| The paper studied the screening of the yeast with high vitalisticβ-Glycosidase, 18 strains of yeasts isolated from herdsman-made koumiss were used. The same proportion of yeast (1%) was inoculated into the milk with the same volume, and cultivated for 48h. The measurements of lactose content and pH value were made respectively before cultivation and in 12h, 24 hand 48h after cultivation. By comparison of the changes in lactose content and pH values, one strain with high vitalisticβ-Glycosidase was screened out, named as J20-3. The lactose content of the milk was 59.69mg/mL before inoculation, and it changed to 17.64mg/mL in 12h after inoculation, accounted for 29.55% of that before inoculation; it changed to 1.87mg/mL in 24h after inoculation, accounted for 3.13% of that before inoculation. After 24h, the lactose content did not change nearly. Based on the tests, the constitution of moiety medium, lactose, yeast juice and peptone were optimized by Box-Behnken method, three-factor and three levels Response Surface Methodology was designed and 15 experiment points were gotten. The optimization results were 18.31g for lactose, 5.23g for yeast juice and 10.43g for peptone in the 1000ml moiety medium. The enzyme activity of crude enzyme was determined by Gist & Brocades method. The activity ofβ-Galactosidase were decided by content ofβ-Galactosidase at 37℃and pH of 7.3 per thalline dry weight,and the content was 23.11IU/g. The thalline dry weight was detected every 2 hours after yeasts J20-3 cultivation within 24 hours at 30℃. The thalline dry weight vs. time referred to Logistic model was used. The parameter Cm=1.04kg/m3, Co=0.32kg/m3 were got. Experimental data were plot LN (Cm/C0-1) +LN [Cx/ (Cm-Cx)] vs. time. The slope was K=0.194, the intercept=0.04. The modelCx=0.32*EXP (0.04+0.194*t)/ {1-[0.32/1.04-EXP (0.04+0.194*t)]} was definited. As the proportional error between experimental data and theoretical value was about 5%, the model was fit.
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