Study On High-density Cultivation And Freeze-drying Starters Of Lactic Acid Bacteria

With the rapidly development of fermented dairy product industry,it's significant for promoting the industrial of lactic acid bacterias(LAB) freeze-drying starters and enhancing the competence of our national starters.In this paper,the nicer strains was isolated and optimum collocation;the technical conditions of its enrichment cultures, concentrated isolation and freeze-drying were studied;the enrichment mediums and cryoprotectants were screened,in order to produce the LAB freeze-drying starters,at last the clabbering characteristics and store stability were examined of the starters.Clabbering characteristics of Streptococcus thermophillus S800 and Lactobacillus acidophilus L621,which isolated from the commercial LAB starters abroad,were studied.2:1 is the optimum proportion of those two strains for mixed fermented in milk.Compositions of the optimum enrichment mediums for L621 was:0.5%lactose,3 %peptone,30%carrot juice,30%mushroom juice,1%beef extract,1%yeast extract, 0.3%K₂HPO₄,1 mL/L Tween80.The cell density of L621 could reach 3.72×10⁹ cfu/mL in this medium on 37℃,within the 18 h.The compositions of the optimum enrichment mediums for S800 was:0.5%lactose,3%bean peptone,30%tomato juice,30% mushroom juice,1%beef extract,1%yeast extract,0.5%K₂HPO₄,0.3%NaAc.The cell density of S800 could reach 1.04×10⁹ cfu/mL in this medium on 37℃after18 h;Experiment results by the automatic fermenter demonstrated that:the optimal temperature of L621 was 37℃,the optimal pH was 5.5,In those optimal conditions,the cell density of L621 could reach 6.21×10⁹ cfu/mL within the 12 h by adding feeding as 10%lactose and 20%yeast extract.The optimal temperature and pH of S800 was 37℃and 5.5.In this optimal condition,,the cell density of S800 could reach 3.29×10⁹ cfu/mL within the 12 hour by adding feeding as 10%lactose and 20%yeast extract.The optimum centrifugal force and time were 6000 r/min and 10min,the harvest rate was 95.8%;for S800,the optimum conditions were 6000 r/min and 10 min,and the harvest rate was 93.5%.The optimal thickness of freeze-dry specimen was 0.5 cm(the corresponding volume was 2 mL),the concentration of skim milk was 10%.the final optimum cryoprotectant was determined as 10%skim milk,3%fucose and 2%glycin using orthogonal experiment.When the thickness of freeze-dry specimen was 0.5cm,with the protection of optimum cryoprotectant,the livability about freeze-drying of L621 was 77.01%,S800 was 83.67%;the cell density of the freeze-drying production was 2.31×10¹⁰ cfu/g and 1.26×10¹⁰ cfu/g,respectively.The freeze-drying starters was stabilization on the cell density of live bacteria and clabbering characteristics,after conserving 3 months in 4℃.When fermenting the milk with the starter,the milk could clabber after 4 hours and 20 mins in 42℃,the pH of the yohourt was about 4.8,the acidity was about 80°T,the clabbering characteristics was finer.