| Recent studies have demonstrated that applied hydrostatic pressure can affect the functional properties of whey protein isolate (WPI). In this work, the effects of applied hydrostatic pressure on the tertiary and secondary structure of whey proteins were investigated by spectroscopic and rheological techniques to elucidate the molecular basis of such pressure-induced changes in protein functionality. The individual protein components of WPI and various samples of WPI obtained from different sources were subjected to different single-cycle pressure treatments of up to 400 MPa in 100 MPa increments with 30-min holding time as well as to pressures ranging from 450 to 650 MPa without a holding time. Electrospray ionization-mass spectrometry, circular dichroism, and Fourier transform Raman spectroscopic studies of pressure-treated samples of beta-lactoglobulin, the major protein component of WPI, revealed significant changes in tertiary structure. Fourier transform infrared spectroscopic studies revealed that the secondary structure of beta-lactoglobulin was also sensitive to applied pressure and holding time. The secondary and tertiary structure of alpha-lactalbumin, the second most prevalent protein in WPI, was unaffected by applied hydrostatic pressure. The spectroscopic behaviour of the various samples of WPI subjected to pressure treatment was variable and indicated that the response of WPI to applied hydrostatic pressure is dependent on the method used to isolate the WPI from whey. The rheological profiles of beta-lactoglobulin, alpha-lactalbumin, and WPI samples after various pressure treatments were also recorded. Both beta-lactoglobulin and WPI exhibited marked increases in viscosity with increasing pressure, whereas alpha-lactalbumin remained solutions exhibited no significant change in viscosity. These studies have furthered the understanding of the effects of applied hydrostatic pressure on the molecular structure and rheological properties of WPI proteins. |
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