Storage of milk powders under adverse conditions. 2. Influence on the content of water-soluble vitamins.

Abstract

Storage of milk powder under unfavourable conditions accelerates the normally slow deterioration in nutritional quality. The effects of such storage on the water-soluble vitamin composition were examined. (a) Spray-dried whole milk containing 25 g water/kg was stored at 60 degrees and 70 degrees and sampled weekly to 9 weeks. (b) Spray-dried whole milk and skimmed milk were adjusted to contain 40 and 100 g water/kg and stored at 37 degrees in nitrogen and in oxygen. Samples were taken for analysis at intervals during storage. The samples were analysed for eight B-complex vitamins and ascorbic acid, and also for total lysine, 'reactive lysine' and 'lysine as lactulosyl-lysine'. Storage at 60 degrees caused rapid destruction of folic acid (53% loss at 4 weeks) and slower loss of thiamin, vitamin B6 and pantothenic acid (18% at 8 weeks). There was no change in the content of riboflavin, biotin, nicotinic acid and vitamin B12. At 70 degrees the rate of destruction of the four labile vitamins was much increased; 18% or less survived at 4 weeks. At 37 degrees and 40 g water/kg there was little change in total and 'reactive' lysine during storage for 57 d. Lactulosyl-lysine was demonstrably present but at low concentration. There was considerable loss of folate (72%) and ascorbate (91%) during storage for 30 d in O2, but no significant loss in N2. Thiamin fell by approximately 12% in 57 d, equally in O2 and N2. The content of the remaining vitamins was unchanged. At 100 g water/kg there were progressive Maillard changes. During 27 d in N2 the colour changed from cream to pale brown, but in O2 there was no perceptible colour change. Total lysine fell by 20% in 27 d, and 'reactive lysine' by 30%. Folate was stable during 16 d in N2, but largely (94%) destroyed in O2. Ascorbic acid was also destroyed in N2 as in O2. Thiamin fell by 41% in 27 d, equally in O2 and N2. Vitamin B6 was more labile, especially in N2, falling by 71% in 16 d. With skimmed-milk powder containing 100 g water/kg, storage at 37 degrees in O2 and N2 gave much the same results as for the corresponding whole-milk powder. The presence of milk fat had no marked effect on the stability of the water-soluble vitamins. Destruction of vitamins was clearly linked to the progress of Maillard-type reactions and was strongly influenced by time and temperature of storage, moisture content and, in some instances, by the presence of O2.