Why Sauces Split: The Science of Emulsions in Vinaigrettes, Mayonnaise and Butter Sauces

You whisk a vinaigrette, and minutes later the oil is floating on top of the vinegar. You make mayonnaise, and suddenly it turns from thick and creamy to a greasy, runny mess. You finish a pan sauce with butter, and it separates into a puddle of fat. These problems have a common cause: a broken emulsion. Many of the most delicious sauces in the kitchen, from vinaigrettes and mayonnaise to hollandaise, beurre blanc and creamy pasta sauces, are emulsions, mixtures of fat and water that normally refuse to stay together. Understanding why they form and why they break makes it much easier to make them reliably and to rescue them when they go wrong.

What is an emulsion?

An emulsion is a mixture of two liquids that do not naturally mix, in which tiny droplets of one liquid are dispersed throughout the other. In cooking, these liquids are usually fat, such as oil or butter, and water, which includes vinegar, lemon juice, stock, wine and the water in egg yolks.

There are two types:

• Oil-in-water emulsions, where droplets of oil are suspended in water. Mayonnaise, vinaigrettes, hollandaise and milk are examples.
• Water-in-oil emulsions, where droplets of water are suspended in fat. Butter is a natural example.

Why oil and water separate

Water molecules are attracted to each other, and oil molecules are attracted to each other, but oil and water molecules do not attract each other. When you shake oil and water together, the oil breaks into droplets, but these droplets quickly merge again and float to the top, because oil is less dense than water. Left alone, every simple mixture of oil and water separates.

To keep an emulsion stable, you need two things: small droplets and a helper called an emulsifier.

Emulsifiers: the peacemakers

An emulsifier is a molecule with two different ends. One end is attracted to water, the other to fat. When you whisk an emulsion, emulsifier molecules surround the oil droplets, with their fat-loving ends facing into the oil and their water-loving ends facing out into the water. This forms a protective layer that keeps the droplets from merging back together.

Common emulsifiers in the kitchen include:

• Egg yolks, which contain lecithin and proteins. They are the key to mayonnaise, hollandaise and Caesar dressing.
• Mustard, which contains compounds in its seed coat that help stabilize vinaigrettes.
• Honey and garlic, which add some thickness and stabilizing compounds.
• Starch, such as the starch in pasta water, which thickens the water and helps keep fat droplets suspended.
• Milk proteins, in cream and cheese.
• Gelatin and stocks, which add body to pan sauces.

Thickeners and stabilizers

Besides emulsifiers, thickening the water phase also helps stabilize an emulsion. When the water is thicker, oil droplets move more slowly and are less likely to merge. That is why reduced stocks, starch and finely pureed vegetables help sauces stay together.

Vinaigrettes: temporary and stable

A basic vinaigrette is a mix of oil and acid, usually in a ratio of about three parts oil to one part vinegar or lemon juice. Without an emulsifier, it separates quickly. This is called a temporary emulsion, and it is fine for many salads, as long as you shake or whisk the dressing right before using it.

To make a more stable vinaigrette:

1. Whisk vinegar or lemon juice with salt, a spoonful of Dijon mustard, and optionally a little honey and minced shallot or garlic.
2. Slowly drizzle in the oil while whisking continuously, or shake everything together vigorously in a jar.
3. The mustard and the small droplets created by whisking keep the dressing creamy for longer.

If it separates, simply whisk or shake it again.

Mayonnaise: a permanent emulsion

Mayonnaise is a remarkable emulsion: about three-quarters oil suspended in a small amount of water, held together by egg yolk.

Basic method:

1. Whisk one egg yolk at room temperature with a teaspoon of mustard, a pinch of salt and a teaspoon of lemon juice or vinegar.
2. Add neutral oil drop by drop at first, whisking constantly. The beginning is the most important stage. Adding oil too quickly overwhelms the emulsifier and the sauce breaks.
3. Once the mixture thickens and looks creamy, you can add the oil in a thin, steady stream.
4. Continue until you have added about 3/4 to 1 cup (180 to 240 ml) of oil for one yolk.
5. Season with more lemon juice and salt. A little water can thin it if it becomes too thick.

An immersion blender in a narrow jar makes mayonnaise in seconds: put all the ingredients in the jar, let the oil settle on top, press the blender to the bottom and turn it on, slowly lifting it as the emulsion forms.

Because homemade mayonnaise uses raw egg yolk, it carries a risk of Salmonella. Use pasteurized eggs, especially for children, older adults, pregnant people and people with weakened immunity. Keep homemade mayonnaise refrigerated and use it within a few days.

Why mayonnaise breaks and how to fix it

Mayonnaise breaks when:

• Oil is added too quickly at the start.
• Too much oil is added for the amount of yolk.
• Ingredients are too cold, making it harder for the emulsion to form.
• The mixture becomes too warm.

How to fix it: Place a fresh egg yolk, or a teaspoon of mustard or a tablespoon of warm water, in a clean bowl. Slowly whisk the broken mayonnaise into it, drop by drop at first, exactly as if it were oil. The new emulsifier will capture the oil droplets again and the sauce will thicken.

Butter sauces: hollandaise and beurre blanc

Hollandaise is an emulsion of melted butter in egg yolks and lemon juice. Beurre blanc is an emulsion of cold butter whisked into a reduction of wine, vinegar and shallots. Both are delicate because they are sensitive to temperature.

• Too hot: Egg yolks in hollandaise can scramble above about 160 °F (71 °C), and butter sauces separate when the butter becomes too hot and its emulsion breaks.
• Too cold: The butter solidifies, and the sauce becomes thick and greasy.

Make these sauces over gentle heat, such as a bowl set over barely simmering water, and keep them warm, but not hot, until serving. A thermos or a warm spot near the stove works well.

To fix a broken hollandaise: Whisk a tablespoon of warm water or a fresh egg yolk in a clean bowl, then slowly whisk in the broken sauce. For beurre blanc, whisk a little cold cream or cold water into the sauce off the heat.

Pan sauces and pasta sauces

Many quick sauces are emulsions too. When you finish a pan sauce by swirling in cold butter off the heat, the butter melts gradually and emulsifies into the reduced liquid, creating a glossy sauce. If the pan is too hot, the butter melts too quickly and separates into a greasy layer.

In pasta dishes like cacio e pepe or aglio e olio, starchy pasta water acts as the emulsifier and thickener. Tossing pasta vigorously with oil or cheese and a splash of pasta water creates a creamy, cohesive sauce. If the sauce looks oily, add more pasta water and toss again. If cheese clumps, the pan was too hot; add cheese off the heat and use finely grated cheese.

Cream sauces and dairy

Cream is itself an emulsion of fat droplets in water, stabilized by milk proteins. Heavy cream, with a high fat content, is quite stable and can be boiled. Milk, half-and-half and yogurt contain less fat and more proteins, which curdle more easily when boiled, especially with acid or salt.

To avoid curdling:

• Use full-fat dairy for sauces that will be heated.
• Add acidic ingredients like lemon juice or wine after the dairy has been incorporated, or cook them first.
• Temper yogurt by whisking a little hot liquid into it before adding it to the pot.
• Heat gently and avoid boiling milk-based sauces.

General rules for stable emulsions

1. Start with an emulsifier, such as egg yolk, mustard, starch or a little cream.
2. Add fat slowly at the beginning, especially when making mayonnaise or hollandaise.
3. Whisk vigorously to create small droplets.
4. Control the temperature: not too hot, not too cold.
5. Keep the ratio of fat to water reasonable. Too much fat for the available water and emulsifier will cause the sauce to break.

Frequently asked questions

Why does my vinaigrette separate? Oil and vinegar naturally separate. Adding mustard or honey and whisking or shaking vigorously helps it stay mixed longer. Shake it again before serving.

Can I save broken mayonnaise? Yes. Whisk the broken mixture slowly into a fresh egg yolk, a little mustard or a tablespoon of warm water.

Why does my cream sauce curdle? It may have boiled too hard, contained too much acid, or been made with low-fat dairy. Use heavy cream and heat gently.

What is the best oil for mayonnaise? Neutral oils like canola, sunflower or grapeseed oil produce mild mayonnaise. Extra-virgin olive oil can taste bitter when blended at high speed; mix it with neutral oil or whisk it in by hand.

Why does pasta water help sauces? It contains starch released from the pasta. The starch thickens the water and helps fat and water combine into a smooth, glossy sauce.

The bottom line

Many great sauces are emulsions: mixtures of fat and water held together by emulsifiers like egg yolk, mustard, starch and dairy proteins. Make them by adding fat slowly to the emulsifier, whisking vigorously and controlling temperature. When a sauce breaks, start again with a fresh emulsifier and slowly whisk the broken sauce into it. With these principles, vinaigrettes stay creamy, mayonnaise stays thick, butter sauces stay glossy and pasta sauces cling beautifully.