LOW pH INACTIVATION OF POLYPHENOLOXIDASE
The effects of low pH on polyphenoloxidase in apple juice, grape juice, and water was investigated. The study used HCl and high pressure CO2 to lower the pH of the products. Apple polyphenoloxidase was inactivated to 99%, 63% and 34% of initial value after 20 min at adjusted pH values of 2.00, 2.25 and 2.50, respectively. Higher pH values resulted in slower inactivation. In a pilot plant study, juice was acidified with HCl to pH 2.00 for 45 min and subsequently raised to the original pH with NaOH. Polyphenoloxidase activity decreased by 88% and filtration of the juice was improved. Sensory analysis of the juice indicated that the acidified samples had a lighter but less preferred color and a disagreable salty taste. No aroma difference was recognized.
Carbon dioxide was used to pressurize apple juice and water to 31,000 kPa at 45 C. The pH of the apple juice was lowered from 4.2 to 3.7 and the pH of the water was lowered to 2.8. Carbon dioxide at 31,000 kPa resulted in near total inactivation of of polyphenoloxidase in water, after 20 min, 15 min, and 10 min at 35 C, 45 C, and 55 C, respectively. High pressure CO2 treatment of apple juice at 45 C resulted in an increase in polyphenoloxiade activity in the first hour. However, a decrease in activity was seen after the first hour and resulted in almost complete inactivation after 4 hrs. The control samples decreased by only 20 percent.