The effect of ultrasonic treatments on turbidity, microbial load, and polyphenol oxidase (PPO) activity of plum nectar

摘要

This study examined the effects of ultrasound on several quality parameters of plum nectars, such as turbidity, microbial load, PPO activity, phenolic content, and physicochemical quality. The ultrasonic treatment was applied to the plum nectars at different treatment temperatures (40, 50, 60, and 70 °C) and durations (2.5, 5, 7.5, and 10 min). Ultrasound processing caused a significant increase in the turbidity level and stability of the plum nectars, which were correlated with increases in treatment temperature and duration. Similar effects were also observed in reduction of microbial load. Ultrasonic treatments at 7.5 min at 60 °C and 5 min at 70 °C reduced the total yeast and mold counts (< 1 log cfu/mL). The total mesophilic-aerobic microorganism (TMAB) count was also reduced by up to 2 log. The maximum inactivation of polyphenol oxidase (PPO) was ~46 % at 70 °C. No dramatic trends were observed for the pH, brix, and the total phenolic compound content of the plum nectar, while the flavonoid content dramatically increased. The ultrasonic treatment caused a slight positive color change by increasing the L* value of the plum nectars. In conclusion, the ultrasonic treatment was important process for increasing the turbidity quality and for inactivation of yeasts and molds, but it was not effective at inactivating the PPO enzyme.