Effect of heat treatment, water activity and storage temperature on the oxidative stability of whole milk powder

The oxidative status of high-heat, medium-heat and low-heat whole milk powder was investigated at moderately accelerated storage conditions, with exposure to atmospheric air at 25 or 45 degrees C and at three water activities (0.11, 0.23 and 0.33 at 25 degrees C, and 0.11, 0.17 and 0.31 at 45 degrees C) for 2 months using: (i) electron spin resonance spectrometry for measurement of the level of free radicals, (ii) determination of thiobarbituric acid reactive substances (TEARS) as a measurement of secondary lipid oxidation products, (iii) size-exclusion HPLC for measurement of fluorescent protein polymerization products, and (iv) sensory evaluation. Lipid oxidation was affected greatly by storage temperature, with a maximum level of free radicals being detected after 47 days at 45 degrees C, and with the highest level in low heat powder, irrespective of water activity. The sensory quality dropped to an unacceptable level for low-heat powder within 33 days of storage, as confirmed by measurement of TEARS, and the increasing TEARS value was parallelled by a decrease of 'free' thiol groups to an unmeasurable level in low-heat powder, In contrast to medium-and high-heat powders, in which the initial level of free thiol groups was only reduced by one-third after 63 days of storage. In contrast to common beliefs, initial powder quality was retained best at water activities between 0.11 and 0.23, where powders showed no significant differences in sensory quality, irrespective of preheat treatment. No difference in sensory score was found for storage at different water activities at 25 degrees C, whereas storage at water activity 0.31 at 45 degrees C markedly affected all powders. This effect was ascribed to a combination of increased autoxidation rare and increased Maillard reaction rate giving rise to protein polymerization, in effect decreasing the powder solubility, as often seen during storage under tropical conditions. (C) 1997 Elsevier Science Ltd. All rights reserved.