beta-Lactoglobulin (beta-lg) forms fibrils when heated at 80 degrees C, pH 2, and low ionic strength (<0.015 mM). When formed at protein concentrations <3%, these fibrils are made up of peptides produced from the acid hydrolysis of the beta-lg monomer. The present study investigated the effects of the polyhydroxy alcohols (polyols) glycerol and sorbitol (0-50% w/v) on beta-lg self-assembly at pH 2. Glycerol and sorbitol stabilize native protein structure and modulate protein functionality by preferential exclusion. In our study, both polyols decreased the rate of beta-lg self-assembly but had no effect on the morphology of fibrils. The mechanism of these effects was studied using circular dichroism spectroscopy and SDS-PAGE. Sorbitol inhibited self-assembly by stabilizing beta-lg against unfolding and hydrolysis, resulting in fewer fibrillogenic species, whereas glycerol inhibited nucleation without inhibiting hydrolysis. Both polyols increased the viscosity of the solutions, but viscosity appeared to have little effect on fibril assembly, and we believe that self-assembly was not diffusion-limited under these conditions. This is in agreement with previous reports for other proteins assembling under different conditions. The phenomenon of peptide self-assembly can be decoupled from protein hydrolysis using glycerol.