Mayonnaise is a stable emulsion of oil, egg yolk, and an acid, either vinegar or lemon juice. The proteins and lecithin in the egg yolk serve as emulsifiers in mayonnaise.
It can contain as much as 80% vegetable oil, usually soybean but sometimes olive oil. Water makes up about 7% to 8% and egg yolks about 6%. Some formulas use whole eggs instead of just yolks. The remaining ingredients include vinegar (4%), salt (1%), and sugar (1%).