Mechanisms of structure formation in particulate gels of β-lactoglobulin formed near the isoelectric point

Particulate gels are known to be formed by bovine beta-lactoglobulin near the isoelectric point when partial unfolding is allowed to occur under heating. The aggregation process of the protein has been investigated within the context of a nucleation and growth process by preparing gels under precisely controlled thermal histories. This was achieved using a Differential Scanning Calorimeter (DSC) to provide controlled heating rates, and known final temperatures and incubation times. The resulting particulate gels were characterized by their particle size and polydispersity using Environmental Scanning Electron Microscopy (ESEM), which permits hydrated samples to be observed. Particle size was found to decrease with increasing final temperature, with the aggregation taking longer to reach completion for lower temperatures. Particle size was also found to decrease with increasing heating rate. This system could be modelled as evolving via nucleation and growth by taking into account the fact that the concentration of the aggregating species was varying as a function of temperature as well as time. The intrinsic tryptophan fluorescence as a function of temperature was used as a guide to the fraction of unfolded protein in solution, thereby permitting successful comparisons between the model predictions and the particle sizes to be made.