Studies Of Cathepsin L On The Mechanism Of Surimi Gel Degradation

In this study, carp(Cyprinus carpio) was used as a research material. To improve the gel strength of surimi products, cathepsin L, one of the endogenous enzymes, which could cause surimi gel degradation (modori phenomenon) was studied. After a series of treatments using biochemical experimental methods, cathepsin L was purified and its characteristics were also analyzed to clarify the mechanism of modori phenomenon. In addition, y-polyglutamic acid as an effective surimi gel enhancer was selected. It could both improve surimi products' flexibility and increase their commodity value. Therefore, the usage of y-polyglutamic acid in carp surimi processing was expected to bring along the rapid development of freshwater fish processing industry.In this study, the following works were endeavoured.(1) Purification and characterization of cathepsin L from the dorsal muscles of carp (Cyprinus carpio).Purified cathepsin L was obtained by 18.27 fold with a recovery rate of 1.70% according to the calculation. SDS-PAGE gel electrophoretic pattern showed that ionic exchange fraction V was a single component and its molecular weight was measured to be 36 kDa. This single component was found to have strong hydrolysis ability of Cat L specific substrate of Z-Phe-Arg-MCA, while almost no hydrolysis of Cat B specific substrate of Z-Arg-Arg-MCA and Cat H specific substrate of L-Arg-MCA. These results fully testified that the fraction obtained was purified Cat L and it belonged to the family of cysteine proteinases containing thiol. Its optimal temperature and pH were 50℃and 5.5, respectively.(2) Research on the mechanism of surimi gel degradation caused by cathepsin L.Surimi gel samples added with Cat L and its inhibitors (E-64 and leupeptin) had no significant difference in color parameters (P>0.05). The gel strength of surimi samples added with Cat L decreased by 24.33% compared with the control group, but those added with both Cat L and inhibitors increased 13.70% and 21.60%, respectively. Cat L is proved to be one of the enzymes contributed to the gel degradation. According to the SDS-PAGE electrophoresis and scanning electron microscopy patterns, Cat L did participate in the phenomenon of gel degradation, decomposing surimi protein, destroying protein network, making the hydrophobic grouping exposed, causing water loss and ultimately reducing the quality of surimi gel. However, the inhibitors (E-64 and leupeptin) added into the surimi inhibited the activity of cathepsin L, reducing the hydrolysis degree of surimi protein and thereby increasing the surimi gel strength.(3) Effect of y-polyglutamic acid on the surimi gel properties.As a newly edible food additive, y-Polyglutamic acid (y-PGA) could increase the surimi gel strength significantly (P<0.05) with a rarely adding amount, and slightly effect its whiteness. Compared to the one-stage heated surimi, two-stage heating method could significantly increase the surimi gel strength (P＜0.05). The results of response surface optimization showed that the surimi had the highest (281.66 g-cm) gel strength with the addition of 0.54%o y-PGA when preheating at 52.6℃for 39 min using the two-stage heating method. Among the three influencing factors, the adding amount of y-PGA had the highest effect on the surimi gel strength, followed by the preheating temperature and incubating time. Surimi gel prepared under the optimum conditions compared with the control group, had much tighter internal structure, smoother and uniform surface without cavities, indicating that y-polyglutamic acid could improve surimi gel strength by effecting the inner structure of surimi gel network.