Dry heat induced whey protein-lactose conjugates largely improve the heat stability of O/W emulsions

Whey protein–lactose conjugates were prepared by dry heat incubation (60 °C, 74% relative humidity) at 1:1 M ratio of protein amino groups to reducing sugar. Pulsed-field gradient-nuclear magnetic resonance (PFG-NMR) was introduced as a new and absolute method to determine the degree of conjugation without any sample pretreatment, based on the different diffusion behaviour of free and protein-bound lactose. Whereas the TNBS method revealed 73% conjugation, PFG-NMR results indicated that only 37% of lactose was bound to the protein after 8 h incubation. Circular dichroism spectra showed no change of WPI secondary structure upon dry heat incubation, whereas the electrophoretic mobility shifted towards more negative values. The emulsifying and, more importantly, heat stabilising properties of the conjugates were subsequently evaluated in oil/water emulsions. Eight hours of incubation was sufficient to produce conjugates that successfully stabilised the emulsion against heating (80 °C, 20 min), regarding the particle size and viscosity.