The effect of pH on polygalacturonase and pectin methylesterase activity in crude extracts and their thermal inactivation kinetics

Processed tomato products are a billion dollar industry in the United States. Based on the final use tomato products can be processed by hot or cold break. Hot break produces a product with better texture while cold break produces a product with better color and flavor. To preserve texture hot break must inactivate polygalacturonase (PG) and pectin methylesterase (PME), the enzymes responsible for texture degradation. The addition of citric acid to lower the pH from 4.3 to 3.9 could increase the thermal inactivation of PG two-fold at 85.5°C (from 60 to 29 minutes) and PME over four-fold at 72°C (from 180 to 37 seconds) (Sekine et al. unpublished). Furthermore, a change in the pH of tomato juice affects the activity of the crude extract, suggesting that the change in pH causes an irreversible change to the enzymes such as a shift in structure or the ratio of isoenzymes. Acidifying the juice from pH 4.9 to pH 3.9 reduces the crude extract activity by 60% for PME and 54% for PG. This study emphasizes the importance of including pH as a parameter for optimizing tomato processing, because of its strong influence on tomato product quality through its affect on enzyme inactivation.