Ultrasonic spectroscopy study of relaxation and scattering in whey proteinsolutions

Ultrasonic attenuation spectroscopy was used to investigate the influence o f pH on fast chemical reactions and aggregation of whey proteins in aqueous solution. Ultrasonic attenuation spectra (1-100 MHz) of 2.5wt% aqueous sol utions containing either 'native' or 'alkali-denatured' proteins were measu red as a function of pH (2-12). Peaks in the attenuation occurred at pH 2.8 and 11.6 due to proton, transfer equilibria, ie -CO2H <----> -CO2- + H+ an d -NH2 + H+ <----> -NH3+ respectively. Attenuation at other pH values was a ttributed to a hydration relaxation mechanism. Relaxation times for the equ ilibria were of the order of 10(-8)s. There was an additional. attenuation peak at the isoelectric point of the proteins (pH 5) for solutions containi ng 'alkali-denatured' protein, which was due to scattering of ultrasound by aggregated proteins. The particle size distribution of the aggregates coul d be determined using ultrasonic scattering theory to analyse the attenuati on spectra. Ultrasonic spectroscopy is an extremely valuable tool for probi ng the molecular characteristics of proteins in solution. (C) 1999 Society of Chemical Industry.