Yes, mayonnaise is a colloid. Specifically, it’s a type of colloid called an emulsion, where tiny droplets of one liquid are suspended throughout another liquid. In mayo’s case, oil droplets are dispersed in a water-based mixture of egg yolk, vinegar, and lemon juice. This makes it an oil-in-water emulsion, even though oil makes up the majority of the product by weight.
What Makes Mayo a Colloid
A colloid is a mixture where particles of one substance are evenly dispersed throughout another, but the particles are too large to dissolve and too small to settle out on their own. Colloid particles range from about 1 to 1,000 nanometers. Solutions, by contrast, have particles small enough to dissolve completely, while suspensions have particles large enough to eventually sink to the bottom.
Mayonnaise fits the colloid definition because its oil exists as tiny, stable droplets suspended in a watery phase. In commercially made mayo, those oil droplets typically measure between 3 and 7 micrometers in diameter. That’s far too small to see individually with the naked eye, which is why mayo looks smooth and uniform rather than oily. If you could zoom in with a microscope, you’d see millions of individual oil spheres packed tightly together.
Colloids share a few signature traits: they don’t separate on standing (at least not quickly), they can’t be separated by simple filtration, and they scatter light. That light-scattering property is called the Tyndall effect. When light passes through a true solution like saltwater, it travels straight through because the dissolved particles are too small to interfere. In a colloid, the dispersed particles are large enough to deflect light, making the beam visible. Mayo’s opaque, creamy white appearance is itself a result of oil droplets scattering light in all directions.
How Egg Yolk Holds It Together
Oil and water don’t mix on their own. If you simply shook oil and vinegar together, the oil would quickly float back to the top. Mayo stays stable because egg yolk acts as an emulsifier, a substance that sits at the boundary between oil and water and keeps the two from separating.
The key molecule doing this work is lecithin, a type of phospholipid found in egg yolk. Lecithin molecules have a split personality: one end is attracted to water, and the other end is attracted to oil. When you whisk oil into egg yolk, lecithin molecules arrange themselves around each tiny oil droplet, with their oil-loving ends pointing inward and their water-loving ends pointing outward into the surrounding liquid. This creates a protective coating that prevents oil droplets from merging back together. Proteins in the egg yolk also get absorbed at the oil-water boundary, adding a second layer of structural reinforcement.
Why It Contains So Much Oil
Despite being classified as an oil-in-water emulsion (meaning oil is dispersed in water, not the other way around), mayonnaise is overwhelmingly oil. The FDA requires that any product labeled “mayonnaise” contain at least 65% vegetable oil by weight. Most commercial recipes hover around 70 to 80%.
This creates an unusual situation. The oil droplets are so tightly packed that they press against one another, which is what gives mayo its thick, semi-solid texture rather than the runny consistency you might expect from a liquid emulsion like milk. Smaller droplets pack even more tightly. Research measuring different mayonnaise samples found that those with smaller oil droplets (around 4.5 micrometers) were noticeably firmer than those with larger droplets (around 6.8 micrometers).
What Happens When Mayo “Breaks”
A broken mayonnaise is one where the emulsion has failed. Instead of a smooth, creamy spread, you get a greasy, separated mess. This happens when oil droplets lose their protective coating and merge back together, a process called coalescence.
Several things can trigger this. Adding oil too quickly while making mayo from scratch overwhelms the emulsifier, leaving droplets uncoated. Temperature extremes cause problems too: refrigerating mayo below freezing can rupture the emulsifier layer, and excessive heat destabilizes it. Vigorous shaking or stirring after the emulsion has set can also break it apart.
If your homemade mayo breaks, the fix relies on the same science that created the emulsion in the first place. Start a fresh egg yolk in a clean bowl and very slowly whisk the broken mixture into it. The new yolk provides fresh lecithin and proteins to re-coat the oil droplets, rebuilding the emulsion from scratch.
Other Colloid Types for Comparison
Emulsions like mayonnaise are just one category of colloid. The classification depends on what’s dispersed in what:
- Liquid in liquid (emulsion): mayo, milk, salad dressing
- Solid in liquid (sol): paint, blood, muddy water
- Gas in liquid (foam): whipped cream, shaving cream
- Liquid in gas (aerosol): fog, hair spray
- Solid in gas (aerosol): smoke, dust in air
- Gas in solid (solid foam): marshmallow, styrofoam
Butter is an interesting counterpart to mayo. While mayo is oil droplets dispersed in water (oil-in-water), butter is water droplets dispersed in fat (water-in-oil). Both are emulsions and both are colloids, but their continuous phases are reversed, which is why they behave so differently at room temperature.