Effects of High Pressure on the Conformation of Freeze-Dried Soy Protein Isolate: A FTIR Spectroscopic Study

The effect of high pressure (HP) treatment on the conformation of freeze-dried soy protein isolates (SPI) was investigated by Fourier transform infrared (FTIR) spectroscopy. Within the amide I' region (1 600-1 700 cm(-1))of the deconvoluted FTIR curve of SPI, more than 10 bands associated with protein conformation were distinctly observed, attributed to the C = O stretching vibration and to a small extent to C-N stretching vibration of the peptide bonds, respectively. The secondary structure of native SPI is estimated to be composed of 15%-16% alpha-helix, 39%-44% extended strands, 17.5% random coils, and 21%-27% turns. The analyses of intensity and wavenumber of the bands showed that, HP treatment at pressures of 200-400 MPa resulted in the increases in intensity and a "red-shift" (about 2 cm(-1))of these bands. HP treatment at 600 MPa further increased the band intensity of the amide I' region. The analyses of amide II bands showed that HP treatment led to gradual increases in intensity and absolute area of amide II bands, in a pressure-dependent manner. Thus, it is suggested that HP treatment resulted in gradual unfolding of secondary and tertiary structure of SPI, while the structure of denatured proteins underwent a "rebuilding" process after the release of high pressure. These results confirm that the HP-induced modification of SPI is by means of the HP-induced conformational changes.