As the nutritive and organoleptic characteristics of grapes decrease during storage and ripening periods because of their high susceptibility to pathogenic infection and physiological deterioration, the use of US method as a potential technology can inactive foodborne spoilage or pathogenic microorganisms and produce juices with organoleptic and nutritional qualities. Moreover, the optimization of US conditions enabled us to shorten the process duration and to reduce the production costs while maintaining the quality at a relatively high level.