Why Sauces Split: Emulsions and Pan Sauces

What an emulsion is

Oil and water do not mix. An emulsion is an unstable state in which one of them has been broken into tiny droplets and dispersed through the other. Mayonnaise, hollandaise, a cream pasta sauce, the opaque broth of ramen — all emulsions.

Unstable is the operative word. An emulsion is always trying to separate, and what holds it together is an emulsifier.

Three emulsifiers worth knowing

• Egg yolk (lecithin): the most powerful. Mayonnaise, hollandaise, Carbonara.
• Starch: physically traps the droplets between water and fat. Pasta water, potato cooking water, a flour roux.
• Mustard, honey, grated garlic: weak, but genuinely effective — the reason a spoon of mustard goes into a vinaigrette.

Pasta water is not water

Water that has boiled pasta carries dissolved starch, and that starch acts as the glue between olive oil and water. It is why the sauce in Aglio e Olio clings to the strands instead of splitting into a puddle of oil.

Hence the advice to save a cup before draining. Plain water thins a sauce; pasta water thickens it, and it is salted, so it seasons at the same time.

Temperature is usually the culprit

The most common reason an emulsion breaks is heat.

• Egg-based sauces coagulate above about 70°C. Make Carbonara over a live flame and you get scrambled eggs, not sauce. Take the pan off the heat and use only its residual warmth.
• Cream splits if you boil it hard — the fat clumps and the whey separates. Bring it just to the edge of a simmer.
• Cold butter dumped into a hot pan melts and separates at once. Lower the heat and stir in cold butter one piece at a time (monter au beurre). The fat emulsifies into the liquid and the sauce turns glossy.

Acid is the other culprit

Pour lemon juice or wine into cream, or cream into them, and you get different results. Dairy added to a strongly acidic pan curdles immediately. Reduce the acid first to soften it, then add the cream.

Bringing a split sauce back

Things to try before starting over:

1. Off the heat, and let it cool. Most emergencies are simply too much heat. 2. Add a splash of cold water or an ice cube and whisk hard. Works well for the hollandaise family. 3. Whisk a fresh yolk in a clean bowl and dribble the broken sauce into it. You are re-emulsifying from scratch; this has the highest success rate. 4. Add a little starchy water. For a pasta sauce, two tablespoons of pasta water usually fixes it.

Thick, but not emulsions

Some things are easy to confuse. The creaminess of Risotto is starch released from the rice, not an emulsion; a butter sauce on Gnocchi is one, but the softness of Chocolate Mousse is fat crystals and aerated foam. Keeping the mechanisms apart is what tells you which thing to fix when it fails.