The Studies Of Fresh-Keeping Effect And Mechanism Of Tea Polyphenols On Grass Carp

Fresh water fish is a kind of ideal food for people because it is rich in nutrients and good for health. In 2006, the product of fresh water in China was up to 23,500,000 tons and take the first place in the world. However, the technologies of fresh-keep and processing for fresh water fish in China didn't meet the development of acquaculture. The processing rate of fresh water fish was only 6.9% and far behind the worldwide average level of marine products processing (>40%). Tea polyphenols (TP), which is the generic terms of compounds with multiple hydroxyl groups in tea, has many functions of heath care and pharmacological effects, such as antioxidation, bacteriostasis, cancer-preventing, and cholesterol-lowering. In this project, the effects of TP on lipids, proteins, and microbe of fish meat from grass carp were studied to determine the possibility of TP applied in fresh-keeping of fresh water fish. The methodologies and results are described as following:1. The changes of biochemical properties were studied by extractability and ATPase activity measurement, SDS-PAGE analysis, and ultrastructure observation of fish meat from grass carp treated with different concentration TP and stored at 4℃. The results revealed: the extractability and ATPase activity of protein decreased gradually with the time when the fish meat stored at 4℃, and these declines were retarded substantially by addition of TP. When the TP concentration was lower than 0.03% (w/w), the difference was insignificant (P>0.05), whereas the concentration was higher than 0.05%, the difference showed significant (P<0.05). The SDS-PAGE analysis showed the rate of protein degradation of fish meat was decelerated by applied of TP. The changes of ultrasturcture of fish surimi by scanning electro microscope checking confirmed the above observations. These results suggested that there were remarkable protecting effects of TP on biochemical properties of fish meat and the optimal concentration was about 0.05～0.07%.2. The emulsion, gel property, and rheology of fish meat treated with different concentration of TP and stored at 4℃were studied using spectrophotometer, texture analyzer, and rheometer. The results indicated that the addition of more than 0.03% of TP could slow down the decrement of emulsion and emulsification stability of protein extracted from fish meat significantly and presented the does effects. In the meantime, 0.03% of TP increased the gel strength substantially and no does effect appeared. In rehological study, addition of TP reduced the storage and loss modulus (G' and G") compared to the negative control, and the G' and G" valves were the lowest when the TP was 0.03% suggesting that the extent of crosslink of proteins was the lowest.3. The fatty acid composition of grass carp was analyzed using GS-MS and more than 18 fatty acids were identified, including hexadecanoic acid, 9-hexadecenoic acid, 9-octadecenoic acid, 6,9-octadecadienoic acid, 9,12,15-octadecatrienoic acid, octadecanoic acid, 5,8,11,14-eicosatetraenoic acid, 4,7,10,13,16,19-docosahexaenoic acid, 5,8,11,14,17-eicosapentaenoic acid, pentadecanoic acid, heptadecanoic acid, and 5,8,11-heptadecatrienoic acid. There were some fatty acids with odd carbon atoms besides the fatty acids with even carbon atoms common in animals and plants. Among the fatty acids, the saturated were taken 27.84%, the monounsaturated occupied 43.67%, and the polyunsaturated accounted for 28.02%.4. The variations of fatty acids of fish meat treated with TP and stored at 4℃were assayed by GS-MS and the changes of malonaldehyde, the oxidative product of fatty acids, were measured by TBA experiment. The data indicated that there was no change in contents of fatty acids by 5 days after treatment, including tetradecanoate, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, 9-hexadecenoic acid, 9-octadecenoic acid, 6,9-octadecadienoic acid, 9,12,15-octadecatrienoic acid, and 5,8,11-heptadecatrienoic acid. There were obvious decreases in content of these fatty acids from fish meat without TP 10 days after treatment compared to the experimental groups (P<0.05) . TBA analysis showed 0.01% of TP had evident antioxidation on lipids from fish meat, and TBA increased with the TP concentration accretion presenting an unlinear relation. The effects of antioxidation of 0.03% and 0.07% of TP were close, thus, the best choice is 0.03% in practical application.5. 93 bacteria strains were isolated and categorized into several spoilage bacteria classes: Aeromonas, Vibrio, Pseudomonas, Enterobacteriaceae, Flavobacterium, Acinetobacter, Photobacteriaceae, Brevibacterium, and Micrococcus. In the early stage of storage (1～3 days), Gram-positive bacteria were the major strains and accounted for 70%, most of them belonged to Brevibacterium and Micrococcus. Whereas the diversity was increased and Gram-negative bacteria became the dominant taking 71～82% after 3 days, the main strains such as Pseudomonas, Aeromonas, Vibrio and Enterobacteriaceae classed into spoilage bacteria attributing to the meat putrefaction.6. The inhibition of TP on several spoilage bacteria from fish meat were determined by plate counting. TP presented potent inhibitory effect on Brevibacterium, Micrococcus, Aeromonas, Pseudomonas, Vibrio, and Enterobacteriaceae, and the minimum inhibition concentration (MIC) was lower than 1.2 g/L, whereas for most strains the optimal inhibition concentration was about 5 g/L.7. The fresh-keeping effects of potassium sorbate, vitamine C, and TP were determined and showed good function compared to the negative control. Among three antioxidations, TP presented the best effect because the acid value, and peroxide value were the lowest, the total volatile basic nitrogen (TVBN) and total colonies number measured 11 days and 9 days after TP treatment respectively were lower than the first grade fish according to the GB.