Study On Bacterial Diversity Of Acidic Gruel And Its Flavor Quality Formation Mechanism

In this paper,the bacterial community structure,physicochemical and nutritional qualities as well as their association in 98 home-made acidic gruel samples from Guangxi,Shanxi and Inner Mongolia regions were investigated,while the dominant bacterial genera were isolated,identified and deposited using traditional microbiology combined with molecular techniques.Moreover,comparative genomics with multilocus sequence typing of the isolated Lactobacillus reuteri were carried out to explore strain adaptation to the environment.To preliminarily explore the mechanism of flavor formation during acid gruel fermentation,this study further used non-targeted metabolomics to monitor the changes of organic compounds during fermentation on the basis of screening out the isolated edible strains for their growth characteristics and fermentation characteristics.The main research contents and results are as follows:(1)High throughput sequencing revealed that the bacteria in the acid gruel mainly belonged to the genera Lactobacillus,Acetobacter,Bacillus,Clostridium and Weissella.The bacterial community composition and sensory qualities of Shanxi and Inner Mongolia samples were similar,but significantly different(p<0.05)from Guangxi samples.PICRUSt analysis showed that the gene functions in acid gruel were mainly related to carbohydrate and amino acid metabolism,and the dominant genera were significantly correlated with flavor and odor indexes(p<0.05).Acid gruel is rich in amino acids,organic acids and soluble solids,and the content in Guangxi samples was significantly higher than that in Shanxi and Inner Mongolia samples,and the p H range of acid gruel in different regions was 3.00-4.00.(2)Lactic acid bacteria belonging to 24 species in 15 genera were isolated from 98 acid gruel by pure culture technique,resulting in 262 isolates,of which Limosilactobacillus fermentum and Lacticaseibacillus paracasei were the dominant lactic acid bacteria.The sequences of Limosilactobacillus reuteri MH1 showed the highest similarity to only 98.37% to the existing strains of Limosilactobacillus reuteri,with an assembled genome size of 1.99 Mbp,1,927 protein coding sequences,and 38.7 mol% G+ C content of genomic DNA,which also contained 13 GIs,two prophage sequences,one CRISPR as well as one T3 PKS.MH1 differed from the Limosilactobacillus reuteri model strain in morphology,size and carbohydrate decomposition.Comparative genomic and MLST analyses of 211 Limosilactobacillus reuteri isolates of various origins revealed that the housekeeping genes ddl,leu S,pep X and rpl B of Limosilactobacillus reuteri MH1 were subject to purifying selection during evolution,and the influence of isolat e source on the relatedness between different strains of Limosilactobacillus reuteri was greater than that of isolate area.(3)According to the list of "available strains for food" published by the Ministry of health,177 strains from 262 lactic acid bacteria were selected for acid gruel fermentation.The quality of the acid gruel prepared was evaluated using a combination of E-tongue,E-nose and sensory evaluation,considering comprehensively the growth characteristics such as breakdown starch,exopolysaccharide production as well as acid production by strains,Lacticaseibacillus paracasei G5-2 is the most suitable for acid gruel fermentation.(4)The dynamic quality changes of acid gruel fermented by Lacticaseibacillus paracasei G5-2 were evaluated by gas chromatography ion mobility spectrometry and liquid chromatography mass spectrometry.A total of 51 volatile compounds were detected in acid gruel samples at different fermentation times,among which the relative contents of aldehydes were much higher than those of other organic compounds and fluctuated more throughout the fermentation process.LC-MS detected 809 qualitatively quantifiable organics,with 157 pathways annotated to KEGG,of which 13 pathways were significantly enriched,containing 40 key differential metabolites,with L-glutamate,L-aspartate,L-arginine,and L-tyrosine participating in the largest number of key metabolic pathways.The acid gruel fermentation process mainly forms its special flavor through the metabolism of amino acids and their derivatives,starch and sugar metabolism and so on.