Spreading of partially crystallized oil droplets on an air/water interface

The influence of crystalline fat on the amount and rate of oil spreading out of emulsion droplets onto either a clean or a protein-covered air/water interface was measured for β-lactoglobulin stabilized emulsions prepared with either anhydrous milk fat or a blend of hydrogenated palm fat and sunflower oil. At a clean interface, liquid oil present in the emulsion droplets was observed to completely spread out of the droplets unimpeded by the presence of a fat crystal network. Further, the presence of a fat crystal network in the emulsion droplets had no effect on the rate of oil spreading out of the droplets. At a protein-covered interface, the spreading behavior of emulsion droplets containing crystalline fat was evaluated in terms of the value of the surface pressure (ΠAW) at the point of spreading; ΠAW at spreading was unaffected by the presence of crystalline fat. We conclude it is unlikely that the role of crystalline fat in stabilizing aerated emulsions such as whipped cream is to reduce oil spreading at the air/water interface. However, the temperature of the system did have an effect: spontaneous spreading of emulsion droplets at clean air/water interfaces occurred for systems measured at 5 °C, but not for those measured at 22 or 37 °C. Thus, temperature may play a more important role in the whipping process than commonly thought: the entering and spreading of emulsion droplets was favored at lower temperatures because the surface pressure exerted by protein adsorbed at the air/water interface was reduced. This effect may facilitate the whipping process.