The effect of pH and temperature pre-treatments on the physicochemical and emulsifying properties of whey protein isolate

•Emulsifying properties of WPI was highly influenced by solvent conditions.•Decreased size of WPI aggregates led to improved emulsion stability.•The emulsion stability was found to be greatest away from whey protein's pI.

The overall goal of this research was to investigate the emulsifying properties of whey protein isolate (WPI) as a function of pH (3.0, 5.0, 7.0) and temperature pre-treatments (25, 55, 85 °C). Specifically, the physicochemical (charge and hydrophobicity, size, interfacial tension) and emulsifying (emulsification activity (EAI) and stability indices (ESI)) properties were assessed. Overall, hydrophobicity was greatest at pH 5.0/85 °C. Charge was found to decrease linearly from ∼ +40 mV at pH 3.0 to ∼ −40 mV at pH 7.0, where net neutrality was observed between pH 5.1 and 5.4. Aggregate size was greatest at pH 5.0 relative to the other pHs, however it declined in size at pH 5.0 from 1589 nm to 1450 nm and then to 832 nm as temperature pre-treatments increased from 25 °C to 55 °C and then to 85 °C, respectively. Interfacial tension was reduced from ∼28 mN/m to ∼14-18 mN/m for systems in the absence and presence of WPI, respectively, with little differences seen among treatments. EAI was found to increase under conditions where WPI aggregation was reduced, whereas ESI increased under conditions away from its isoelectric point where electrostatic repulsive forces were strongest and under conditions where surface hydrophobicity was reduced.