Degradation of pectins in alkaline conditions: kinetics of demethylation

The demethylation of commercial citrus pectins (86% by weight galacturonic acid, degree of methylation 73) was studied between pH 8 and 11, for temperatures between 15 and 45 °C. The pectins were incubated in hermetically closed tubes in 0.2 M ethanolamine/HCl buffer. After stopping the reaction by adding acetic acid to reach pH ∼ 4.5, the amount of methanol liberated was analysed by gas chromatography. At the higher pH values, the reaction showed marked deviation from the expected pseudo-first-order kinetics with respect to methyl-esterified carboxyl groups. This deviation could be accounted for by the changes of pH that occurred during saponification as the buffer salt was used up by the reaction. The rate constant of the reaction was calculated using the initial steps of the reaction and a constant pH approximation (k = 25 mol−1 L min−1) or using an integrated rate equation taking into account the pH variations (k = 27 mol−1 L min−1). The energy of activation was determined using the Arrhenius equation (Ea = 42–49 kJ mol−1).