Pectin hydrolysis: Effect of temperature, degree of methylation, pH, and calcium on hydrolysis rates

The rate of acid hydrolysis of pectin at pH 3 decreased as the degree of pectin methylation (DM) increased. Acid hydrolysis rates for polypectate (<5% DM) declined as the pH was raised from 2 to 6. Pectin (35% and 70% DM) hydrolyzed more slowly than polypectate below pH 3.5, but degradation rates then increased because beta-elimination became the dominant reaction above pH 3.8. Temperature effects on the hydrolysis rates at pH 3 of pectin samples from different sources, as indicated by values for the entropy and enthalpy of activation for this reaction, were very different from the effect of temperature on cucumber tissue softening at the same pH. The results indicated that pectin hydrolysis is not the primary reaction responsible for nonenzymatic plant tissue softening at acid pH. Though calcium ions strongly inhibit plant tissue softening at acid pH, calcium ions did not inhibit acid hydrolysis of pectin.