Study On The Relationship Between The Changes Of Endogenous Enzymes And Quality Of Decapterus Maruadsi During Dry-salted Processing

Traditional dry-salted fish has been favored by consumers deeply,the major quality trait of this product is characterized by the unique flavor and resistant to storage.Traditional salted-fish are mainly produced by high salt content(20-30%).Based on health recommendations,consumer demand for less salty meat products,and particularly products with traditional unique flavors,but low salt contents,has gradually increased.Endogenous enzymes could cause a series of quality changes(mainly flavor and texture changes)during the process of salted fish.Lipolysis and lipid oxidation directly influence the final flavor of processed fish.Lipase(acid lipase,neutral lipase,phospholipase)is mainly used for the degradation of lipid.Lipoxygenase(LOX)is mainly used for the oxidation of lipid.The protein could be degraded to small peptides and free amino acids.The endogenous proteases that could degrade proteins into polypeptides and free amino acids are cathepsins(B,L,H),dipeptidases(DPP,DPP)and aminopeptidase(AAP,RAP,LAP).In this study,the relationship between endogenous enzyme changes and quality changes during the processing of pickled fish was studied.Therefore,changes in endogenous enzyme activity,the lipid oxidation parameters thiobarbituric acid reactive substances(TBARS)and peroxide value(POV),fatty acid composition,free amino acids,and volatile flavor substances were examined during dry-salted fish processing using the traditional method with high salt(HS)and the new method with low-salt and lactic acid bacteria(LAS).Furthermore,differences in the development of endogenous enzyme,lipolysis-oxidation and protein degradation in LAS and HS fish,the relationship between endogenous enzyme and quality,and the mathematical comprehensive quality evaluation model which associated with endogenous enzymes were also assessed.The research work and conclusions are as follows:(1)Lipolytic enzyme activity levels decreased substantially in the process compared to the initial activity levels in fresh fish.Neutral lipase(in HS and LAS)demonstrated the fastest decline rate throughout processing,followed by phospholipase,with acid lipase exhibiting the lowest activity.The activity of phospholipase,neutral lipase and acid lipase were 32.76%,17.48%,57.29%(in HS)and 37.07%?25.24%?67.76%(in LAS)compared with the levels in fresh fish.LOX activity(in HS and LAS)increased firstly and decreases at the last stage.Although LOX activity showed a decreasing trend,the activity in HS and LAS increased by 25% and 34.8%,respectively,in the final product compared with initial activity levels.Our study indicating that LAS is more suitable than HS with respect to lipase and LOX.(2)The trends in POV and TBARS during HS and LAS processing increased in the first three stages and decreased in the final stage.POV decreased rapidly at the last stage.POV and TBARS did not differ significantly between the finished products(in HS and LAS,p >0.05),indicating that oxidation degree in HS and LAS is similar?Compared with EPA,DHA exhibited the opposite trend during the entire HS and LAS processes.The final content of DHA and EPA in HS was lower than content in LAS.Thus,the LAS method improves the nutritional value of salted fish products.The decrease in DHA at the last two stages may reflect increased oxidation.The contents of major volatile compounds,which mainly generated in the drying period in products(HS and LAS)were aldehydes,alcohols,ketones and hydrocarbons.Compared with the product of HS,floral flavors were improved and fruity,almond-nutty flavors were increased.2.Comparison of endogenous protease and protein degradation and free amino acid production in low-salt lactic acid-fermented fish and highly salted fish(1)In general,Cathepsin B and L activity showed a increasing trend and the activity in the final product was significantly higher than that of fresh fish(p < 0.05 in HS and LS),and cathepsin B was more stable than cathepsin L.HS method has inhibitory effects on cathepsin H activity,however LAS method could stimulate cathepsin H activity HS.Cathepsin H activity did not differ significantly between the finished products(in HS and LAS).DPP?and ? activity showed a decreasing trend during the processing of HS and LAS,and the activity in the final product LAS was significantly higher than that of HS.The activity of aminopeptidase(AAP,RAP,LAP)could be determined at the whole stages of HS and LAS.The residual activity of aminopeptidase in HS was slightly higher than that of LAS.RAP 1,Comparison of endogenous lipase and lipolysis-oxidation in low-salt lactic acid-fermented fish and highly salted fish showed the highest stability in the processing of salted fish.The development of AAP and LAP activities during the processes were greatly influenced by environment.(2)The trends in NPN and P.I.during HS and LAS processing increased in the whole stages.The final value of P.I.in HS was higher than value in LAS(P>0.05).Thus,the LAS method could reduce the degree of protein degradation of cured fish products.The trends content of total free amino acid content(TFAA)and taste amino acid content(PFAA)during HS and LAS processing were same and increased significantly at the last stage.The final content of TFAA in LAS was lower than content in HS,but the content of PFAA(%)in LAS was higher than content in HS.Thus,the LAS method helps to enhance the taste substances of salted fish products.3.Revealed the relationship between the changes of endogenous enzymes and quality in Low-salt Lactic Acid-Fermented Fish and Highly Salted FishPrincipal component analysis indicated that lipolytic enzyme had accelerating effects on the hydrolysis of salted fish(in HS and LAS).Lipolytic enzyme activity(in HS and LAS)was closely related to the PUFA content,and MUFA,SFA contents were not closely related to lipolytic enzyme activity.LOX had a significant accelerating effect on the oxidation in LAS,but had little effect in HS.LOX had a positive correlation with the generation of volatile flavor compounds.The LOX activity of the LAS method at the end of the drying process was significantly higher than that of the HS method,which significantly enriched the volatile flavor compounds of the LAS method.The effect of endogenous lipase on flavor and nutritive value in LAS was larger than in HS.Cathepsin B and L had larger influence than cathepsin H on protein degradation and flavor substance formation in HS and LAS,but dipeptidyl peptidase and aminopeptidase have limited effect on protein degradation and flavor substance formation.The activity of cathepsin B in HS was greater than the activity in LAS,which resulted in the increase of the TFAA content in HS,but the PUFA ratio did not increase.confirming that compound lactic acid bacteria could accelerate the process of lipid oxidation.Thus,LAS method was more conducive to the preservation of nutritive value and the formation of taste substances due to the fermentation of compound lactic acid bacteria.4.Established the comprehensive evaluation model for the quality of salted fishThe raw data of all endogenous enzymes and flavor quality indexes during the whole processes were included in the factor analysis.Four principal components in HS and three principal components in LAS were extracted,explaining 100% and 96.11% of the total variance,respectively.The mathematical model of synthetical quality evaluation in the two method of salted fish was constructed by the contribution rate of each principal component as the proportion of the total contribution rate.Comprehensive evaluation model for HS method:Z=0.6284F1+0.256F2+0.0686F3+0.047F4=0.6284(-0.817X1-0.716X2+0.885X3+0.885X4-0.735X6-0.549X7+0.816X9+0.836X10+0.891X11+0.724X12+0.928X13-0.889X14-0.834X15-0.999X16-0.974X17+0.706X18+0.940X19+0.526X20+0.896X21+0.811X22)+0.256(0.688X2+0.883X5+0.573X6+0.825X7+0.914X8+0.549X9+ 0.513X10+0.650X12-0.618X18)+0.0686(0.758X20)+0.047(-0.545X22)Comprehensive evaluation model for LAS method: Z=0.6019F1+0.2802F2+0.079F3=0.6019(-0.71X1+0.902X3+0.834X4-0.786X6-0.607X7-0.538X8+0.768X9+0.807X10+0.893X11+0.677X12+0.945X13-0.855X14-0.603X15-0.978X16-0.889X17+0.986X18+0.918X19+0.626X20+0.823X21+0.699X22+0.2802(0.871X2-0.549X4+0.851X5+0.587X6+0.787X7+0.821X8+0.631X9+0.577X10+0.714X12-0.748X15)+0.079(0.743 X20-0.7X22)...