雪梨多酚氧化酶酶学特性及其果汁褐变控制技术研究

雪梨在加工过程中极易发生酶褐变,为抑制雪梨汁的酶褐变,试验以雪梨为原料制备多酚氧化酶(Polyphenol oxidase,PPO)粗酶提取液,加入邻苯二酚为底物,采用分光光度法对其PPO的酶学特性及不同护色剂对PPO活性的影响进行了研究;并选择L-半胱氨酸、D-异抗坏血酸钠、抗坏血酸3种添加剂对雪梨汁进行护色,通过响应面优化试验确定雪梨汁最优护色组合。结果表明:雪梨PPO的最适pH为4.5,最适温度为30 ℃;雪梨PPO具有一定的热稳定性,随着温度的提高,抑制PPO活性所需要的时间逐渐减少;雪梨PPO催化底物邻苯二酚的酶促反应动力学与米氏方程高度符合,最大反应速率Vmax=217.39 U/min,米氏常数Km=0.152 mol/L。雪梨汁加工的最优护色剂组合为:6.56 mmol/L的L-半胱氨酸、4.58 mmol/L的D-异抗坏血酸钠和6.18 mmol/L的抗坏血酸,在此条件下对雪梨汁褐变的抑制率可达90.82%。

Study on Enzymatic Characteristics of Polyphenol Oxidase in Snow Pear and Inhibiting Technology of Fruit Juice Browning

Snow pear is prone to enzymatic browning during processing.In order to inhibit the enzymatic browning of snow pear juice,snow pear was used as raw material to prepare polyphenol oxidase (PPO) crude enzyme extract.Using catechol as substrate,the enzymatic characteristics of PPO and the effect of different color-protecting agents on PPO activity were studied by spectrophotometry.Three additives of L-cysteine,sodium D-isoascorbate and ascorbic acid were selected to protect the color of snow pear juice.The optimal color protection combination of snow pear juice was studied by response surface optimization test.The results showed that the optimum pH and temperature of pear juice PPO were 4.5 and 30 ℃ respectively.The PPO of snow pear had certain thermal stability.With the increase of temperature,the time required to inhibit PPO activity decreased gradually.The kinetics of the enzyme-catalyzed reaction of PPO was in accord with Michaelis-menten equation.The maximum reaction rate Vmax was 217.39 U/min and the Michaelis constant Km was 0.152 mol/L.The optimal color protectors for snow pear juice processing were the combination of 6.56 mmol/L L-cysteine,4.58 mmol/L sodium D-isoascorbate and 6.18 mmol/L ascorbic acid.Under these conditions,the inhibition rate of snow pear juice browning was 90.82%.