Denaturation and aggregation processes in thermal gelation of whey proteins resolved by differential scanning calorimetry

Gelation of globular proteins on heating involves two separate stages. The first is partial unfolding (denaturation) of the native globular structure; the second is intermolecular aggregation. Denaturation involves dissociation of intramolecular bonds (non-covalent and, in some cases, disulfide) and is therefore an endothermic process. The aggregation step involves formation of new bonds between protein molecules, and would therefore be expected to give a differential scanning calorimetry (DSC) exotherm on heating, but numerous previous studies of the thermal gelation of whey proteins, carried out on conventional (fast scanning) DSC calorimeters (typical sample mass similar to 15-50 mg), have shown only endothermic transitions. In the present work, however, we have observed the endothermic (denaturation) and exothermic (aggregation) processes in thermogelation of whey protein isolate (WPI) as separate transitions in DSC heating traces recorded on a Setaram microcalorimeter (sample mass similar to 850 mg). Under conditions where aggregation occurs much more slowly than denaturation (low protein concentration; low ionic strength) the two transitions are well resolved, with the exotherm from aggregation following the endotherm from denaturation. The position of the exotherm, however, appears to be time-dependent rather than temperature-dependent. On reduction in heating rate, the apparent peak-maximum temperature of the aggregation exotherm decreases towards the (essentially constant) position of the denaturation endotherm, and, at sufficiently low scan rates, the exotherm becomes obscured by the more intense endotherm. Progressive displacement of the exotherm into the temperature range of the denaturation endotherm also occurs in response to changes that accelerate intermolecular aggregation and accompanying gelation (addition of salt; increasing protein concentration). The absence of a detectable exotherm in previous studies using conventional calorimeters is attributed to the much smaller sample mass than in the Setaram instrument, giving much faster heat transfer, which may cause the exothermic heat flow from the slow aggregation process to be swamped by the endothermic heat flow from the more rapid denaturation process. (c) 2006 Elsevier Ltd. All rights reserved.