Mayonnaise is the mother of all cold emulsified sauces — a suspension of oil droplets in a continuous phase of egg yolk and acid, held stable by the lecithin in the yolk acting as a surfactant. The science is precise: one large egg yolk can emulsify up to 250ml of oil, though practical limits keep the ratio closer to 180ml for a stable, spoonable result. The yolk is placed in a bowl at room temperature with Dijon mustard and a pinch of fine salt. Oil — neutral for classical mayonnaise, though olive oil is used for Provençal aïoli — is added drop by drop while whisking continuously. The first 50ml are critical: the emulsion is most fragile at the start, when the oil-to-yolk ratio is low and the lecithin molecules have not yet formed a complete shell around each droplet. After the emulsion catches — the mixture suddenly thickens and turns pale — oil can be added in a thin stream. Once complete, white wine vinegar or lemon juice is whisked in for brightness. The finished mayonnaise should be firm enough to hold peaks, ivory-white to pale yellow depending on the egg and oil, and taste of egg, oil, and acid in balance. If it tastes primarily of oil, the acid is insufficient. The temperature of all components matters: cold eggs from the refrigerator emulsify poorly. Allow the yolk to reach 20°C before starting. A broken mayonnaise — oily, thin, separated — can be rescued by starting a fresh yolk in a clean bowl and whisking the broken mixture in drop by drop, exactly as if it were oil.
Room-temperature yolk (20°C) — cold eggs emulsify poorly. Add oil drop by drop for the first 50ml — the emulsion is most fragile at the start. One yolk emulsifies up to 180ml oil for stable results. Acid added at the end — vinegar or lemon for brightness. Rescue broken mayo by whisking into a fresh yolk drop by drop.
A teaspoon of warm water added to the yolk before you begin oil addition creates a more stable initial emulsion — the water gives the lecithin more room to work. For a lighter mayonnaise, fold in a tablespoon of boiling water at the end — it thins the texture without breaking the emulsion. Handmade mayonnaise keeps 3-4 days refrigerated; the acid content inhibits bacterial growth, but it will never be as shelf-stable as commercial products with preservatives.
Adding oil too fast at the start — the emulsion breaks before it forms. Using cold-from-fridge eggs — lecithin is less effective below 15°C. Using extra-virgin olive oil exclusively — its polyphenols cause bitterness when emulsified; blend with neutral oil. Neglecting the acid finish — bland mayonnaise tastes only of oil.
Escoffier, Le Guide Culinaire; Larousse Gastronomique; McGee, On Food and Cooking