A study of alcohol-induced gelation of β-lactoglobulin with small-angle neutron scattering, neutron spin echo, and dynamic light scattering measurements

Gelation of β-lactoglobulin (β-Lg) in various alcohol–water mixtures with 0.1 M (M = mol L−1) hydrochloric acid was investigated with small-angle neutron scattering (SANS), neutron spin echo (NSE), and time-resolved dynamic light scattering (TRDLS) measurements. The β-Lg in alcohol–water solutions undergoes gelation at specific alcohol concentrations where the alcohol-induced α-helical structure of β-Lg is stabilized. The SANS profiles showed that β-Lg exists as a single molecule at a low alcohol concentration. With increasing alcohol concentration, the profiles indicate a power law behavior of ∼1.7 when the samples gelate. These behaviors were observed in all alcohol–water mixtures used, but the alcohol concentrations where the SANS profiles change shift to a lower alcohol concentration region with an increase in the size of the hydrophobic group of the alcohols. Apparent diffusion constants, obtained from the intermediate scattering function (ISF) of NSE and the intensity time correlation function (ITCF) of TRDLS, mainly depend on the viscosity of alcohol–water mixtures before gelation. After gelation, on the other hand, the ISFs of gels do not change appreciably in the range of the NSE time scale, indicating the microscopically rigid structure of β-Lg gel. The ITCF functions obtained from TRDLS follow a double exponential decay type before gelation, but a logarithmic one (exponent α = 0.7) after gelation. It is most likely that the alcohol-induced gelation undergoes a similar mechanism to that for the heat-induced one at pH = 7 where β-Lg aggregates stick together to form a fractal network, although the gelation time is faster in the former than in the latter.