Abstract: For a better understanding of the mechanism of alkali demethylation of pectin and the effect of temperature and pH on the degree of esterification (DE) and demethylation rate of pectin, high methoxyl citrus pectin (DE = 77.8%) was investigate for demethylation kinetics and thermodynamic characteristics at different temperatures (10, 20, 25 and 30 ℃) and pH (10.0-11.3). The pH was accurately controlled by sodium hydroxide in sodium bicarbonate/sodium carbonate buffer. DE and methoxyl group concentration were studied during the demethylation process. The results showed that the alkali demethylation followed a first-order kinetic reaction model. Temperature, pH and their interaction had remarkable effects on the demethylation process, while the effect of pH was more notable. The demethylation reaction rate constant (k) and pH showed an exponential relationship and k sharply increased when pH varied lightly from 11.0 to 11.3. At pH 10.7, 11.0 and 11.3, the activation energy (Ea) were 136.97, 122.37 and 104.20 kJ/mol, the activation enthalpy (ΔH) were 134.52, 119.92 and 101.75 kJ/mol, the activation entropy (ΔS) were 141.86, 100.28 and 44.81 J/(mol·K), and the free energy of activation (ΔG) were 92.76, 90.40 and 88.56 kJ/mol, respectively.

Key words: pectin, alkali demethylation, demethylation rate, kinetics, thermodynamics