Emulsification is the technique of suspending fat droplets in a water-based liquid (or vice versa) to create a stable, creamy sauce. Mayonnaise, aioli, hollandaise, béarnaise, beurre blanc, and vinaigrettes are all emulsions — each using a different fat (oil, butter), a different continuous phase (egg yolk, vinegar, wine reduction), and a different emulsifier (lecithin in egg yolk, mustard, garlic). Understanding emulsion science means understanding WHY these sauces work and — critically — how to rescue them when they break.
An emulsion needs three things: a fat phase (oil, butter), an aqueous phase (vinegar, lemon juice, water), and an emulsifier (egg yolk lecithin, mustard, garlic). The emulsifier coats fat droplets and prevents them from recombining. Mayonnaise: egg yolk + acid + oil streamed in SLOWLY while whisking constantly. The first tablespoon of oil is critical — it must be incorporated drop by drop. Once the emulsion catches (thickens visibly), oil can be added faster. Aioli: traditionally garlic pounded to a paste, then oil added drop by drop — no egg. The garlic provides the emulsifying power. Hollandaise: warm egg yolks whisked with a reduction of vinegar and water, then warm clarified butter streamed in. Temperature must stay between 60-75°C — too hot and eggs scramble, too cool and butter solidifies.
The universal rescue for broken emulsion: start with a fresh egg yolk (for mayo/hollandaise) or a tablespoon of warm water (for vinaigrettes) in a clean bowl. Whisk vigorously while adding the broken sauce drop by drop — you're re-forming the emulsion around a new stabiliser. For foolproof blender mayo: one egg, one tablespoon of acid, one teaspoon of mustard, one cup of oil — everything in the blender, immersion blender straight to the bottom, blend and slowly pull upward. Mayo in 30 seconds. For aioli: a proper Provençal aioli has no egg — just garlic, salt, and oil. It's more fragile but the garlic flavour is pure and intense.
Adding oil too fast at the start of mayonnaise — the emulsion hasn't formed yet. Using cold eggs for hollandaise — they need to be warm. Getting water into mayonnaise — even a drop can break it. Making hollandaise over too-high heat. Using extra virgin olive oil for mayonnaise in a food processor — the blades shear the oil and release bitter polyphenols (use light olive oil or a neutral oil, or whisk by hand with EVOO). Not having a rescue plan when the emulsion breaks.