Free and membrane-bound xanthine oxidase in bovine milk during cooling and heating.

The effect of cold storage (5 C, 24 h) and heat treatment (60 C, 5 min) of milk on activities of free and membrane-bound xanthine oxidase has been studied. Both treatments enhanced total xanthine oxidase activity in milk. Activity of membrane-bound xanthine oxidase increased and free xanthine oxidase decreased in buttermilk while it increased in skim milk on cold storage. Heat of milk increased free and membrane-bound xanthine oxidase activities in both buttermilk and skim milk. The state of xanthine oxidase activity in skim milk from reconstituted milk, which was prepared by mixing xanthine oxidase inactivated skim milk and fresh cream, showed that only the free enzyme migrated from the cream phase to skim milk on cold storage. Very little xanthine oxidase activity was detectable in skim milk on heat treatment of the reconstituted milk sample. The overall increased activity of xanthine oxidase in milk during cold storage or heat treatment may not be due to the release of fat globule membrane enzyme to skim milk.