The effect of high-power ultrasound treatment on enzymes' activity, physicochemical attributes (total soluble solids, pH, viscosity, turbidity, particle size distribution and colour) and carotenoids' content of carrot juice was investigated. The treatments were carried out at 20 kHz (0.95, 2.38, 3.80 W/ml power) in an ice bath for 2, 4, 6, 8, 10 min. The polyphenol oxidase and pectin methylesterase activity were decreased by 43.90 and 37.95% at 3.80 W/ml power and 10 min exposure time, respectively. With the increase of power and time, the effect of high-power ultrasound on the inactivation of enzymes was getting stronger. However, high-power ultrasound had no inactivation effect on peroxidase activity under all treatment conditions. The visual colour differences were not obvious after high-power ultrasound. The pH, total soluble solids and particle size distribution of carrot juice were not significantly affected (p > 0.05) under all treatment conditions, while turbidity was increased and carotenoids' content was decreased. The viscosity of carrot juice was decreased by 1.27% at 0.95 W/ml power and 8 min, while it was increased by 2.29% at 2.38 W/ml power and 8 min. The value of viscosity was negatively correlated with the activity of pectin methylesterase (Pearson's r = -0.481, p < 0.05). According to these results, we could conclude that the optimal treatment condition was 3.80 W/ml for 10 min. Overall, high-power ultrasound treatment inhibited browning, maintained taste and nutritional value and improved stability of carrot juice. Therefore, this technology could well be an option for processing of carrot juice and laid the theoretical foundation for the production of carrot juice and carrot compound beverage.
Keywords: Ultrasound processing; carotenoids; enzymes; fruit juices; viscosity.