Mayonnaise is an oil-in-water emulsion stabilised by the lecithin in egg yolk — a thick, stable sauce in which oil droplets are suspended in an acid-yolk medium. It is the foundational cold emulsified sauce from which aioli, rémoulade, and dozens of derivatives descend. The technique is not difficult when the principles are understood: the emulsion must be established in the first tablespoon of oil before the addition rate increases, and everything must be at room temperature.
- **Room temperature ingredients.** Cold egg yolk cannot flex its lecithin molecules to coat fat droplets efficiently. Cold oil forms larger droplets that coalesce. Room temperature is the only correct starting state. - **Acid first.** A small amount of lemon juice or white wine vinegar in the yolk before any oil creates the water phase into which fat will be dispersed. - **Oil drop by drop to begin.** The emulsion must be established in the first tablespoon of oil — this requires the oil to be added in individual drops while whisking vigorously. Once the mixture is visibly thickened and pale, the addition rate can increase to a thin, steady stream. - **Mustard as additional emulsifier.** A teaspoon of Dijon adds lecithin (from the mustard seed) and makes the emulsion more stable and more forgiving. Decisive moment: The first tablespoon of oil. If the emulsion is not established here — if the mixture remains thin and liquid rather than thickening — add no more oil. Add another yolk and restart, treating the existing mixture as the oil phase. Sensory tests: **Sight:** The emulsion establishing: the mixture changes from transparent yellow to opaque, pale yellow as the oil droplets are coated and suspended. **Texture:** Properly emulsified mayonnaise is thick — it falls from the whisk in a ribbon that holds its shape. Thin, pourable consistency means incomplete emulsification.
— **Broken mayo (greasy pools of oil):** Oil added too fast before emulsion established, or ingredients too cold. Rescue: new yolk in a clean bowl, whisk the broken mixture in drop by drop.
Jacques Pépin's Complete Techniques