Emulsification

Oil and water repel each other, so left alone they separate into layers. An emulsifier — a molecule with a water-loving (hydrophilic) end and a fat-loving (hydrophobic) end — coats the surface of each tiny droplet and keeps them from merging back together. Egg yolk (lecithin), mustard, and the proteins in dairy are common emulsifiers. Vigorous agitation breaks the fat into droplets small enough to stay suspended; the emulsifier locks them in place.

• Turns thin liquids into thick, clingy, glossy sauces without flour or starch.
• Carries fat-soluble flavour evenly through a dish instead of leaving an oil slick.
• A broken emulsion looks curdled or greasy and coats food poorly — the difference between a silky and a split sauce.

1. 1Add the oil slowly, especially at the start — a thin stream while whisking lets each droplet get coated before the next arrives.
2. 2Start with the emulsifier and water phase together (yolk + a little acid), then build in the fat.
3. 3Have everything near room temperature; cold fat resists breaking into fine droplets.
4. 4To rescue a broken sauce, start fresh with a new yolk or a spoon of water and whisk the broken mixture back in drop by drop.

• The mixture thickens and turns opaque and glossy as it emulsifies.
• A broken sauce looks grainy, thin, or shows free pools of oil.
• Properly emulsified, it coats the back of a spoon evenly.

• Adding the fat too quickly at the start.
• Using fridge-cold ingredients that won't disperse.
• Overheating a warm emulsion (hollandaise) until the proteins seize and the fat weeps out.

• Heat Transfer

  Warm emulsions live in a narrow temperature band — too hot and they break.

• Acidity & Balance

  Acid in the water phase both stabilizes and seasons the emulsion.

1. 1.Harold McGee, On Food and Cooking — Sauces
2. 2.J. Kenji López-Alt, The Food Lab — Emulsions

Confidence: high