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Selection of Strain, Growth Conditions, and Extraction Procedures for Optimum Production of Lactase from Kluyveromyces fragilis

Department of Food Science and Technology, University of California, Davis 95616

ABSTRACT

Forty-one strains of Kluyveromyces fragilis (Jörgensen) van der Walt 1909 varied 60-fold in ability to produce lactase (ß-galactosidase). The four best strains were UCD #55–31 (Northern Regional Research Center NRRL Y-1196), UCD #C21(–), UCD #72–297(–), and UCD #55–61 (NRRL Y-1109). Biosynthesis of lactase during the growth of K. fragilis strain UCD #55–61 was followed on both lactose and sweet whey media. Maximum enzyme yield was obtained at the beginning of the stationary phase of growth. Best lactase yields from K. fragilis UCD #55–61 were obtained with 15% lactose and an aeration rate of at least .2 mmol oxygen/liter per min. Supplementary growth factors were unnecessary for good lactase yields when yeast was grown on whey media. Best extraction of lactase from fresh yeast cells was obtained by toluene autolysis (2% vol/vol) at 37 C in .1 M potassium phosphate buffer, pH 7.0, containing .1 mM manganese chloride and .5 mM magnesium sulfate. The enzyme was concentrated and purified partially by acetone precipitation. At least 95% of the enzyme activity of the concentrated solution was retained after storage for 7 days at 22 C, for 3 wk at 4 C, and for 6 wk at –20 C.