| Inoculating lactic acid bacteria can easily induce and monitor natural fermentation during the process of paocai, shorten the fermentation and reduced the risk of failure.Therefore, isolation and screening of lactic acid bacteria(LAB) with excellent fermenting properties are crucial. Tailored lactic acid bacteria starters can completely exploit the potential of vegetables, which enhances the sensory, nutritional and shelf life properties.The key of the pattern which can be changed from traditional fermentation to inculated fermentation is the isolation and screening of outstanding LAB. Compared with liquid stater culture of LAB, Direct Vat Set(DVS) has some advantages such as higher concentration of cell viabilites and fermentation activity, easy storage and transportation.It can also reduce formation, cost and improve the stability and safety of products in industrial production.The aim of this study was to isolate and screen LAB with excellent acidification,nitrite depletion and antimicrobial activity. Then appropriate single and combined cryoprotectants were selected to retain the cell viabilites during the process of freez-drying. The storage stability and fermented activity of DVS were studied as well.This research was expected to provide theoretical support to the large scale and standardized preparation of DVS. The results of this study were as follows.1.93 strains were isolated and purified from homemade and commercial paocai with the MRS medium. Streak plate technique was used. Then 81 strains were tested as gram-postive and 12 strains were gram-negative. The 81 gram-postive strains were also catalase negative. Then the 81 strains were proved as lactic acid bacteria preliminarily.2.The medium was made of radish juice and 4% salt brine in equal proportion. The pH meter and Acid-base titration method were used to test pH values and titrable acidity.The strains with the faster produce rate of the acidification and higer titrable acidity were screened from the above mentioned 81 strains. It turned out that 5 strains from 68 strains that isolated from homemade paocai could reduce the medium’s pH values more quickly in 6 hours and produced higher total acid in 72 hours. There was significant differencebetween the 5 strains and the CK1 strain(P<0.05).3.The Griess reaction was used to study the ability of nitrite depletion of the 5strains that above mentioned. The result indicated that all of the 5 strains had strong ability of nitrite depletion. In the first 2 days, the trends of sodium nitrite depletion was logarithmic. More than 50% was degradated in 12 hours. After 48 hours, there was only5% sodium nitrite was left. In the last 4 days, the sodium nitrite was degraded slowly until totally disappeared. Moreover, the sodium nitrite depletion was related with the decline of pH values.4.Agar diffusion method was used to study the antimicrobial activity of the 5strains that above mentioned. The results showed that the cell-free supernatant(CFS) with low pH values(pH=3.59 ~ 3.75) of the 5 strains had obvious inhibition effects to107cfu/mL E.coli ATCC8739 and S.aureus ATCC 29213.But there was no inhibition effect when the concentration of E.coli ATCC8739 and S.aureus ATCC 29213 reached109cfu/mL. The treatment to the CFS under low temperature(4℃)diffusion for 24 h could enhance the antimicrobial activity of CFS.5.The five excellent strains were identified as lactobacillus plantarum according to the result of physical and chemical determination and Berger bacteria identification manual, and Common bacteria system identification manual.6.The software of Minitab17.0 was applied to optimize the parameters of bacteria centrifugal collection. The centrifugal rate, time and temperature were 6000r/min, 20 min,10℃ separately. And the centrifugal yield was 98.26±6.52% under this condition.7.Relative cell viabilities of freeze dried LP cells in the presence of different single cryoprotectant under different concentrations were studied. The result showed that inulin,sucrose, D-trehalose and skim milk except glucose were promising protectants. And inulin at a concentration of 9% was found to show best preservative effect(93.63%) and followed by the 12% of sucrose which could protect 90.94% cell alive. However,supplementation of the drying medium with glucose at any concentration was found the lowest cell viability of LP during freeze drying which were below 60%.8.The LP cell survival was tested with various combinations of cryoprotectants including 9% of inulin, 12% of sucrose, 3% of D-trehalose and 3% of skim milk. Amongthe combinations, the maximum cell viability of LP was observed 95.7±0.97% in the combination of inulin and sucrose, followed by 95.47±3.43% inulin, sucrose and trehalose.9.The storage stability and fermentation activity of DVS prepared with IS and IST were study at different temperatures. The results showed that low temperatures(-18℃)contributed to the process of storage and high fermentation activity. In addition, the presence of trehalose facilitated the long-time storage. |
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