Effects Of Different Strains On Lipid Changes During Processing Of Suan Yu

Suan yu is a traditional low-salt fermented freshwater fish product in the southwestern region of China,and has the characteristics of aromatic flavor and long storage period.Microbial fermentation technology can remove the odor of fish,soften the fishbone,improve product safety,thus effectively solved the problems of freshwater fish processing,and has great potential for development.At present,researches on fermented fish products by domestic and foreign scholars mainly focus on microbial population analysis,strain screening,processing technology,flavor and bio-safety.Lipid content and composition are important indicators for evaluating the quality of foods,but the changes in lipids during fish fermentation are still unclear.Lactobacillus plantarum 120,Staphylococcus xylosus 135 and Saccharomyces cerevisiae 31 are the dominant strains in traditional Suan yu.In this paper,common carp was separately inoculated with single strains and mixed strains(three strains were mixed in equal ratios),the non-inoculated batch was the control.Five batches of Suan yu samples were obtained,including CO batch(control),LP batch(inoculated with Lactobacillus plantarum 120),SX batch(inoculated with Staphylococcus xylosus 135),SC batch(inoculated with Saccharomyces cerevisiae 31)and LSS batch(inoculated with mixed strains).Focusing on the impact of inoculation of different strains and processing on lipid composition and fatty acid distribution,lipid degradation and lipid oxidation in fish meat,to provide the theoretical basis for the nutrition and quality control of fermented fish product.Firstly,the lipids of Suan yu samples at different processing stages were extracted and subjected to lipid fractionation and fatty acid analysis.By measuring the changes in lipid composition and fatty acid distribution of Suan yu during processing,the effects of different fermentation methods and processing processes on the lipid profile of Suan yu were explored.The results showed that the neutral lipids of Suan yu accounted for more than 84% of the total lipids,whose fatty acid composition was similar to that of total lipids,and the MUFA content was the most abundant.The proportion of PUFA especially DHA(C22:6)in phospholipids was significantly higher.The MUFA and PUFA ratios of free fatty acids increased significantly after fermentation.Compared with fresh fish,the increase of free fatty acid content and the decrease of phospholipid content were observed in five batches of Suan yu at the end of fermentation.Among them,the SX batch had the highest free fatty acid content(9.64 g/100 g total lipids),CO batch had the lowest content(6.20 g/100 g total lipids),the phospholipid loss in SC batch was the lowest and the content was 8.68 g/100 g total lipids;In addition,the UFA/SFA ratio of free fatty acids in CO batch(1.12)was significantly lower than that in other four batches(about 1.5),and the UFA/SFA ratio of phospholipids in SC batch(1.31)was significantly higher than that in other four groups(0.36~0.71),indicating that the three strains especially S.xylosus 135 could promote lipid degradation,and S.cerevisiae 31 could inhibit phospholipids oxidation.The contents of PUFAs which have important physiological functions in total lipids of Suan yu slightly decreased after drying.During the fermentation,the contents of these PUFAs in CO batch showed a decreasing trend and was significantly lower than that in starter culture batches;For the starter culture batches,the contents of linoleic acid(C18:2n6)and ?-linolenic acid(C18:3n3)in the SX batch continuously increased to the highest,and the contents were 20.3% and 3.00% respectively after fermentation;In the pre-fermentation(fermentation for 2 weeks),the contents of eicosatetraenoic acid(C20:4n6),eicosapentaenoic acid(C20:5n3)and docosahexaenoic acid(C22:6n3)were the highest in SC batch,then the contents of these PUFAs in SX and LSS batches began to be higher than that in other two batches,LSS batch had the highest contents(1.10%,1.54% and 4.85%)after fermentation;The content of docosapentaenoic acid(C22:5n3)in LSS batch increased the most,and the content was 0.95% after fermentation,which was even higher than that in fresh fish.The UFA/SFA ratio of total lipids in LSS batch was the highest at the end of fermentation,which was 2.99.Therefore,the synergistic effect of the three strains,especially S.xylosus 135 and S.cerevisiae 31,significantly increased the nutritional value of Suan yu.Secondly,the activities of acid lipase,neutral lipase and phospholipase were used as indicators,combined with the analysis of physicochemical and microbial colonies,the changes of lipase activity and the influencing factors during the processing of five batches of Suan yu were studied.The results showed that the activities of acid lipase,neutral lipase and phospholipase all decreased rapidly during fish drying and fermentation.However,the inoculation of S.xylosus 135 could slow down the rate of decline of lipase activities during fermentation.The correlation analysis showed that the activities of the three lipases had significant positive correlation with water content,water activity and pH,and had significant negative correlation with salt content and total acidity.Finally,the acid value,peroxide value,TBA value,lipoxygenase activity and volatile flavor composition were used as indicators,the effects of fermentation methods and processing on lipid oxidation of Suan yu were explored.The results showed that the acid value,peroxide value and TBA value of Suan yu showed an upward trend,the acid value of SX batch was the highest,the peroxide value and TBA value of CO batch were the highest.Lipoxygenase activity showed a downward trend and was positively correlated with environmental pH.N-hexanal,ndecanal,2,4-heptadienal and 2,4-decadienal,which are mainly from the oxidation of unsaturated fatty acids,were found to have the highest levels in the CO batch at the end of fermentation,and had low levels in the SC and LSS batches.The acid content of SX batch was significantly higher than that of other four batches.The results showed that the inoculation of fermentation strains had different effects on lipid oxidation during fish fermentation.The three strains,especially S.cerevisiae 31,showed strong antioxidant capacity against unsaturated fatty acids.