Study On Quality Improvement Technology And Frozen Storage Stability Of Low-salt Fermented Bream

Fermented fish product is a traditional aquatic food with typical local characteristics,which is deeply loved by consumers because of its unique flavor.However,in order to ensure food safety,traditional fermented fish products usually have the characteristics of high salt or high acid.On the one hand,high salt intake may lead to cardiovascular diseases.With the upgrading of nutritional and healthy consumption demand,salted and fermented foods gradually develop to low salt;On the other hand,under the condition of high acid,the protein will be denatured and the water-holding capacity will decrease,which will lead to the change of the taste of the product during the ripening process,and the over-acid product will also affect the acceptance of consumers.How to ensure the safety and flavor quality of fermented fish products under the condition of low salt and low acid is a challenge.Traditional fermented fish products are complex fermentation systems with multiple flora,involving a variety of fermentation microorganisms.Changing environmental factors to influence microbial fermentation process is an important way to regulate the quality of fermented food.In this paper,taking bream as the research object,aiming at the improvement of the edible quality of traditional salted bream,the fermentation conditions were optimized,and a stage temperaturecontrolled fermentation process was developed.On this basis,the optimal microbial strains were inoculated,and the effects of microbial inoculation and fermentation on the microbial flora structure,physical and chemical characteristics and quality improvement of bream during low-salt fermentation were studied,and the quality stability of bream during frozen storage was further explored.This study has important practical significance for the quality improvement and industrial production of traditional fermented fish products in China.Firstly,the effect of fermentation temperature on the quality of low-salt fermented bream was studied.The bream was fermented at 4 ℃,10 ℃,20 ℃,20 ℃-10 ℃ and 10 ℃-20 ℃.The physicochemical characteristics,flavor and safety of bream were analyzed,and the effects of different temperature conditions on the quality of fermented bream were evaluated.Results showed that after 4 days of fermentation,the moisture content of samples in each fermentation group decreased by 3.34%～18.99%,and the p H was between 6.34～6.62.Putrescine,cadaverine and histamine were the main biogenic amines,accounting for 31.31%～81.91% of the total biogenic amines.Compared with fermentation at 4 ℃,higher fermentation temperature promoted protein degradation and microbial growth,but compared with fermentation at 20 ℃,low temperature(10 ℃)and stage temperature-controlled fermentation(20 ℃-10 ℃ and 10 ℃-20 ℃)effectively inhibited the growth of Pseudomonas and coliform bacteria and the accumulation of volatile base nitrogen(TVB-N)and biogenic amines.Among them,the total amount of TVB-N and biogenic amine in the fermentation group was reduced by 31.46% and39.39% respectively by controlling the temperature(20 ℃-10 ℃)at 20 ℃ for 2 days and then at 10 ℃ for 2 days.After 4 days of fermentation,a total of 138 volatile compounds were detected in 7 groups of fermented fish.Alcohols were the main flavor compounds in fermented fish,and the contents of alcohols,esters and ketones were the highest in 20 ℃-10 ℃ fermented samples,3949.23±39.44 μg/kg,312.46±24.18 μg/kg and 695.69±72.89 μg/kg,respectively.Results showed that the process of stage temperature-controlled fermentation(20 ℃-10 ℃)could effectively inhibit the accumulation of biogenic amines and the growth of spoilage bacteria,promote the formation of volatile flavor compounds,and improve the quality of lowsalt fermented bream.Secondly,the effect of inoculation fermentation on the quality of fermented bream.was studied.Based on the analysis of microbial growth and esterase activity,two microbial strains(Lactobacillus pentosus Lp-1 and Saccharomyces cerevisiae Sc-2018)suitable for low salt and low temperature fermentation were selected.On this basis,the effects of the selected strains on the physicochemical,biochemical,microbial and flavor characteristics of fermented bream.were studied.Results showed that compared with the natural fermentation,the inoculation of Lp-1 and Sc-2018 promoted the growth of lactic acid bacteria and yeast respectively,and decreased the p H value.Putrescine,cadaverine and histamine decreased by 22.78%～48.03%.After 4 days of fermentation,the TVB-N and biogenic amines of the Lp-1 inoculation fermentation group decreased by 36.82% and 34.78% respectively,and the inhibition effect of Sc-2018 inoculated fermentation was the second.During the fermentation process,the acid protease activity of Lp-1 inoculation group was higher,while the neutral protease activity of Sc-2018 inoculation group was higher,which promoted protein degradation.The contents of TCA-soluble peptide,amino acid nitrogen and total free amino acid in inoculated fermentation group were higher than those in natural fermentation group.A total of 95 volatile compounds were identified in the three groups of samples,among which the content of alcohols was the highest,accounting for 40.23%～53.78% of the total volatile compounds.Inoculated fermentation promoted the formation of flavor compounds such as ethanol,ethyl acetate and 3-hydroxy-2-butanone.The analysis of microbial diversity showed that the microbial diversity and the abundance of harmful microorganisms in the inoculated fermentation group were lower than those in the natural fermentation group,and it had obvious inhibition effect on spoilage microorganisms,especially Lp-1 inoculated fermentation.Macrococcus,Acinetobacter,Shewanella and Glutamicibacter were positively correlated with p H value,TVB-N and biogenic amine content,which may be important microorganisms affecting product quality and safety.Finally,the quality stability of low-salt fermented bream during frozen storage was studied.Moisture content,water retention,lipid oxidation and TVB-N were used as indexes to evaluate the quality changes of low-salt fermented bream during frozen storage.Results showed that after frozen storage for 3 months,compared with the unfermented group,the cooking loss of the natural fermentation group and Sc-2018 inoculation fermentation group decreased by 70.07%and 71.55%,respectively.Due to the lower p H value,the water holding capacity of the sample of Lp-1 inoculated fermentation was poor.With the extension of frozen storage time,the drip loss,TVB-N and TBARS content of each group of samples increased slowly,and there was no significant difference compared with that before frozen storage,indicating that low-salt fermented bream had good stability during frozen storage.