Bulk self-aggregation drives foam stabilization properties of whey protein microgels

Whey protein microgels (WPM) produced by controlled heat treatment of native whey protein isolate were characterized by a spherical and compact shape as well as a low polydispersity index. They exhibited maximum self-aggregation close to their isoelectrical pH (IEP) of 4.0–5.5, leading to a size increase from about 270 nm to several microns. The bulk viscosity of the WPM was increasing linearly with the concentration in WPM until an exponential increase was visible for a protein concentration ranging between 9.5 and 13.4 wt%. The intrinsic viscosity (∼10 mL g−1) and specific volume (∼4 mL g−1) of whey protein microgels were very close to those reported for casein micelles. WPM dispersions were able to reduce surface tension, but likely this phenomenon was due to the release of some surface-active material from the WPM. Stable foams were produced at 5 wt% WPM content and pH 5.0. In these conditions, the liquid drainage rate was strongly reduced as well as the disproportionation of air bubbles. It was concluded that WPM were able to stabilize foams close to their IEP by in situ self-aggregation leading to an arrest of the drainage enabling to counteract the gas flux responsible for bubble disproportionation. To our knowledge, this the first time that such mechanism has been described for food-grade protein based particles.