柑橘果胶碱法脱酯动力学特征

Demethylation Kinetics of Citrus Pectin in Alkaline Solution

为研究碱法脱酯机理以及温度和p H值对果胶酯化度(degree of esterification,DE)与脱酯速率的影响,以高酯柑橘果胶(DE为77.8%)为原料,研究不同温度(10、20、25℃和30℃)和不同p H值(10.0~11.3)条件下的脱酯动力学和热力学特征。为更精确控制p H值,在Na HCO3/Na2CO3缓冲体系中以Na OH调节果胶溶液p H值,以酯化度和甲氧基浓度为指标研究碱法脱酯规律。结果表明:温度和p H值都对碱法脱酯有显著影响,但p H值的影响大于温度,且它们的交互作用明显。果胶碱法脱酯符合一级动力学反应模型。脱酯速率常数(k)与p H值呈幂指数关系。当p H值从11.0轻微增加到11.3时脱酯速率显著增大。p H值为10.7、11.0和11.3时,脱酯活化能分别为136.97、122.37 k J/mol和104.20 k J/mol,脱酯活化焓分别为134.52、119.92 k J/mol和101.75 k J/mol,脱酯活化熵分别为141.86、100.28 J/(mol·K)和44.81 J/(mol·K),脱酯活化自由能分别为92.76、90.40 k J/mol和88.56 k J/mol。

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(10,20,25and30℃)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 from11.0to11.3.At pH10.7,11.0and11.3,the activation energy(Ea)were136.97,122.37and104.20kJ/mol,the activation enthalpy(ΔH)were134.52,119.92and101.75kJ/mol,the activation entropy(ΔS)were141.86,100.28and44.81J/(mol·K),and the free energy of activation(ΔG)were92.76,90.40and88.56kJ/mol,respectively.