苹果汁中维生素C热降解动力学研究

为了探索温度对苹果汁中维生素C降解的影响以及维生素C的降解动力学规律,采用可逆的一级反应模型,对加热温度30、40、50、60、70℃条件下苹果汁中还原型维生素C(AA)和氧化型维生素C(DHA)含量的变化及降解动力学进行了研究,结果表明:60℃的AA反应速率最大,DHA反应速率基本随着温度升高而增大;温度升高,可逆反应速率常数增大,其它降解途径速率常数变化不明显;AA无氧氧化速率与温度有一定关系,50℃的AA无氧氧化速率最大,70℃的AA无氧氧化速率最小;AA和DHA热降解反应活化能均低于一般化学反应活化能,说明苹果汁中维生素C热稳定性较差。维生素C的降解主要是有氧氧化反应,苹果汁中维生素C无氧降解速率比有氧降解速率慢;温度升高,苹果汁中维生素C有氧降解速度加快,无氧降解速度变化不明显。在苹果汁加工工艺中应尽量隔绝氧气,采用高温(70℃)短时热处理。对成品苹果汁应采用低温贮藏。

Thermal Degradation Kinetics of Vitamin C in Apple Juice

In order to grope for the influence of temperature on degradation and thermal degradation kinetics of the vitamin C in apple juice,the thermal degradation kinetics of Ascorbic Acid(AA) and Dehydroascorbic Acid(DHA) at 30,40,50,60 ℃ and 70 ℃ in the apple juice were investigated,using the reversible first-order kinetic model.The results showed that the maximum reaction rate of AA was at 60 ℃,DHA reaction rate usually was increased as temperature increasing.Reversible reaction constant was increased as temperature increasing although reaction rate constants of other pathways of degradation did not vary obviously.Anaerobic oxidation rate of AA was related to the temperature,the maximum anaerobic oxidation rate of AA was 50 ℃.Under 70 ℃ anaerobic oxidation rate of AA was the lowest.The thermal degradation activation energy of vitamin C was lower than those of general chemical reactions,indicating that heat stability of vitamin C in apple juice was very low.The aerobic oxidizing reaction was the primary degradation pathway of vitamin C degradation.The speed of anaerobic degradation of vitamin C in juice was slower than that of the aerobic degradation.As temperature was increased,the aerobic degradation rate of vitamin C was accelerated;the anaerobic degradation rate did not change significantly.Oxygen isolation should be guaranteed in apple juice processing.Short time treatment of high temperature(70 ℃) should be adopted in thermal processing,and refined apple juice should be stored at low temperature.