Screening Of Lactic Acid Bacteria With Antimicrobial Activity And The Application In Malt Manufacturing

The Lactic acid bacteria A14-2 isolated from sauerkraut has a strong inhibitory activity against Aspergillus,Fusarium and bacteria and doesn't produce biogenic amines.Taking F.graminearum as indicator fungal to analyze antimicrobial activity substances in supernatant of LAB A14-2,We found that the effective antimicrobial active substances were mainly organic acid and also included protein substances.The analysis of organic acids in supernatants of LAB A14-2 by HPLC showed that the content of lactic acid,acetic acid,citric acid and phenyl lactic acid was 13.51 g·L^-1,4.19 g·L^-1,3.3 g·L^-1,and 0.09 g·L^-1,respectively.Through the inhibition test of single organic acid,it was known that acetic acid had the strongest antibacterial activity.Physiological and biochemical characteristics and molecular identification of LAB A14-2 showed that it was Lactobacillus plantarum.To achieve higher density culture of L.plantarum A14-2,it was necessary to optimize the wort culture medium and L.plantarum A14-2 culture conditions.The optimization conditions were as follows:at the beginning of saccharification,we added neutral protease with 1800U·g^-1 to malt,diluted the Sugar concentration to 8°P at the end of saccharification,added 1.25%diammonium hydrogen citrate,and incubated L.plantarum A14-2 with 8%inoculum size to the wort culture medium at 30°C for 20 h.We measured the OD value was 5.92 and the cell density reached 3.1×10⁹ cfu·mL^-1.The organic acids produced by the metabolism of L.plantarum A14-2 in wort medium mainly include lactic acid and citric acid.Through inhibiting the growth of Fusarium and DON biosynthesis experiments showed that the lactic acid had the strongest antifungal activity,but the mixed acid had a stronger antifungal effect than singal organic acid.Comparing the result between culturing F.graminearum with L.plantarum A14-2 and lactic acid,L.plantarum A14-2 bacteria had a stronger inhibitory effect on the F.graminearum growth and DON biosynthesis than the supernatant and it had no concern with cell adsorption.Electron microscopic observations revealed that the lactic acid bacteria supernatant had an adverse effect on the growth of Fusarium spores and mycelia.We studied the effect of extra L.plantarum A14-2 on DON biosynthesis during malting,and the results showed that the addition of supernatant of L.plantarum A14-2 was more effective at reducing DON biosynthesis than the addition of the same amount of L.plantarum A14-2 liquid at the beginning of steeping.The addition of F.graminearum resulted in a dramatic increase in DON content at the start of steeping.When the added amount of F.graminearum reached 3×10⁶ spores/200 g barley,the DON content exceeded the national detection limit standard.Based on the above method,adding 10 mL supernatant of L.plantarum A14-2 reduced DON content by 44.6%.Through the determination of malt indicators,it could be known that the addition of F.graminearum increased the turbidity and filtering difficulty of the wort.However,the addition of the supernatant of L.plantarum A14-2was beneficial to the filtration of the wort and had little effect on other malt indicators.