Studies On The Mechanism Of Enzymatic Browning For Yam And Its Control

Browning easily occurred in yam storage and processing, which would affect its merchandise appearance and edible quality. To explore the mechanism of yam browning and the technology of browning control, the physiological and biochemical indexes and isozymes of different breeds of Wenzhou yam were researched. The browning substances of heavily browning breed (Dioscorea alata L. baiyuan) had been analyzed. The effects on PPO activity of the factors such as pH vaiue, temperature and substances concentration and enzyme kinetics had been researched. The effects of different storage ways of Dioscorea alata L. baiyuan and Dioscorea batatas Decne.zhen on browning and the technology of browning control also had been studied. The main results were as follows:1. The browning in different breeds of Wenzhou yam occurred differently. Water contents of Dioscorea alata L. baiyuan, Dioscorea batatas Decne.zi and Dioscorea batatas Decne.zhen were relatively high but water contents of Dioscorea alata L.hongyuan was relatively low. The degree of browning and the contents of all phenolics were that Dioscorea alata L. baiyuan> Dioscorea alata L.hongyuan> Dioscorea batatas Decne.zi > Dioscorea batatas Decne.zhen. The contents of all phenolics was positively correlated with the degree of browning(r=0.983). The activity of PPO was that Dioscorea alata L. baiyuan> Dioscorea batatas Decne.zi>Dioscorea alata L.hongyuan>Z)Dioscorea batatas Decne.zhen, and the activity of PPO was also positively correlated with the degree of brovvning(r=0.909). The activity of POD was different from that of PPO and the activity of POD was that Dioscorea batatas Decne.zhen> Dioscorea alata L.baiyuan.> Dioscorea alata L.hongyuan> Dioscorea batatas Decne.zi, and it correlated little with the degree of browning. The activity of SOD was negatively correlated with the degree of browning(r= - 0.860), but the content'of MDA was positively correlated with the degree of browning(r=0.990).The isozyme charts of PPO, POD and SOD indicated that the isozyme content, varieties and physical and chemical character of different breeds existed difference. Dioscorea alata L. baiyuan had more PPO isozyme bands and they were dyed the darkest. There were eight types of POD isozyme bands in the four breeds. The four breeds had the P4 band in common and the P4 band was dyed the heaviest in Dioscorea batatas Decne.zhen. There were nine types of SOD isozyme bands in the four breeds. Dioscorea batatas Decne.zhen had all the SOD isozyme bands. The results of isozyme experiments of the four breeds were the same as that of the experiments of enzyme activity. The breed having severe degree of browning had more PPO isozyme bands and they were dyed the darkest. The breed having light degree of browning had more SOD isozyme bands and they were dyed the darkest.The results of thin-layer chromatography showed that the Rf value of phenol extractive in yam was the same as catechol. So it could be considered that the main ingredient of phenols in yam tuber was catechol.2.The optimum pH value of PPO of the yam was 5.8 to 6.0, and the optimum temperature was 20癈. When the added enzyme content was under 200 u L, enzyme activity would increased with the enzymic content. The enzyme kinetics showed that when the substrate was catechol, Km would be 1.82mol / L, Vmax would be 1.11( A OD / min). The PPO activity could be inhibited at different degrees. Among the inhibitors, NaHSO3, NaiS^Gs, L-Cys and Vc were strong inhibitors. 0.1% Na2S2OrcouIcf almost^ completely inhibited the PPO activity and the valid inhibitory concentration of Vc, L-Cys and NaHSO3 was 0.05%, 0.05% and 0.06% respectively.3. The degree of browning and PPO activity were in a general trend of increasingduring both room temperature storage and low-temperature storage. The activity of PPO during room tempetature storage was higher than that during low-temperature storage. And the activity of PPO was positively correlated with the degree of browning during room temperature storage(r=0.891). It showed that the increasing of PPO act...