Raman spectroscopy of heat-induced fine-stranded and particulate β-lactoglobulin gels

Molecular structural changes due to heat-induced gelation of β-lactoglobulin were investigated by Raman scattering spectroscopy. Transparent fine-stranded gels were prepared from 15% w/v β-lactoglobulin at pH 7 or 2 by heating at 80 °C for 60 min. Opaque particulate gels were formed at pH 7 with 0.1 or 0.3 mol/dm3 NaCl or at pH 5.4. Heat-induced gelation caused a general increase in disordered secondary structural fractions and more strongly hydrogen bonded tyrosine residues. The intensity of the tryptophan vibrational band around 760 cm−1 increased on the formation of fine-stranded gels but decreased on the formation of particulate gels. A significant involvement of hydrophobic interactions on forming particulate gels was suggested by the intense band at 1345 cm−1, assigned to CH bending vibrations. Secondary structures appeared to be preserved better in particulate gels than in fine-stranded gels: a decrease in α-helix was more pronounced in fine-stranded gels, while considerable fractions of β-sheet structures remained in both types of gels, consistent with known heat-resistance of the β-barrel structure in β-lactoglobulin. Raman spectroscopy allows discrimination between gel network types at certain amino acid side chain levels as well as the secondary structure level.