Emulsion Science: Lecithin and Stabilizers
Oil and water are immiscible — left alone they separate immediately. An emulsion is a stable dispersion of one liquid in another, maintained by an emulsifier. Emulsifiers are molecules with a dual nature: one end is hydrophilic (attracted to water) and the other is lipophilic (attracted to oil). The emulsifier aligns at the interface between oil droplets and the surrounding water phase, surrounding each oil droplet and preventing it from coalescing with other oil droplets. The most important culinary emulsifiers are: lecithin (found in egg yolk — specifically phosphatidylcholine — the workhorse of mayonnaise and hollandaise); mustard (which contains mucilage compounds and is used in vinaigrettes); and milk proteins (casein — used in dairy-based sauces). Emulsions are either oil-in-water (O/W) or water-in-oil (W/O). Mayonnaise is an O/W emulsion: tiny oil droplets suspended in the aqueous egg yolk phase. Butter is a W/O emulsion: tiny water droplets suspended in the fat matrix. Temperature, oil addition rate, and ratio all affect emulsion stability. Adding oil too quickly overwhelms the available emulsifier molecules before they can coat new droplets — the emulsion breaks. The ratio of emulsifier to oil has a practical ceiling: mayonnaise can incorporate up to approximately 80% oil by volume; exceeding this produces a greasy, broken sauce regardless of technique.