Food emulsions: what milk, mayo and butter have in common

Milk, mayonnaise and butter look like unrelated products. All three are emulsions: one phase dispersed inside another, held together by a fragile balance. Understanding that balance is the difference between a reformulation that works and one that separates on the shelf.

What is a food emulsion?

An emulsion is a mixture of two liquids that don't mix, usually water and oil, in which one is dispersed as droplets inside the other. There are two main types:

• Oil-in-water (O/W): fat droplets dispersed in a water phase. Examples: milk, mayonnaise, dressings, creamy beverages.
• Water-in-oil (W/O): water droplets dispersed in a fat phase. Examples: butter, margarine, some spreads.

Emulsions are thermodynamically unstable: over time, the phases tend to separate. What keeps them stable are emulsifiers, droplet size, the viscosity of the continuous phase and, in some cases, networks of fat crystals.

Three emulsions, three stability mechanisms

Milk: oil-in-water

Fat globules are stabilized by their natural membrane and by milk proteins. Homogenization reduces globule size, which slows cream from rising to the surface.

Mayonnaise: oil-in-water

Oil droplets are stabilized by lecithin and egg yolk proteins. Droplet size and pH define texture and stability. A mayonnaise with smaller droplets is thicker and more stable.

Butter: water-in-oil

It forms when cream is churned and the phases invert. Here fat is the continuous phase, and stability depends on a network of fat crystals that traps the water droplets. More water or a more liquid fat means fewer crystals: the texture softens and it can weep water. And if the temperature rises, the network melts.

Why no change in an emulsion is ever isolated

Reducing fat, changing the protein, adjusting pH or modifying the process rarely affects a single variable. Droplet size, viscosity and stability all move at once:

• Reducing fat in a dressing changes the phase ratio and viscosity. It has to be offset with hydrocolloids or fibers, which changes mouthfeel.
• Changing the protein (for example, from egg to plant protein in a mayonnaise) changes its ability to coat droplets and how it behaves with pH. We explain it in food proteins: whey, caseinate or pea.
• Adjusting pH can push a protein toward its isoelectric point, where it loses charge and stops stabilizing.
• Modifying the process (shear rate, temperature, order of addition) directly changes droplet size.

How emulsion stability is evaluated

In product development, stability isn't assumed: it's measured. Common tools include droplet size distribution, viscosity, centrifugation tests, temperature cycling and accelerated shelf-life studies. Phase separation, creaming or oiling off are signs that the system wasn't designed well.

Reformulating means redesigning the system

Change one ingredient and you have changed the ecosystem. In an emulsion, reformulating isn't swapping one line on the ingredient list: it's redesigning the whole system so it becomes stable again with the new composition.

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