Why do emulsions like mayonnaise and hollandaise break?

Mayonnaise, hollandaise, béarnaise, beurre blanc, aioli and a shaken vinaigrette are all the same physics. Oil and water do not mix, so the only way to combine them is to smash the oil into droplets small enough to stay suspended and then coat every droplet in something that stops it touching its neighbours. That coating is the emulsifier — lecithin from egg yolk, mustard, the milk proteins in butter and cream. Understanding what the coating is doing tells you every way a sauce can fail and exactly what the rescue has to achieve.

Step by step

1. Picture the sauce as coated droplets, not as a mixture.A stable emulsion is a field of small oil droplets, evenly spaced, each wrapped in emulsifier. Thickness comes from crowding — the droplets are packed so tightly that they cannot flow past each other. That is why mayonnaise is thicker than the oil it is made from.
2. Cause one: more oil than the emulsifier can cover.One egg yolk holds roughly 200ml of oil comfortably as mayonnaise. Push far past that and there is not enough lecithin to coat the new surface area, so droplets meet and merge. The sauce thins first, then breaks. Adding a second yolk raises the ceiling.
3. Cause two: oil added faster than it can be broken up.The whisk is what makes the droplets small. Pour oil in a stream at the start and it arrives as a pool rather than as droplets, and there is nothing to coat. This is by far the most common failure in a home kitchen and it happens in the first thirty seconds.
4. Cause three: heat.Warmth thins the film around each droplet and makes the droplets move faster and collide harder. It is why hollandaise, held warm, is fragile and mayonnaise, kept cold, is not. Above about 85°C in a yolk-based sauce the protein also coagulates into grains and the sauce is finished as a sauce.
5. Cause four: cold, in a butter emulsion.Chill a beurre blanc and the butterfat inside each droplet crystallises, and the crystals puncture the film from inside. This is why a beurre blanc cannot be refrigerated and reheated, and why a mayonnaise made with fridge-cold ingredients often refuses to come together at all.
6. Rescue by building a new emulsion, never by whisking the broken one.Put a fresh emulsifier — a yolk, a teaspoon of mustard, or a tablespoon of warm water — in a clean bowl, whisk one teaspoon of the broken sauce into it until thick, then add the rest a spoonful at a time. You are treating the split sauce as if it were the oil.

Tips

• Everything at the same temperature is worth more than any other precaution. A fridge-cold yolk meeting room-temperature oil is a classic cause of a mayonnaise that never starts.
• Mustard is both a flavour and a genuine emulsifier. A teaspoon in a mayonnaise or a vinaigrette makes it measurably more stable.
• A little water in the mixture is not a fault. Emulsions need a water phase; a sauce that is nearly all oil has nowhere for the droplets to sit and is right on the edge of breaking.
• A stick blender makes smaller droplets than a whisk, which is why blender mayonnaise is very stable and very thick.

Common mistakes

• Whisking harder at a sauce that has already split — The droplets have already merged. Agitation without a fresh emulsifier just moves the two layers around and often makes the pools larger.
• Adding more oil to a sauce that looks thin — Thin usually means the emulsifier is at its limit. More oil is the one thing guaranteed to finish it off.
• Treating a hot and a cold emulsion as the same problem — A hollandaise usually breaks from heat and needs cooling before it can be rebuilt. A mayonnaise usually breaks from rate or temperature mismatch and needs warmth.

Questions people ask