One of the most important roles of emulsifiers is to facilitate the formation of small emulsion droplets during homogenization of the oil and aqueous phases. An effective emulsifier should rapidly adsorb to the freshly formed droplet surfaces, reduce the interfacial tension by an appreciable amount to facilitate droplet disruption and provide a protective coating that prevents the droplets from aggregating with their neighbors [2]. Protein emulsifiers differ in the rate at which they adsorb to droplet surfaces during homogenization, in the minimum amount that is required to saturate the droplet surfaces, and in their ability to protect droplets against coalescence under different environmental conditions [19]. For example, casein micelles adsorb more rapidly to droplet surfaces than individual casein molecules during high pressure valve homogenization, but more protein is required to saturate the droplet surfaces for casein micelles than individual casein molecules. The amount of total protein present also plays a major role in determining the stability of the droplets to aggregation during homogenization. It is convenient to divide the influence of emulsifier concentration on droplet size into two regions
The influence of emulsifier type (whey protein isolate, casein and Tween 20) on the stability of oil-in-water emulsions to fat crystallization and ice formation have recently been carried out [21], [22] and [24]. These studies have shown that dairy proteins (WPI and casein) provide better protection against droplet coalescence than small molecule surfactants when the oil phase is partially crystalline, but that they can only provide a limited degree of protection against droplet coalescence when both water and oil crystallization occurs. This has been attributed to the ability of the proteins to form relatively thick interfacial membranes that are difficult to penetrate by fat crystals. The stability of emulsions to water crystallization can be improved by incorporating cryoprotectants (e.g., sucrose) or by creating multiple layers of emulsifiers around the oil droplets