1. Whole-milk powders containing 25 g water/kg were stored for up to 9 weeks in sealed aluminium containers at elevated temperatures. Lysine and other essential amino acids were measured by chemical and microbiological methods.2. Storage at 60' resulted in the progressive formation of lactulosyl-lysine. After 9 weeks, 30% of the lysine groups were present in this form. The powders still retained their natural colour and the levels of tryptophan, methionine, cyst(e)ine and leucine remained unchanged.3. Storage at 70° resulted in the formation of lactulosyl-lysine followed by its complete degradation with the development of browning. Available tryptophan, methione, leucine and isoleucine decreased progressively during storage.4. The different methods for lysine determination gave widely dissimilar results. The direct fluorodinitrobenzene (FDNB) technique and reactive lysine from furosine were considered to be the most reliable methods. The FDNB-difference, dye-binding lysine, Tetruhymena and Pediococcus methods all seriously underestimated reactive or available lysine in heat-damaged milk powders. Tetrahymenuand Pediococcus appeared to utilize lactulosyl-lysine as a source of lysine.5. The results are discussed in relation to storage and distribution of milk powders in hot climates.During prolonged storage of milk powders, Maillard reactions between milk proteins and lactose may reduce the level of lysine and other essential amino acids. The loss of lysine, the most sensitive amino acid to Maillard damage, increases with increase in temperature and moisture content (Erbersdobler, 1970) and hermetic packaging and cool storage conditions are necessary to reduce the reaction to a minimum. Lysine has been reported to be reasonably stable for at least 1 year in low-moisture milk powders stored in moisture-resistant containers at temperatures up to 37 ' (Henry et al. 1948;Rolls & Porter, 1973; Womack & Holsinger, 1979). In temperate climates, distribution and storage would therefore appear to present little possibility of large losses of amino acids. In hot countries, however, where distribution networks are often less developed, milk powders may be subjected to high temperatures (above 40') at some stage during their transport and storage. Little information is available on the losses of nutrients under these more extreme conditions. In the present paper, we report the losses oflysine and of other amino acids in low-moisture milk powder (25 g water/kg) during storage at high temperatures, and high-moisture powder (100 g water/kg) stored at 37' . At the same time, we have extended earlier studies (Finot & Mauron, 1972;Finot, 1973;Hurrell & Carpenter, 1974 on the ability of different lysine assay methods to accurately predict the extent of lysine damage after Maillard reactions. The dye-binding lysine and furosine techniques, and microbiological methods with Tetrahymena and Pediococcus, have been evaluated.
Test materialsSpray-dried whole-milk powder containing 25 g water/kg was packed under air into aluminium cans (4...