Effects of heating on the secondary structure of proteins in milk powders using mid-infrared spectroscopy

Milk powder is an important source of protein for adults and children. Protein is very sensitive to heat, which may influence people's usage of nutrients in milk powder. In this study, we describe the temperature induced secondary structure of protein in milk powders. In this study, whole milk powder containing 24% protein and infant formula containing 11% protein were heated from 25 to 100 degrees C. Attenuated total reflectance (ATR) spectra in the mid-infrared range 400-4,000 cm(-1) were used to evaluate the heat effect on the secondary structure of protein in these 2 milk powders. The spectral changes as a function of temperature were maintained by difference spectra, second-derivative spectra and Gauss curve-fitted spectra. The secondary structures of protein in the whole milk powder began to change at 70 degrees C and in the infant formula at 50 degrees C. The beta-sheet and beta-turn structures in the whole milk powder both decreased in the range of 70 to 85 degrees C, whereas alpha-helix structures increased. The loss of beta-sheet and beta-turn may contribute to the formation of alpha-helix in the whole milk powder. In infant formula powder, the beta-sheet structure showed a decrease and then increase, whereas the beta-turn structure showed an increase and then decrease in the range of 50 to 75 degrees C, and no change was found for alpha-helix structures. This implies that heating may induce the transformation from beta-sheet to beta-turn. Overall, whole milk powder had better temperature stability than infant formula powder, probably because of the lower content of lipid in the former than in the latter. These results help us understand the thermal stability of protein in milk powder.