Abstract: Fourier transform infrared spectroscopy was used to investigate protein conformation changes in pasteurized milk (65 ℃/30 min and 80 ℃/15 s), yoghurt (95 ℃/5 min) and ultra-high temperature sterilized milk (137 ℃/5 s) in comparison with raw milk as a control. The aim was to examine the effect of heat treatments on the secondary structure of milk proteins. The results showed that heat treatments could cause interactions between milk proteins, resulting in damage to the spatial structure of milk proteins and intramolecular hydrogen bonds. Different heat treatment conditions could result in different degrees of red shift of milk protein amide Ⅰ bands to lower wavenumbers, suggesting that intermolecular hydrogen bonds were formed between hydrophobic amino acid residues exposed during denaturation. Meanwhile, the secondary structure composition of milk proteins changed significantly after heat treatments; α-helix content decreased significantly (P < 0.05), random coil content increased significantly (P < 0.05), and both β-sheet and β-turn content decreased after an initial increase, indicating that the ordered structure was partially transformed to random coil. Protein aggregation occurred after thermal denaturation, and β-sheet and β-turn played an important role in the formation of heat-induced protein aggregates.

Key words: Fourier transform infrared spectroscopy, heat treatment, milk protein, thermal aggregation, secondary structure