OXIDATIVE CHANGES OF MILK FAT IN DRY MILK STORED UNDER VARIOUS CONDITIONS

Oxidative changes of milk fat in whole dry milk during 50 days of storage under various conditions were examined. Whole dry milk with 26.47 % of fat and insolubility index 1.27 was taken as a sample. Whole dry milk was manufactured by roller drying in YOG s. r. o. Bojkvice. Whole dry milk was stored in desiccators at temperature 37 degrees C in thermoregulator under various water activities (0.23 and 0.82). Water activity was made by 100 cm(3) of saturated salt solution. Water activity 0.23 was made by saturated solution of potassium acetate and water activity 0.82 was made by saturated solution of potassium bromide. The milk powder was stored for 50 days. The sample with water activity 0.82 became dark brown during storage thanks to products of Maillard reaction. The oxidative changes were examined as a content of hydroperoxides, TBARS (thiobarbituric reactive substances), peroxide value, neutralization number, content of conjugated dienes and fatty acids composition. The content of hydroperoxides, TBARS and fatty acids, especially unsaturated fatty acids (oleic acid and linoleic acid) decreased during storage. Neutralization number and peroxide value increased during storage. All chemical parametrer were significantly changed during 50 days of storage under various water activities.