Making Stabilised Emulsion Sauces

How oil becomes part of a smooth sauce

Oil and water are immiscible: they do not normally mix. Vinegar contains water, so oil and vinegar also separate into layers when left standing. Yet these ingredients can be combined into a smooth sauce by forming an emulsion — tiny droplets of one liquid dispersed through another.

Whisking breaks oil into small droplets and spreads them through the watery liquid. This process is called emulsification. The oil has not dissolved: it is still present as droplets, but they are small enough for the mixture to look uniform.

A simple oil-and-vinegar dressing demonstrates a temporary emulsion. Shaking or whisking brings the ingredients together, but the droplets gradually join together again and the dressing separates. It needs mixing again before serving.

Keeping the droplets apart

A stabilised emulsion resists separation because it contains an emulsifier. Egg yolk contains an emulsifier called lecithin, which makes it useful in mayonnaise and hollandaise.

An emulsifier has a part attracted to water and a part attracted to fat. It acts at the boundary between an oil droplet and the surrounding watery liquid, helping prevent droplets from joining together. Whisking creates the droplets; the emulsifier helps keep them separate. Both are important.

In mayonnaise, oil droplets are dispersed through the watery ingredients supplied by the egg yolk and vinegar or lemon juice. As more oil is incorporated as small droplets, the sauce becomes thick and creamy rather than forming a separate oily layer.

Making mayonnaise: gradual addition and continuous whisking

Mayonnaise is a useful way to demonstrate the technical skill of making a stabilised emulsion. For one batch, use 3 egg yolks, 2 teaspoons of vinegar or lemon juice, ½ teaspoon of English mustard, 250 ml of olive oil and a small pinch of salt. Have 1 teaspoon of boiling water available to thin the sauce if needed.

Use a large bowl and a balloon whisk. Set the bowl on a damp cloth so that it stays steady while you whisk and pour.

1. Separate the egg yolks from the whites and place the yolks in the bowl.
2. Add the vinegar or lemon juice, mustard and salt. Whisk these ingredients together before adding the oil.
3. Add only a small amount of oil and whisk until it is fully incorporated. There should be no visible pool of oil before you add the next amount.
4. Continue adding oil a little at a time while whisking thoroughly. This gives you time to disperse each addition into small droplets rather than allowing a large amount of oil to collect together.
5. Continue until the oil is incorporated and the sauce is smooth, thick and uniform. These are the signs that the emulsion has formed successfully.
6. Adjust the flavour with a little more vinegar or mustard if needed. If the mayonnaise is too thick, carefully whisk in a little of the measured boiling water.

The important control is not simply how long you whisk: it is coordinating small additions of oil with thorough whisking. Adding oil faster than you can incorporate it can cause the sauce to split, leaving visible oil and a less uniform mixture.

Working safely

Mayonnaise is not cooked, so use pasteurised egg yolks where possible, particularly when serving people who are more vulnerable to foodborne illness. Wash your hands after handling eggs, use clean equipment and prevent raw egg from contaminating other foods. Keep the finished mayonnaise refrigerated rather than leaving it at room temperature.

Take care with boiling water: add only a small amount and avoid splashing. Thinning mayonnaise with boiling water does not cook the whole sauce or make raw egg safe.

Applying the skill to hollandaise

Hollandaise uses melted butter rather than oil, but the same principle applies: disperse the fat gradually and use egg yolk to stabilise the droplets.

Whisk egg yolks with a little reduced white wine vinegar in a heatproof bowl over a pan of simmering water. Whisk continuously until a golden, foam-like mixture forms. Remove it from the heat, then gradually whisk in melted, skimmed butter until the sauce has a thick, mayonnaise-like consistency. A little warm water can loosen it if necessary.

Gentle heating and continuous whisking matter here: excessive heat can cook the yolks into lumps instead of producing a smooth sauce. Keep the bowl above, rather than in contact with, the simmering water, and use oven gloves when handling the hot bowl. Avoid the rising steam. The successful result is a smooth, thick sauce without separate pools of butter.

Formation and stability

• Whisking: breaks oil or melted butter into tiny droplets dispersed through a watery liquid.
• Egg-yolk lecithin: helps prevent droplets joining together and separating.
• Mayonnaise and hollandaise: stabilised emulsions.
• Simple vinaigrette: temporary emulsion; separates on standing.

Mayonnaise technique

Whisk yolks, vinegar or lemon juice, mustard and seasoning → add oil a little at a time → whisk each addition in fully → continue until smooth and thick.