Mutation Screening High Acid-Producing Activity Stain In Yogurt Manufacture

Lactic acid bacteria starter play a important role in yogurt manufacture industry. The performance of lactic acid bacteria starter effect the quality of yogurt directly. It is important to improve the taste of yogurt to screen out fine strain which has excellent characters of acid-producing and slime-producing. This experiment employ mutation screening select out high acid-producing bacteria from Streptococcus thermophilus (S.t) and Lactobacillus bu!garicus(L.b).Firstly select two stains : TM111-S and B-3 from S.t and L.b as start stain. The acid-producing ability of the two strain was higher than others. The final acidity of yogurt ferment by the two bacteria was 57.65 T and 54.43 T respectively.Because the -galactosidase produce by starter is a endoenzyme it must abstract after the cell wall broken. This experiment compare the different method of cell wall broken and research on the influence of temperature and time on condition of cell wall broken. The optimum condition of cell wall break is 45% chloroform, 45'C preservation 8-12 hours. The activity of 3 -galactosidase was mensurate by the method provided by AOAC. The starter can produce -galactosidase throughout the process of yogurt making . With yogurt ferment extend the activity of the enzyme get higher. Lactose in the milk was decompose by P -galactosidase and the pH value was decrease gradually .Ultraviolet and NTG were employed to mutate TM111-S and B-3.Fristly TM111-S was exposed in ultraviolet 150s obtaining a new stain the acid-producing ability of the new stain can reach 57.85 T increase 3.05 % then before . When TM111-S was disposed by 3g/L NTG 60mins can obtain a new stain which ability of acid-producing can reach 58.02 T increasing 6.52% then before. Then disposed the new stain by UV+NTG four times obtaining a new stain TM111-S-UV-N which acid-producing can reach 83.66 T increase 30.64% than before. Secondly B-3 was exposed in ultraviolet 120s obtaining a new stain which acid-producing ability can reach 55.96 T increase 2.81% . When B-3 was disposed by 3g/L NTG 60mins can obtaining a new stain which acid-producing can reach 57.83?T increase 6.26%. Then disposed the new stain by UV+NTG four times obtaining a new stain B-3-UV-N which acid-producing can reach 82.65 T increase 30.03% .Employed the new stains B-3-UV-N and TM111-S-UV-N to manufacture yogurt,the coagulation time was shorter obvious Iy,the texture was fine and the viscosity was improved.