Heat-induced gelation of whey proteins observed by rheology, atomic force microscopy, and Raman scattering spectroscopy

Rheological and structural transitions during heat-induced gelation of whey proteins were investigated using mechanical spectroscopy, atomic force microscopy (AFM), and Raman scattering spectroscopy. β-Lactoglobulin aqueous solutions containing 0.1 mol/dm3 NaCl at pH 7 exhibited solid-like mechanical spectra before gelation. Heating such a solution resulted in the formation of an opaque gel that exhibited frequency independent View the MathML source values, an indicative of self-similar network structures. Translucent gels were formed in the absence of added salts at pH 7 and their View the MathML source values were frequency dependent. AFM images of heat-induced gel precursors revealed that these aggregates were composed of ellipsoidal primary particles, regardless of the concentration of added NaCl, confirming that aggregation occurs in two-step: the formation of primary aggregates and the subsequent aggregation of the primary aggregates. The size of primary aggregates and the rate of aggregation increased with increasing NaCl concentrations. Thus, transitions from translucent to opaque gels with increasing ionic concentrations are likely to be caused mainly by kinetic effects without accompanying fundamental changes in the two-step aggregation mechanism. Raman scattering spectroscopy allowed discrimination between these two gel types based on secondary structures in denatured β-lactoglobulin molecules: a decrease in the α-helix structure content was more pronounced in translucent gels, while considerable fractions of β-sheet structures remained in both types of gels. A significant involvement of hydrophobic interactions in the formation of opaque gels was suggested by an intense band assigned to hydrophobic side chains.