The repartition of whey protein microgels and caseins between fat droplet surface and the continuous phase governs the heat stability of emulsions
Résumé
Whey protein microgels (WPM) are highly stable on heating and form Pickering-like emulsions exhibiting longterm resistance to coalescence. However, the heat stability of WPM emulsions is unclear and depends on the emulsion characteristics (WPM concentration, presence of non-microgel proteins such as caseins). The objective of this study was to investigate the heat-stability of WPM emulsions in the presence of caseins in order to clarify the role of the two types of proteins in the mechanism of heat-stabilization/destabilization. Emulsions (30 wt% milkfat) containing WPM (from 2.34 to 6.84 wt%) and non-microgel proteins, mainly caseins (from 0 to 0.81 wt %), were prepared at pH 7. A comparison of the emulsions before and after heating at 120 °C up to 30 min allowed their classification in three categories: viscous emulsions that gelled on heating (emulsions E1), fluid emulsions that gelled on heating (emulsions E2) and fluid emulsions even after heating (emulsion E3). A characterization of the fat droplet surface (protein interfacial load, protein composition, structure of the adsorbed entities by transmission electron microscopy) indicated emulsions E1 and E2 had WPM adsorbed at the fat droplet surface but their structural rearrangement did not explain the heat-induced modifications. Emulsions E3 had a fat droplet interfacial layer composed of caseins whereas the WPM were only located in the continuous phase. These results demonstrate that the WPM do not contribute to emulsion heat-stability but they do not induce heat instability as long as they are not situated at the fat droplet interface. Knowing the total interfacial area, the amount of caseins to obtain heat stable emulsions containing WPM can be evaluated because the caseins dominate WPM adsorption at the fat droplet surface. This knowledge will allow controlling the stability of dairy emulsions in a wider whey protein concentration range than when using native whey proteins.
