Abstract: The effect of thermal pretreatments at different temperatures (70, 80, 85, 90, and 100 ℃) on the in vitro
digestibility of soy protein isolate (SPI) was investigated using various protein analytical techniques. The curves of degree
of hydrolysis (DH) showed an upward and then downward trend with pretreatment temperature or time. FTIR and Raman
spectral analysis showed that thermal pretreatments resulted in a higher α-helix content and a lower β-sheet content in
digestion products of SPI compared with the un-pretreated sample, while no significant changes in β-turn or random coil
were observed. The α-helix content rose firstly and then fell, whereas the opposite trend was noted for β-sheet content. The
Raman spectral peaks of tyrosine revealed small and irregular changes, leading to a conclusion that heat-treated tryptophan
residues were more close to “embedding state”.

Key words: soy protein isolate (SPI), in vitro digestion, pepsin, heat treatment, structural characteristics