REACTIVITY OF MILK CHOLESTEROL WITH BACTERIAL CHOLESTEROL OXIDASES

We tested three bacterial cholesterol oxidases on cholesterol in milk and milk fractions to probe cholesterol reactivity. Pseudomonas fluorescens cholesterol oxidase oxidized cholesterol rapidly in homogenized/pasteurized milk or sonicated raw milk but catalyzed cholesterol oxidation only slowly in intact raw milk. Pasteurization had little influence on oxidation rates. Oxidases from Streptomyces and Rhodococcus species were significantly less active in milk than the Pseudomonas enzyme. The Pseudomonas oxidase oxidized cholesterol in all raw milk fractions tested, providing the fractions were first disrupted by sonication or treatment with lipase. Cholesterol oxidation in homogenized milk depended on temperature and enzyme concentration. A maximum of 85% of the initial whole milk cholesterol (initially 100-138-mu-g/mL) was oxidized in 96 h, giving final cholesterol concentrations of 20-30-mu-g/mL. Sonicated skim milk cholesterol concentrations (initially 17-20-mu-g/mL) were reduced to 4-6-mu-g/mL.