Oil and vinegar naturally separate because oil and water are immiscible; tiny droplets of oil coalesce and rise because they are less dense. Food scientists explain that a stable vinaigrette requires something to reduce interfacial tension and keep droplets dispersed. Harold McGee, author of On Food and Cooking, describes how emulsifiers and mechanical shear work together to create and maintain emulsions. Cornell University Department of Food Science notes that both the choice of emulsifier and the mixing method influence stability.
Practical emulsifiers besides mustard
Egg yolk contains lecithin, a powerful natural emulsifier that binds water and oil; a single yolk can stabilize many vinaigrettes. Commercial alternatives include soy lecithin powder and mayonnaise, which is essentially an already-stable egg-yolk emulsion. Thickeners such as xanthan gum act differently: in very small amounts they increase viscosity and slow droplet movement, making separation less obvious while adding a slightly different mouthfeel. Sweet syrups like honey or agave also help because their soluble sugars and viscosity assist droplet dispersion, though they contribute flavor.
J. Kenji López-Alt, Serious Eats, emphasizes that technique often matters as much as ingredient choice. Rapid shear from an immersion blender or whisk creates smaller oil droplets, increasing surface area for emulsifier action and improving stability. Shaking in a closed jar works for immediate use but tends to separate more quickly.
Ratios, temperature, and repair
A commonly used guideline is about three parts oil to one part acid for a balanced vinaigrette; altering this modifies both flavor and how readily an emulsion forms. Warmer ingredients blend more readily because viscosity falls with temperature, while very cold oils are harder to emulsify. If a vinaigrette breaks, whisking in a teaspoon of warm water or adding an egg yolk or a spoonful of mayonnaise will often rebind it.
Cultural and territorial nuances matter: French vinaigrette traditionally uses mustard as both flavor and stabilizer, while Mediterranean preparations often rely on vigorous mixing and the natural viscosity of olive oil. Choosing an oil with a slightly higher viscosity or incorporating a neutral emulsifier lets you create a stable dressing without mustard, maintaining the textures and flavors appropriate to the cuisine. Experiment with small adjustments in emulsifier type, mixing method, and temperature to find the balance you prefer.