The Science of Emulsification: Mayonnaise, Vinaigrette and Hollandaise

An emulsion is a mixture of two immiscible liquids (typically oil and water) in which one is dispersed as tiny droplets throughout the other. The two types:

**Oil-in-water emulsions:** Tiny oil droplets suspended in a continuous water phase. Examples: mayonnaise, hollandaise, milk, cream soups. The water phase is dominant — these feel light and flow.

**Water-in-oil emulsions:** Tiny water droplets suspended in a continuous oil phase. Examples: butter, margarine. The oil phase is dominant — these are solid or semi-solid at room temperature.

The reason oil and water resist mixing is surface tension and polarity. Water molecules are polar (charged), attracting each other strongly. Oil molecules are nonpolar, attracting each other. When mixed, the two phases always separate — unless an emulsifier is present.

**Emulsifiers** are amphiphilic molecules: they have a water-loving (hydrophilic) end and an oil-loving (lipophilic) end. When an emulsifier is present, it positions itself at the interface between oil and water droplets — hydrophilic end facing water, lipophilic end facing oil — coating each droplet and preventing them from coalescing back together. The emulsifier doesn't create energy; it lowers the energy cost of maintaining the interface between the two liquids.

Mayonnaise is an oil-in-water emulsion stabilised by egg yolk lecithin. The process of making it illustrates every emulsification principle:

**Ingredients and their roles:** • **Egg yolk** — the emulsifier. Contains lecithin that coats oil droplets. • **Oil** — the dispersed phase (75% of the final product) • **Acid (lemon juice or vinegar)** — the continuous water phase; also denatures some yolk proteins which aids stability and adds flavour • **Mustard** — secondary emulsifier (mucilage in mustard provides additional stabilisation) and flavour • **Salt** — flavour and slight viscosity effect

**Method (the technique explained by physics):** 1. Start with oil addition at a tiny drip — each drop must be coated with emulsifier before the next is added. Add too fast and you outpace the emulsifier's ability to coat droplets — the emulsion breaks. 2. Once the emulsion is established (after the first 3–4 tablespoons of oil), you can add oil faster — there's now a stable matrix to absorb new droplets. 3. The finished mayo is thick because the oil droplets are so densely packed (75% of volume) that they're pressing against each other — the viscosity is mechanical, not chemical.

**Why mayonnaise breaks and how to fix it:** • Adding oil too fast — outpaces emulsification • Ingredients too cold — lecithin is less effective below 10°C; ingredients should be room temperature • Too much oil per yolk — each yolk can only stabilise ~200ml of oil

**To rescue broken mayo:** Start fresh with a new yolk in a clean bowl; slowly whisk in the broken mayo as if it were oil. The new yolk re-emulsifies the broken mixture.

Hollandaise is a warm oil-in-water emulsion — butter (clarified) dispersed in an egg yolk base with acid. It is technically more challenging than mayonnaise because heat destabilises emulsions, requiring careful temperature management.

**Classic hollandaise method:** 1. Reduce white wine vinegar with peppercorns (the 'gastrique') — this is the acid component 2. Whisk egg yolks with the gastrique over a bain-marie (bowl over barely simmering water) until the mixture is thick, pale and tripled in volume — the yolks are being gently cooked and their proteins begin to set, increasing viscosity 3. Add clarified butter (butter with milk solids and water removed — pure butterfat) slowly while whisking continuously, exactly as with mayonnaise 4. Season; keep warm at 60–65°C

**Why temperature matters:** Above 70°C, egg proteins scramble and the sauce breaks irreparably into sweet scrambled eggs in butter. Below 55°C, the emulsion is unstable and the sauce may separate. The working window is narrow — 60–65°C.

**Stabilised hollandaise:** Adding a small amount of starch (½ tsp cornflour per 3 yolks) creates a buffer that allows higher temperatures without scrambling. This is widely used in professional kitchens where holding sauce for service requires robustness.

**Béarnaise** is hollandaise with a tarragon and shallot reduction instead of plain vinegar — a flavour variation, not a different technique.

Vinaigrette is a temporary or semi-permanent emulsion — it doesn't contain a powerful emulsifier like lecithin, so it separates over time. The classic ratio is 3:1 oil to acid.

**Making a more stable vinaigrette:** • **Mustard** — acts as a weak emulsifier and significantly extends stability; a teaspoon of Dijon can keep a vinaigrette together for 10–15 minutes after shaking • **Garlic** — contains surface-active compounds that aid temporary emulsification • **Honey** — viscosity reduces droplet mobility and slows separation • **Emulsification method** — a jar (rather than bowl and whisk) allows more vigorous mixing; blending with an immersion blender creates very small droplets that take longer to coalesce

**Permanent vinaigrette:** Add an egg yolk to a classic vinaigrette and blend — this creates a mayo-like permanent emulsion with a dressing consistency. Often used in restaurant prep.

**Pan sauce emulsions:** When you deglaze a pan with wine and finish with cold butter (monter au beurre), you're creating a temporary oil-in-water emulsion. The key: butter must be cold, and the sauce must not boil after butter is added (high heat breaks the emulsion). The result — a glossy, velvety sauce — is pure emulsification chemistry.

**Caesar dressing:** A classic oil-in-water emulsion using anchovy paste and egg yolk as emulsifiers, with lemon juice as the water phase. The anchovy provides umami and additional amphiphilic proteins that aid stability.

**Pasta water technique:** The starch in pasta cooking water is an emulsifier. When you toss pasta with the cooking water and olive oil or butter, the starch helps create a temporary emulsion that coats the pasta — this is the technique behind cacio e pepe, carbonara and aglio e olio.

**Beurre blanc:** A warm butter emulsion made without egg yolk — the emulsification comes from the proteins and phospholipids naturally present in whole butter (clarified butter cannot make a stable beurre blanc). Reduction of wine and shallots creates a concentrated water phase with dissolved sugars that aids stability.

**Chocolate ganache:** An emulsion of fat (cocoa butter + dairy fat) in water (cream). It can break if cream is too hot (fat separates) or too cold. An immersion blender can rescue a broken ganache by breaking down fat droplets and re-dispersing them.

**Ice cream:** A complex oil-in-water emulsion stabilised by egg yolk lecithin (in custard-based ice creams) with additional stabilisers. The churning process creates tiny ice crystals and incorporates air — producing the smooth, scoopable texture.