| High-activity directed vat set are essential for fermented dairy products and fermented vegetable protein beverage products,and the development of suitable strains is the key to achieving directed vat set production.Based on the pre-laboratory study,this experiment examined the biological characteristics of 7 strains of suitable strains of soybean yoghurt and explored the potential characteristics of the strain.A strain of soybean yoghurt strain SCB0469 was selected to optimize its culture conditions and high-density medium.The freeze-drying survival rate was used as an indicator to optimize the SCB0469 freeze-dried protective agent,and a formula with high survival rate was obtained,which laid a foundation for the development of bean yogurt direct-injection starter.The main results are as follows:(1)The resistance and drug resistance of suitable strains of soybean yoghurt.It was found that 7 strains of lactic acid bacteria could not grow in MRS liquid medium with p H?2.5;however,5 strains such as SCB0119,SCB0151,SCB0351,SCB0531,SCB0641 could grow in a liquid environment of p H 3.5MRS.Therefore,SCB0119,SCB0151,SCB0351,SCB0531,and SCB0641 have certain acid resistance.The seven suitable strains of soybean yoghurt could not grow in an environment with a bile salt concentration of?0.2%.SCB0119 and SCB0151 have relatively strong ability to resist bile salts and can grow in 0.1%bile salts.The K-B paper method was used to find that the tested strains were all multi-drug resistant strains,and showed resistance to three major antibiotics that inhibit cell wall,protein and nucleic acid synthesis.(2)The amount of extracellular polysaccharide produced by the 7 strains was 49.8?90.1mg/m L,and the amount of extracellular polysaccharide produced by SCB0351 and SCB0469 was significantly higher than that of other strains(P<0.05).The lactic acid bacteria metabolites of 7 strains of lactic acid bacteria were inhibited by E.coli and Staphylococcus aureus by Oxford Cup agar diffusion method.The inhibition zone of SCB0119 on E.coli reached 17 mm,the inhibition zone of Staphylococcus aureus reached 14mm;the inhibition zone of SCB0297 to E.coli reached 16mm,and the inhibition zone of Staphylococcus aureus reached 15 mm;Among them,SCB0119,SCB0151,SCB0297,SCB0531,SCB0641 and other five strains have higher bacteriostatic effects than SCB0351 and SCB0469.However,the ectopolysaccharide produced by the seven strains of lactic acid bacteria was found to be bacteriostatic,and the extracellular polysaccharides produced by the seven strains had no bacteriostatic effect.(3)The soybean yoghurt strain SCB0469 was identified.It was found by morphological observation that the strain could form white colonies with a diameter of 1?2 mm on the MRS medium,and the edges were neat and opaque.It was spherical or ovoid by microscopic observation;SCB0469 was a Gram-positive bacterium by Gram staining,and it was found to be Lactococcus lactis by 16Sr DNA alignment on the NCBI website.(4)High-density culture conditions(temperature,inoculum size,initial p H)and medium composition(nitrogen source,carbon source,growth factor)of SCB0469 were optimized.The results showed that the culture conditions were p H 6.4,37°C,and the number of viable bacteria was the highest at 1%inoculation.The yeast extract is 30 g/L,the maltose is 30 g/L,the magnesium sulfate heptahydrate is 0.62g/L,the dipotassium hydrogen phosphate is 2g/L,the diammonium hydrogen citrate is 2g/L,and the sodium acetate is 5 g/L.Tween-80 is 1.5 m L.It has been verified that the number of viable cells of SCB0469 can reach 4.5×10~9 CFU/m L under this formula.(5)The optimal formulation of the lyoprotectant was obtained by single factor test,Plackett-Burman,steepest rise test and central optimization combination design:sodium glutamate was 9 g/100m L,lactose was 14 g/100m L,and mannitol was 6g/100m L.The lyophilization survival rate of SCB0469 was verified to be 93.36%±1.27%under this protective agent regimen. |
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