Effect of Thermal Treatment on the Quality of Cloudy Apple Juice

Apple juice from eight different varieties of apples was heated at high-temperature (60−90 °C) and short-time (20−100 s) (HTST) combinations. To determine the effect of heating conditions on enzymatic browning and cloud stability in apple juices, the activity of polyphenol oxidase and pectinesterase was analyzed and correlated with the thermal treatment conditions and the quality of the juice. Additional investigations included the measurement of pH value, soluble solid content, titratable acidity, color, and turbidity after 3 and 6 months. The results showed that HTST treatment at 80 °C already inactivated polyphenol oxidase, whereas pectinesterase activity was reduced to half and could even at 90 °C not be inactivated completely. In fact, highest residual pectinesterase activity was found at 60 °C. Heating at 70 °C caused stable pectinesterase activity and even a slight increase for 50 and 100 s heating times. Turbidity and lightness increased after HTST treatment. In particular, differences in cloud st...