A vinaigrette is the simplest emulsion in cooking — oil suspended in acid (vinegar or citrus) with the help of an emulsifier (usually mustard). Understanding how and why it works unlocks dozens of cold sauces: vinaigrettes, Caesar dressing, rémoulade, ravigote, chimichurri, salsa verde (Italian), chermoula, and every herb sauce. The principle is identical across cuisines: fat + acid + flavouring, held together by mechanical action and an emulsifying agent.
Classic French ratio: 3 parts oil to 1 part vinegar, with a teaspoon of Dijon mustard as emulsifier. The mustard contains lecithin which coats oil droplets and prevents them from recombining. Whisk the acid and mustard first, then stream oil in slowly while whisking constantly. For Italian salsa verde and Argentinian chimichurri: the emulsion is broken deliberately — the oil and herbs remain loosely combined, not creamy. This is a different technique with a different goal. Temperature affects stability: cold ingredients emulsify more easily.
The food processor trick for stable vinaigrette: blend everything except the oil, then stream oil in with the motor running. For broken-style herb sauces (chimichurri, salsa verde): finely chop herbs by hand, combine with oil, acid, and garlic — do NOT blend. The texture should be loose and rustic. Honey is both sweetener and emulsifier — a teaspoon dramatically improves vinaigrette stability. The best vinaigrette is the one that matches the salad: light lemon-olive oil for delicate greens, robust red wine-shallot for hearty lettuces.
Adding oil too fast — the emulsion breaks. Not enough emulsifier. Using olive oil only in a food processor — the blades shear the oil and release bitter polyphenols. Under-seasoning — vinaigrettes need more salt than you think. Making too far ahead — fresh vinaigrettes lose brightness. Using the same ratio for every acid — lemon juice is more acidic than most vinegars, so you need less.