Screening Of Antifungal Lactic Acid Bacteria And Study Of Its Antifungal Properties

The lactic acid bacteria that can reduce pH value and produce Antibacterial material had a long history in food and fodder field. The lactic bacteria that were applied in food had important pratical value, such as many studies have found that mould and yeast can be inhibited by it in recent years. In this paper , Agar-well diffusion method were used to select the antifungal LAB which came from 60 stains screened form Northeast kimchi and traditional dairy product . Then the antifungal properties of the screened LAB AST18 was studied, that including fermentation properties,physical and chemical properties of the antifungal material,the isolation and purification of the antifungal material and the applications in the yogurt production process as adjunct culture . when the AST18 was used as adjunct culture ,the Anti-fungal properties,physical and chemical and sensory evaluation of the yogurt were also studied. The main results are as follows:(1) Agar-well diffusion method was used to select the antifungal LAB which from 60 strains preserved in laboratory. There are 7 strains had better antifungal properties among 60 stain LAB. Of which the LAB of best antifungal activities was identified as Lactobacillus casei by 16S r DNA method. The 7 screened LABS all can produce PLA through HPLC detecting. Although the PLA from fermentation broth can reach highest content 16.24 mg l/L and the minimum content 6.86 mg l/L , the test result showed PLA content have no relations with antifungal activities. The MRS broth pH was adjusted to 3.88 by acetic acid,lactic acid and hydrochloric acid separately, but there is no inhibition zone. The further research about antibacterial material needs to be done to further identify antibacterial material.(2)The effects of different incubation time,incubation temperature and aeration condition on the growth and antibacterial properties of AST18 were studied, The optimum growth conditions and antibacterial activities was at 37°C and at the initial pH of 6.5-7.0 and by shaking culture . by studying the different sensitivity of culture supernatants of Lactobacillus casei AST18 with pH,heat treatment and protease treatment, the results showed the antibacterial activities of the culture supernatants of Lactobacillus casei AST18 was high at the pH of 2.0-4.0, it was vulnerable to heat treatment and even lost bioactivity by above 100℃heat treatment ,but it was not affected after treatment of trypsin and pepsin.(3)The part 2(4.5-7.5min)and part 4(11.4-13.4min)showed the biggest antifungal activities. Through the primary research, the high level of lactic acid was the main antifungal materials in part 2. Part 4 was analysed by GC-MS. There are three organic compounds departed: cyclic peptidesctclo-(Leu-Pro),2,6-diphenyl-piperidine and small molecular substances of pyrazine.(4)The physical and chemical properties and sensory properties were evaluated when the Lactobacillus casei AST18 as Secondary fermentation agent was used in yogurt. The result showed the addition of Lactobacillus caseiAST18 had little effection on viscosity,water-retaining capability and sensory evaluation of yogurt, and the weak acid yield ability, and little effection on pH value and titratable acidity of yogurt. The AST18 can be used as secondary fermentation and its addition had little effection on physical and chemical properities and sensory evaluation. At the same time, AST18 addition can increase the number of Lactobacillus in yogurt and reduce the number of Lactococcus, such as, Compared with the control group the 2% addition AST18 can reduce by 1.0 Log (cfu/mL) and other test groups reduce by 0.5 Log (cfu/mL). This may because of the Antagonism of Lactobacillus and Lactococcus.(5) The shelf life experiment and mouldproof accelerated testing were done at 4℃and 28℃. The test result showed, the mould growth situation can be totally inhibited by addition 2% Lactobacillus caseiAST18 in yogurt at shelf life experiment and mouldproof accelerated testing at 4℃; the mould growth situation can be delayed by addition 2% Lactobacillus caseiAST18 in yogurt at shelf life experiment and mouldproof accelerated testing at 28℃...