Lactic Acid Metabolism In The Microbial Community Of Zhenjiang Aromatic Vinegar

Lactic acid,one of the predominant non-volatile organic acids in traditional fermented vinegar,plays an important role in prolonging the storage,coordinating the taste of vinegar.However,the microbial mechanism of lactic acid formation in acetic acid fermentation stage is still lacking in-depth study.The difference in the contents of vinegars belong to different types and D/L-lactic acid content in Zhenjiang aromatic vinegar during acetic acid fermentation were analyzed.The structure and dynamic succession of microbiota which participate in lactic acid metabolism were revealed.Lactate metabolism capacity of functional bacteria strains were verified and D/L-lactic acid content was regulated in acetic acid fermentation in situ and in vitro fermentation,respectively.The main conclusions of this study are as follows.The study aimed to reveal the configuration and determine contents of D/L-lactate in vinegar and vinegar Pei according to the D/L-lactate detection kit.The results showed that the contents of D/L-lactic acid increased first and then decreased during the acetic acid fermentation stage,and the content ratio is close to 1:1.However,the content of L-lactic acid was slightly lower than D-lactic acid,which might be due to the uneven proportion of D/L-lactic acid in raw materials.Based on the database of metagenomics sequencing of vinegar Pei,and five key enzymes were identified.The synthesis of D/L-lactic acid was mainly regulated by D-LDH and L-LDH,while DLD,LLDH and LOD were mainly responsible for the degradation of D/L-lactic acid.The results showed that the microbiota participate in DLD and LLDH synthesis were mainly distributed in Acetobacter,in which A.pasteurianus occupied an absolute dominant position.Rhizobium and Agrobacterium are the main contributors to LOD synthesis,but the pathway was relatively weak due to the low content of these two genera in vinegar Pei.Many species scattered in the genus Lactobacillus were the main lactic acid producers,relative abundance was above 95.0%.L.reuteri,L.pontis,L.fermentum and L.helveticus possessing D-LDH gene might be the major D-lactic acid producing microorganisms,while L.helveticus,L.fermentum,L.pontis,L.casei and L.acetotolerans having L-LDH gene might mainly contribute to the production of L-lactic acid.Based on sequencing analysis results,8 functional bacteria strains including 7 Lactobacillus strains and 1 A.pasteurianus strain from vinegar Pei were slected to verify the lactate metabolism capacity.L.casei 21M3-1,L.plantarum F,L.helveticus 3-2 and L.fermentum 10M1-5 had strong and fast acid-producing ability,lactic acid production reached 18.81±0.52 g×L^-1,18.75±1.20 g×L^-1,14.43±0.51 g×L^-1,9.86±0.81 g×L^-1 at the end of fermentation.L.casei 21M3-1 and L.pontis 68M17-2 produced high optical purity L-lactic acid which the proportion reached 98.0% and 84.9%,while other Lactobacillus strains produced D/L-lactic acid with similar proportion.A.pasteurianus G3-2 could utilize lactic acid to produce other secondary metabolites,and its ability to utilize D/L-lactic acid was similar.Therefore,L.casei 21M3-1?L.plantarum F?A.pasteurianus G3-2 was respectively added into the Pei,the physicochemical indexes were detected and changes in bacterial community structure during fermentation were analyzed to evaluate the effect of functional microbe augmentation in vinegar Pei.The results showed that A.pasteurianus could degrade D/L-lactic acid.However,due to the high relatively abundance of A.pasteurianus in the system,the bioaugmentation effect was not significant.Bioaugmentation of Lactobacillus could increase the content of lactate in vinegar Pei,and the change of D/L-lactic acid content was regulated by the D/L-lactate-producing ability of the functional strain.