Emulsification in Food Science: Lecithin, HLB, and Mayonnaise Physics

A Jar of Mayonnaise Defies Thermodynamics — and Stays Stable for Months

Oil and water do not mix. This is not merely an observation — it is a thermodynamic reality rooted in the hydrophobic effect: water molecules preferentially form hydrogen bonds with each other rather than accommodate nonpolar hydrocarbon chains from oils. Left alone, oil and water separate rapidly. Emulsification forces them together in a stable dispersed system where one liquid exists as tiny droplets inside another. Standard supermarket mayonnaise contains roughly 70–80% oil dispersed as droplets of 1–5 micrometers in an aqueous continuous phase — and it remains stable on a shelf for months. Understanding why requires understanding emulsifiers, droplet physics, and the conditions that cause emulsions to fail.

What Emulsifiers Do: Amphipathic Molecules at the Interface

An emulsifier is a molecule with two distinct regions: a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. These amphipathic molecules migrate to the oil-water interface and form a thin film around each oil droplet. The hydrophobic tail inserts into the oil droplet; the hydrophilic head faces the aqueous phase. This interfacial film:

• Reduces interfacial tension between oil and water, making it energetically favorable to create small droplets
• Provides a physical and electrostatic barrier that prevents droplets from coalescing
• Stabilizes the dispersed state by preventing adjacent droplets from merging

Lecithin — a phospholipid abundant in egg yolk — is one of the most effective natural emulsifiers. Each lecithin molecule has a phosphate-choline head group (strongly hydrophilic) and two fatty acid tails (hydrophobic). Egg yolk contains approximately 10% lecithin by weight, which is why egg yolk is the emulsifier in mayonnaise, hollandaise, and aioli.

Hydrophilic-Lipophilic Balance (HLB)

The hydrophilic-lipophilic balance (HLB) is a numerical scale (0–20) developed by W.C. Griffin in 1949 to describe the relative affinity of an emulsifier for water versus oil. The HLB value predicts which type of emulsion an emulsifier will stabilize:

Lecithin has an HLB of approximately 4 when extracted from soybeans (more lipophilic) and about 8 when modified (lysolecithin). Mono- and diglycerides (common in commercial bread and ice cream) have HLB values of 2–5. Polysorbate 80 (Tween 80), widely used in commercial ice cream and salad dressings, has an HLB of approximately 15, making it strongly hydrophilic and effective at stabilizing oil-in-water emulsions.

Mayonnaise: A Case Study in Emulsion Physics

Commercial mayonnaise is a concentrated oil-in-water emulsion with 70–80% oil by volume. The oil phase exists as droplets of 1–5 micrometers stabilized by egg yolk lecithin and proteins. The process of making mayonnaise involves:

• Creating an initial coarse emulsion by slowly adding oil to aqueous egg yolk under shear
• High-shear mixing (whipping) reduces droplet size to the micrometer range
• Acidification (vinegar or lemon juice) reduces pH to approximately 4, improving stability and inhibiting microbial growth
• Salt reduces water activity and enhances flavor; also affects protein conformation at the interface

The high oil content makes mayonnaise a "concentrated emulsion." At 70% oil by volume, droplets are packed so tightly that the emulsion becomes a semi-solid gel — this is why mayonnaise has yield stress (it holds its shape on a knife) despite being mostly liquid oil.

Homogenization: Forcing Stable Emulsions

Commercial homogenization creates emulsions by forcing a coarse mixture through a narrow gap or nozzle at high pressure — typically 150–300 bar (2,200–4,400 psi) for dairy homogenization. The shear forces rupture large fat globules into droplets of 0.1–1 micrometer. At this scale, Brownian motion and interfacial adsorption of milk proteins stabilize the droplets against coalescence. Homogenized milk does not separate a cream layer because the fat globules are too small and too well-coated with casein proteins to float to the surface.

Breaking an Emulsion

Emulsions fail by several mechanisms. Coalescence occurs when emulsifier films rupture and adjacent droplets merge. Flocculation is reversible aggregation of droplets without merging. Creaming (or sedimentation) is buoyancy-driven separation of the dispersed phase without coalescence. Ostwald ripening is the thermodynamically driven growth of larger droplets at the expense of smaller ones.

Deliberately breaking an emulsion — as in clarified butter (ghee) production — uses the same principles in reverse: applying heat, removing emulsifiers (milk proteins are skimmed off), and allowing phase separation to proceed to completion.