INFLUENCE OF STORAGE, HEAT, AND HOMOGENIZATION UPON XANTHINE-OXIDASE ACTIVITY OF MILK

Xanthine oxidase activity in milk was determined by measuring the rate of formation of vanillic acid from vanillin. Raw milk received at a dairy plant had .208 units xanthine oxidase activity per ml and after 24-h storage at 4 C, .228 units per ml. Upon further storage activity decreased. Heating the fresh raw milk in a water bath to 55 C increased xanthine oxidase activity to .236 units per ml. Partial inactivation of the enzyme occurred when milk was heated at 60, 65, or 70 C for 5 min, and destruction was almost complete with heat at 75 C for 5 min. Raw milk heated at 48 C for 5 min and homogenized at pressures between 70.3 and 281.2 kg/cm2 had xanthine oxidase activities which were a linear function of pressure and showed that each additional kg/cm2 pressure resulted in additional xanthine oxidase activity of .16 milliunits per ml of milk. © 1978, American Dairy Science Association. All rights reserved.