Mayonnaise, hollandaise and beurre blanc are the same problem with three solutions: how do you get fat and water into one sauce without them separating. Understand the emulsion, and nothing ever splits again, or at least you know how to repair it.

An emulsion is a mixture of two liquids that normally don’t mix, such as oil and water. By dividing one into tiny droplets in the other, and through an emulsifier that keeps the droplets stable, a smooth, creamy sauce results.

What is an emulsifier?

An emulsifier is a substance that holds on to both sides: one part attracts water, another fat. That way it keeps the droplets together. In the kitchen the main emulsifiers are:

egg yolk, thanks to lecithin: the base of mayonnaise and hollandaise;
mustard, which makes the emulsion extra stable;
milk protein and butter, as in beurre blanc;
honey in a vinaigrette.

Cold emulsions

Vinaigrette: a temporary emulsion

The simplest emulsion is a vinaigrette: about 3 parts oil to 1 part vinegar or lemon juice. Whisk them vigorously until they mix, preferably with a teaspoon of mustard as an emulsifier. Without an emulsifier the mixture stays bound for a few minutes and then separates again. That is a temporary emulsion. Just whisk it again before serving.

Mayonnaise: a stable emulsion

Put 1 egg yolk with 1 teaspoon of mustard, a splash of vinegar or lemon juice and a pinch of salt in a bowl.
Whisk everything smooth.
While whisking constantly, add the oil drop by drop, the first few tablespoons drop by drop.
When the sauce thickens, you can add the oil in a thin stream.
Add about 200 to 250 ml of oil per yolk in this way and season.

The secret is starting slowly. If you add the oil too fast, the yolk can no longer coat the droplets and the sauce splits. All ingredients are preferably at room temperature.

Mayonnaise contains raw egg. Use fresh eggs and keep the sauce covered in the fridge, for no more than two days. Pregnant women and vulnerable people preferably use pasteurised egg.

Warm emulsions

Hollandaise

Hollandaise is a warm emulsion of egg yolk and melted butter, seasoned with lemon juice and a pinch of salt. It is one of the five mother sauces.

Melt 150 to 200 g of butter and let it cool slightly. If you like, skim off the foam.
Whisk 3 egg yolks with 2 tablespoons of water in a bowl over a pan of gently simmering water (bain-marie).
Whisk until the yolks are pale, thick and foamy. The mixture must not get too hot.
Take the bowl off the pan and whisk the butter into the yolks drop by drop.
Season with lemon juice, salt and a pinch of cayenne pepper.

The temperature is crucial. Above about 70 °C the yolks set and the sauce splits. Keep the hollandaise warm at about 60 to 65 °C, so not below 60 °C, and serve it within the hour. Don’t keep or eat it the next day.

Béarnaise is a derivative: a reduction of vinegar, shallot and tarragon replaces the lemon juice. It goes with grilled meat.

Beurre blanc

Boil 2 finely chopped shallots with 100 ml of white wine and 2 tablespoons of white wine vinegar until about 2 tablespoons remain.
Turn the heat low and whisk in cubes of cold butter, one by one, about 150 to 200 g in total.
Let each cube almost melt before you add the next.
Strain if you like, season with salt and a pinch of pepper and serve immediately.

Beurre blanc is very sensitive to temperature. Keep it warm, but don’t let it boil, and serve it quickly. It goes with fish and shellfish.

Cold or warm: the differences

Sauce	Emulsifier	Fat	Temperature
Vinaigrette	mustard	oil	cold, temporary
Mayonnaise	egg yolk	oil	cold
Hollandaise	egg yolk	melted butter	warm, about 60 to 65 °C
Beurre blanc	milk protein and reduction	cold butter	warm, below boiling point

Why does a sauce split?

The fat was added too fast, making the droplets too large so they merge.
The sauce is too warm, making the egg yolk set or the butter too thin.
The sauce is too cold, making the butter solidify.
There is too little emulsifier for the amount of fat.
The ingredients are not at the same temperature.

Rescuing a split sauce

Put the sauce off the heat.
Put a fresh egg yolk or a teaspoon of mustard, or a tablespoon of warm water, in a clean bowl.
Whisk the split sauce drop by drop into the new mixture, just as you add the oil to a new mayonnaise.
Keep whisking until the sauce is smooth and bound again.

With a beurre blanc or hollandaise that has become too hot, it often helps to whisk a tablespoon of cold water or an ice cube into the sauce.

Practise with a mayonnaise

Put a bowl on a folded tea towel so it doesn’t slide.
Whisk 1 yolk, 1 teaspoon of mustard and 1 teaspoon of vinegar.
Add the oil drop by drop while whisking.
Increase the pace only once the sauce is thick.
Season with salt and lemon juice.

If it doesn’t work, start again with a new yolk and whisk the split sauce into it. That way you learn the repair straight away.

Frequently asked questions about emulsions

Can I make mayonnaise with a stick blender?

Yes, that is the easiest way. Put all the ingredients in a tall beaker, set the blender on the bottom and blend until the sauce forms, then slowly pull the blender upward.

Why does my mayonnaise turn runny?

The oil was added too fast or there was too little emulsifier. Start again with a new yolk.

How long can I keep hollandaise warm?

For no more than about an hour, at 60 to 65 °C, for example in a warming bowl. After that it is no longer safe or tasty.

Can I reheat hollandaise?

That is tricky. You can heat it very carefully in a bain-marie, but it splits easily. Preferably make it fresh.

Are emulsions gluten-free?

Yes, mayonnaise, hollandaise and beurre blanc contain no gluten. They do contain egg, mustard or milk, which are allergens.

Emulsions at a glance

An emulsion is fat and water in one sauce, held together by an emulsifier.
Egg yolk and mustard are the best-known emulsifiers.
Add the fat drop by drop.
Keep hollandaise at 60 to 65 °C.
Don’t boil beurre blanc, keep it warm.
To rescue a split sauce: whisk it into a fresh yolk or water.
Use fresh eggs and mind the allergens.

Remember this

An emulsion isn’t about luck. It is about patience, temperature and order: adding the fat slowly, watching the temperature and knowing how to repair a split sauce. Whoever can do that makes a sauce that looks professional with a few simple ingredients.