The Contribution Of Microorganisms On Lipid Changes And Flavor Formation In Fermented Fish

As a traditional process and preservation technology,biological fermentation not only increases the utilization rate,improves the texture and colour and lustre,prolongs the storage period,also can produce unique ester aroma and dispel the earthy aroma,which has become a hot research topic in the field of freshwater fish processing in recent years.Suan yu is a kind of Chinese traditional fermented fish,and has been studied for several years in our group.However,the contribution of microorganisms on the lipid changes and flavor formation during the fermentation of Suan yu is still unclear.In this paper,we studied the relative roles of microorganisms and endogenous enzymes in lipid degradation and flavor formation during fermentation,and three dominant strains(Lactobacillus plantarum 120,Saccharomyces cerevisiae 31 and Staphylococcus xylosus 135,isolated from Suan yu)were purely cultured to study their contributions on the lipid changes and flavor development during fermentation.This study has important academic significance to help understand the mechanism of quality change during the fermentation of fermented fish,and also provides theoretical guidance for application of fermentation in the freshwater fish processing industry.To investigate the contribution of autochthonous microflora on free fatty acids(FFA)release and flavor development in low-salt fermented fish,three treated groups,including bacteriostatic-acidification group(BAG),bacteriostatic group(BG)and spontaneous fermented fish(CG)were established.Results showed that addition of NaN3 reduced microbial load in BAG and BG below 3.5 log CFU/g after 3 weeks of incubation.Activities of lipases and lipoxygenase declined markedly with increasing time,where BG had the highest activities,followed by CG and BAG.There was a 36.3% higher in the total FFA content in CG than that in BAG,indicating both microbial and endogenous lipases contributed to the FFA liberation in fermented fish while endogenous lipases played a major role.However,compared to BAG and BG,largely higher levels of lipid oxidation and volatile compounds were observed in CG,suggesting that autochthonous microflora dominated the generation of volatile flavor compounds and lipid oxidation in fermented fish.To better understand the role of three dominant strains in lipid changes and flavor development during the processing of fermented fish,sterile fish paste with or without the antioxidant tert-butylhydroquinone(TBHQ)was used to investigate the roles of three strains isolated from traditional fermented fish in free fatty acid(FFA)liberation,lipid oxidation and volatile compounds.The results showed that the addition of 0.03% TBHQ had no significant influence on the strain growth but could effectively inhibit fatty acid oxidation.Staphylococcus xylosus 135(Sx 135)demonstrated the highest liberation of FFAs,followed by Saccharomyces cerevisiae 31(Sc 31),with its released FFAs being almost all polyunsaturated fatty acids(PUFA).Furthermore,the highest amount of PUFA oxidation in FFAs was observed in the samples inoculated with Sx 135(802 mg/kg DM),followed by Lactobacillus plantarum 120(Lp 120,476 mg/kg DM)and Sc 31(388 mg/kg DM).The contents of aldehydes,ketones,alcohols and furans in the fish paste with the addition of TBHQ were significantly lower than those in the fish paste with the absence of TBHQ,and the contents of volatile compounds in the fish paste inoculated with Sx 135 were also the highest.The results suggested that Staphylococcus xylosus may play a major role in the lipolysis,lipid oxidation and flavor development during the processing of fermented fish.Lactobacillus plantarum also contributed to the fatty acid oxidation and flavor formation although it had no obvious lipolysis ability.To better understand the interactions among three strains on the microbial growth,lipid degradation and flavor formation,the acidified fish paste was established to co-culture three dominant strains.Results showed that in the pure culture groups,the FFA content of Sc 31 group was significantly higher than that of the Sx 135 group,indicating that the low pH could inhibit the hydrolysis activity of Sx 135,while the Sx 135 group still had the highest oxidation amount of FFA and concentration of volatile compounds.When co-cultured,Sx 135 enhanced the growth of Lp 120 and Sc 31,while its own growth was inhibited,and the inhibitory effect of Sc 31 was more significant.In the co-culture group of Lp 120 and Sc 31,the number of Sc 31 was higher than that of the pure culture,while the FFA content of the former was significantly lower,indicating that Lp 120 inhibited the lipid hydrolysis activity of Sc 31.The co-culture grouop of Lp 120 and Sc 31 had the lowest PV,TBARS values,FFA content and flavor concentration.The co-culture of strains inhibited the abilities of strains to hydrolyse fish lipids,and the inhibitory effect on flavor formation was more remarkable.