Storage of milk powders under adverse conditions

Abstract

1. 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.2. (a) Spray-dried whole milk containing 25 g water/kg was stored at 60° and 70° 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° in nitrogenand in oxygen. Samples were taken for analysis at intervals during storage.3. The samples were analysed for eight B-complex vitamins and ascorbic acid, and also for total lysine, ‘reactive lysine’ and ‘lysine as lactulosyl-lysine’.4. Storage at 60° 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° the rate of destruction of the four labile vitamins was much increased; 18% or less survived at 4 weeks.5. At 37° and 40 g water/kg there was little change in total and ‘reactive’ lysine during storage for 57 d. Lactulosyl-lysine was demonstrably present butatlow 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 palebrown, 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 16d.6. With skimmed-milk powder containing 100 g water/kg, storage at 37° 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.7. 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.