Formation of shelf stable Pickering high internal phase emulsions (HIPE) through the inclusion of whey protein microgels†
High internal phase emulsions (HIPE) prepared using whey protein microgels (WPMs) as a surfactant were demonstrated to have substantially higher stability than HIPEs prepared using similar loadings of non-gelled whey protein isolate (WPI) or Tween 20. Microgel colloids were prepared from WPI solutions by heat treatment at 85 °C in a narrow pH range (5.8–6.0) to particle sizes of approximately 90, 160 and 350 nm in diameter. ζ-potentials of the WPM increased in negativity with decreasing particle size from −7.4 ± 2.5 down to −21.1 ± 0.9 at 90 nm. All WPMs conferred high stability to corn oil based HIPE when used as an emulsifier. Light microscopy and cryo-scanning electron microscopy showed that both increasing WPM concentration and decreasing WPM particle size produced increasingly smaller and more hexagonally shaped corn oil emulsion droplets; WPI and Tween 20 based HIPE droplets were generally smaller and spherical in shape. The HIPE (75% w/w corn oil) produced with 1% (w/w) WPM as an emulsifier showed stability through 6 months storage at 4 °C at all WPM sizes tested, while the HIPE prepared with 1% (w/w) WPI or Tween 20 exhibited significant creaming. WPM and WPI based HIPE both showed thermal stability at 70 °C and 95 °C while the heating of Tween 20 based HIPE resulted in droplet coalescence and oil-phase separation. HIPE production with WPMs significantly improved the viscoelastic properties of the HIPE, imparting drastic increases in yield stress, critical stress, complex modulus and elastic modulus over HIPE prepared with WPI or Tween 20. The more rigid rheology of the WPM HIPE indicated by these data is likely the primary mechanism driving the improved stability of these emulsions.