Study On The Preparation Technology Of Streptococcus Thermophilus 937 Starter

As people pay more attention to health and quality of life,consumers pay more and more attention to lactic acid bacteria fermented food.In this context,the market demand for lactic acid bacteria starters has also increased significantly.However,at present,the largest proportion of domestic lactic acid bacteria raw materials is foreign companies,accounting for about 85%of the total domestic lactic acid bacteria starter market.This is related to the late start of lactic acid bacteria industry and the lack of independent property rights.Therefore,the development of lactic acid bacteria starter with high live bacteria number and high vitality is of extremely important practical significance,which can not only break the market monopoly of foreign companies,but also protect the domestic intellectual property rights.This paper takes the research object of Streptococcus thermophilus 937,studies the nutrient requirements,optimizes the culture medium and culture conditions,explores the high density culture process,selects the best harvest time of S.thermophilus 937,Then,the amount and storage stability of lyophilization protector S.thermophilus 937 were studied,and finally the lyophilization starter with high viable number and high viability.The results of the study are as follows:(1)The proliferation medium and fermentation conditions of Streptococcus thermophilus 937 were studied.A single deduction experiment was selected for the nutrients required for the growth of S.thermophilus 937.Further through one-factor experiment and response surface optimization experiment,the proliferation medium formulation of S.thermophilus 937 was obtained as follows:Lactose 20 g/L,enzymatically hydrolyzed skim milk 16.60%,soy oligopeptide 18.88 g/L,whey protein powder 1.69 g/L,10 mM histidine,10 mM isoleucine,5 mM tyrosine,1 mM cysteine and 1 mM glutamic acid,2 mg/L niacin,0.5 g/L ascorbic acid,40 mg/L magnesium chloride,2 mg/L calcium pantothenate,4 mg/L thiamine hydrochloride,0.4 g/L calcium chloride,the number of live bacteria could reach to 1.38×109 CFU/mL。Finally,The culture conditions of S.thermophilus 937 were optimized by orthogonal experiments,and the optimization results were:culture temperature of 39℃,initial pH of 6.5,and inoculum of 2%.Under this condition,the number of viable S.thermophilus 937 could reach to 1.75×109 CFU/mL。(3)The high density culture method of S.thermophilus 937 was optimized by 5 L automatic glass fermentation tank,and finally verified by 200 L pilot test.The high-density culture method of S.thermophilus 937 was determined as follows:using 5 L automatic fermentation tank,sterile N2,stirring speed 85 r/min,39℃ fermentation,feeding concentration 4 mol/L,molar ratio 1:1 potassium hydroxide and sodium bicarbonate mixed neutralizer to control the pH value of the fermentation broth 6.5,the lactose concentration of the fermentation broth was reduced to 12 g/L for feeding and maintaining the lactose concentration of the fermentation broth in the range of 12-14 g/L.The number of viable S.thermophilus 937 was as high as 5.87×109 CFU/mL.At the same time,the 200 L pilot test of high density culture of S.thermophilus 937 was carried out,and the number of viable bacteria could reach 4.56×109 CFU/mL.(4)The preparation technology of S.thermophilus 937 starter was studied.The effects of collection time,protective agent addition ratio,centrifugation conditions and storage temperature on the survival rate and bacterial activity of S.thermophilus 937 freeze-dried starter were studied.The results showed that The optimal collection time of S.thermophilus 937 was 8 h,and the viability of lyophilization,lactose dehydrogenase,β-galactosidase,hexokinase,pyruvate kinase and ATPase before and after lyophilization were at a high level,while the growth capacity and fermentation activity of S.thermophilus 937 starter culture were the best.When the feeding speed of the disc centrifuge was 3.5 kg/min and the slag discharge time was 540 s,the highest cell collection rate was 96.95±0.19%.When the best quality ratio of bacteria sludge and protective agent is 1:1,the maximum survival rate of bacteria drying is 94.0±0.5%.When the storage temperature is-20℃,the storage stability of Streptococcus thermophilus 937 starter was poor.The lower the storage temperature,the better the storage of Streptococcus thermophilus 937 starter.When the storage temperature is-20℃,the number of live bacteria is still higher than 1×1011 CFU/g after 210 days,and the fermentation vitality is kept at a high level.