Permanent Emulsions

Mayonnaise is also a mixture of oil and vinegar—in this case, an oil-in-water emulsion—but the two liquids do not separate. This is because the formula also contains egg yolk, which is a strong emulsifier. A substance in the egg yolk called lecithin forms a layer around each of the tiny oil droplets and holds them in suspension so that they do not recombine (see sidebar for more complete explanation).

The harder the mayonnaise is beaten to break up the droplets, the more stable the emulsion becomes. All emulsions, whether permanent or temporary, form more easily at room temperature, because chilled liquid is harder to break up into small droplets.