Activity of Xanthine Oxidase in Dairy Products

Abstract

One-hundred-ninety-five commercially processed dairy products were assayed polarographically for activity of xanthine oxidase. Average activity of 22 of 25 fluid milks ranged from 4.5 to 139.6μ1 oxygen/ml per h. Most of the powdered and evaporated milk products and all butters had no activity. Average activity in ice creams and yogurts ranged from 0 to 40.3μl oxygen/g per h. One-half of the 24 brands of creams contained activity ranging from 4.2 to 404.8μ1 oxygen/ml per h. Approximately two-thirds of the 111 brands and varieties of cheeses contained activities ranging from 0 to 420.4μl oxygen/g per h. If the starting milk in cheese making was pasteurized or if the cheese was processed, enzyme activity was generally low or zero. Cheeses made from either goat's or ewe's milk contained always low or no activity. Activity of fresh raw milk increased 3.7-fold in storage at 4C for 24h while activity declined for longer periods. Fresh unchilled raw milk contained about two-thirds as much activity as processed whole milk, while the latter contained one-half as much activity as the former stored at 4C for 24h. The effect of freezing on xanthine oxidase in raw milk was even greater when stored at 4C. Activity increased steadily from the first 24h at –20C to a 15-fold maximum at 72h. This increase in activity is over 4-fold higher than that obtained by storing raw milk at 4C for the same time. This finding can improve the yield of xanthine oxidase in purification procedures. In contrast, storage of processed whole milk at –20C and treatment with Triton X-100 increased activity only slightly. Commercial processing destroyed about 82% of the activity in raw milk, an amount much greater than reported heretofore.