Mayonnaise Oil Droplet Size and Emulsion Stability

Classical French sauciers were producing stable mayonnaise by the 18th century without understanding why it held together. The mechanistic explanation — that lecithin and lipoprotein fractions in egg yolk coat oil droplets and create a repulsive electrostatic barrier — was mapped out progressively through 20th-century colloid science and consolidated for kitchen application in McGee's On Food and Cooking (2004) and later in Modernist Cuisine (2011).

Mayonnaise is an oil-in-water emulsion: tiny oil droplets suspended in an aqueous phase of egg yolk, acid, and water. Stability is almost entirely a function of droplet size. When you disperse oil into droplets smaller than roughly 1–2 microns, you massively increase surface area, which demands more emulsifier — primarily the phospholipids and LDL (low-density lipoprotein) fractions in egg yolk — to coat and stabilize each droplet. Smaller droplets also move more slowly by Brownian motion, which reduces the frequency of collision and coalescence. That is the physics of why a stick blender or high-speed Robocoupe produces a more stable, whiter mayonnaise than a fork: you are driving droplets smaller and coating them faster. Myhrvold, Young, and Bilet in Modernist Cuisine Vol. 4 quantify this directly, noting that hand-whisked mayonnaise typically produces droplets in the 2–10 micron range, while high-shear blending can drop that to under 1 micron. The whiter, more opaque color you see with machine-made mayo is a direct optical consequence: smaller droplets scatter light more uniformly across the visible spectrum (Mie scattering), breaking the yellowish transparency you get from larger droplets. Acidity matters for two reasons beyond flavour. First, it denatures some egg proteins, freeing more amphiphilic surface area. Second, at a lower pH, the emulsifier molecules carry stronger surface charges, increasing electrostatic repulsion between droplets and slowing coalescence. McGee (2004) notes that the ratio of oil to aqueous phase is the other lever: push beyond roughly 80% oil by volume and you are asking the emulsifier to do more work than it physically can, and the emulsion inverts or breaks. In the kitchen this means that addition rate and shear are the two controllable variables. Add oil too fast early in the process — before you have established a continuous aqueous phase containing sufficient emulsifier — and you get large droplets the moment they form. Large droplets coalesce faster than the lecithin can stabilize them. You break the mayo before it starts. Drip oil slowly at first, then accelerate once the emulsion is established and viscosity climbs. That viscosity change is tactile feedback that the droplet network is self-reinforcing.

Smaller oil droplets release fat-soluble flavour compounds — the grassy esters and aldehydes from the oil, and the sulfurous volatiles from egg yolk — more readily into the aqueous phase at the oil-water interface because surface area is larger. This means fine-droplet mayo carries more aroma to the palate. The acid component (acetic acid from vinegar, citric and ascorbic acids from lemon juice) contributes brightness and suppresses the eggy sulfur notes by shifting equilibrium away from volatile hydrogen sulfide. Mustard, when used, contributes allyl isothiocyanates as flavour but also functions as a secondary emulsifier via its mucilage proteins — its flavour and structural roles are simultaneous. McGee (2004) notes that the fat itself rounds and softens sharp acid notes by coating taste receptor surfaces, which is why a high-oil mayo tastes richer and less acidic than a low-oil version with identical acid content.