Metabolic Regulation Of Phenyllactic Acid By Lactic Acid Bacteria In Fermented Vegetable

Phenyllactic acid(PLA)is a novel antimicrobial compound produced by lactic acid bacteria(LAB).Phenyllactic acid has a broad antimicrobial spectrum against both gram-positive and gram-negative bacteria.Moreover,PLA has an inhibitory activity against a wide range of fungi.As we all know,lactic acid fermentation is the core reaction of fermented vegetable.LAB is the most improtant microorganism of fermented vegetable.Herein,the concentration of PLA in fermented vegetable was determined,and the metabolic of PLA by LAB in fermented vegetable was regulated.This work would have a important significance to extend the shelf life of fermented vegetable.27 kinds of fermented vegetables were detected using HPLC.The results showed 19 kinds of samples contained phenyllactic acid,which led to the detection rate reaching to 70%.The yield of PLA was in the range of 0-50mg/kg.The concentrations of PLA were more than 30 mg/kg for 2 kinds of samples,8-30 mg/kg for 10 kinds and 0-7 mg/kg for 15 kinds.The production of PLA by L.plantarum CNQ7 in fermented coewpea increased slightly at the beginning 12 h,then it significant increased from 12 to 48 h,and finally reached up to peak amount of 0.168 mM in cowpea at 72 h of fermentation.After that,no increase of PLA in coewpea was observed at the time period tested.The metabolic of PLA by L.plantarum CNQ7 in fermented vegetable was regulated through the phenylalanine(Phe)pathway.Phenyllactic acid were increased when biosynthetic precursors(Phe),keto acid acceptor(?-ketoglutaric acid)were present.The concentration of PLA reached to 1.173 mM by adding 15 mM phenylalanine and ?-ketoglutaric acid(?-KG)(the ratio was 1:1),which was 7.88-fold compared to that without regulation.We demonstrated L.plantarum CNQ7 was a GDH-negative bacteria,because it couldn't promote the synthesis of phenyllactic acid when glutamic acid(Glu)instead of ?-ketoglutaric acid were used as a substrate.The presence of citrate(Cit)had a positive effect on the PLA synthesis in L.plantarum CNQ7.Probably,citrate stimulated the production of ?-ketoglutaric acid through the way of citrate-isocitrate.The analysis of lactic acid and acetic acid were performed by Waters HPLC.The increase in the production of lactic acid was greater for the first 48 h,a slight increase being observed from then on.The concentration of lactic acid reached 134 mM by L.plantarum CNQ7 after 168 h fermentation.By contrast with lactic acid,the content of acetic acid was only 0.043 mM(2.606 ?g/mL).Supplied with various compounds such as phenylalanine,?-ketoglutaric acid,glutamic acid and citrate had no perceptible effect on the production of lactic acid and acetic acid.