Emulsify

To emulsify is to beat oil or melted fat into a watery liquid, such as vinegar, lemon juice or stock, so that it breaks into tiny droplets and the two hold together as one smooth mixture.

Oil and water separate if left alone, so most kitchen emulsions use something to help: egg yolk in mayonnaise and hollandaise, mustard in a vinaigrette, or the starch in pasta water when butter or cheese is tossed through spaghetti.

The method is usually the same. Start with the watery part and the helper, then add the fat slowly while whisking hard, a few drops at a time at first, then in a thin stream once it has thickened. Adding the oil too fast is the usual reason mayonnaise splits.

A split emulsion can often be rescued. Put a fresh yolk, or a spoonful of water or mustard, in a clean bowl and whisk the split mixture into it a little at a time.

Warm butter sauces split if they get too hot, so keep the heat low.

The word is the same in British and American English.