Emulsification Science: Why Egg Yolk Stabilizes Mayonnaise, What HLB Value Means, and the Exact Chemistry of Why Hollandaise Breaks at 70°C

What an Emulsion Actually Is

An emulsion is a dispersion of droplets of one liquid within another liquid with which it is immiscible. In culinary contexts, this is almost always oil and water. The two fundamental types:

• Oil-in-water (O/W): Oil droplets dispersed in a continuous water phase. Mayonnaise, hollandaise, vinaigrette (temporarily), cream soups, milk. These feel lighter on the palate because water is the continuous phase you first taste.
• Water-in-oil (W/O): Water droplets dispersed in a continuous oil phase. Butter, margarine, pâté. These feel richer and more coating because fat is what the tongue contacts first.

The thermodynamic problem: oil and water minimize their contact area spontaneously because the molecules at the oil-water interface are in an energetically unfavorable state (they can't hydrogen bond properly in either phase). Left alone, an oil-water mixture will always phase-separate. An emulsifier's job is to lower the interfacial tension at the oil-water boundary — making it energetically cheaper to maintain small droplets than to let them coalesce into large ones.

Surfactants and the HLB System

Surfactants (surface-active agents) are molecules with a hydrophilic (water-loving) head and a lipophilic (oil-loving) tail. They adsorb at the oil-water interface, positioning their lipophilic portion in the oil phase and their hydrophilic portion in the water phase. This reduces interfacial tension and creates a mechanical barrier against droplet coalescence.

The Hydrophilic-Lipophilic Balance (HLB) is a scale from 0–20 developed by W.C. Griffin in 1949 that quantifies where a surfactant sits on the spectrum from completely oil-soluble (HLB 0) to completely water-soluble (HLB 20). The HLB value determines which type of emulsion a surfactant stabilizes:

• HLB 3–6: Water-in-oil emulsifiers (e.g., monoglycerides, sorbitan monostearate — used in butter, chocolate)
• HLB 7–9: The transition range — can stabilize either type depending on conditions. Lecithin (HLB ~8) falls here, which is why egg yolk can work in both mayo (O/W) and butter sauces (W/O context).
• HLB 10–16: Oil-in-water emulsifiers (e.g., polysorbate 80, sodium stearoyl lactylate — used in salad dressings, ice cream)
• HLB 16–20: Solubilizers and detergents (e.g., sodium lauryl sulfate — not food use)

Egg yolk lecithin's HLB of approximately 8 means it sits in the sweet spot for O/W emulsions with good physical stability — it can coat oil droplets effectively while maintaining water as the continuous phase. This is why egg yolk is the most versatile natural emulsifier in the kitchen: it works in both warm (hollandaise) and cold (mayonnaise) preparations, and across a wide pH range.

Mayonnaise: The Physics of 70–80% Oil in Water

Commercial mayonnaise is approximately 70–80% oil by weight — yet it's an oil-in-water emulsion. This seems paradoxical: how can oil be the dispersed phase when it makes up the majority of the mixture? The answer is droplet geometry. At 75% oil by volume, oil droplets are packed so densely that they deform into polyhedral shapes (like a foam) rather than spheres — yet the continuous water phase still coats each droplet via the lecithin film, maintaining the O/W structure.

The key variables controlling mayonnaise stability:

Droplet size: Smaller droplets (1–3 microns) have larger surface area-to-volume ratio, requiring more lecithin per unit of oil. But smaller droplets are dramatically more stable against coalescence — the van der Waals attraction between droplets decreases with the 6th power of distance, so small drops stay separated more easily. The mechanical energy of mixing (whisking, blending) determines droplet size: immersion blenders produce 1–3 micron droplets vs. 3–10 microns from a whisk, which is why blender mayo holds longer.

Lecithin-to-oil ratio: One large egg yolk (approximately 17g) contains roughly 1.2g of phospholipids. This is sufficient to emulsify approximately 250–300mL of oil (the typical mayo recipe) with droplets in the 2–5 micron range. Beyond this ratio, lecithin coverage of new droplet surface becomes insufficient and the emulsion becomes unstable.

Acid: Vinegar or lemon juice in mayo serves two functions: flavor and emulsion stability. The acidic pH (~3.5–4) creates a positive charge on the lecithin head groups, causing electrostatic repulsion between oil droplets — an additional stabilizing force beyond the mechanical surfactant barrier. This is why mayo without acid breaks faster.

Mustard: Dijon or dry mustard contains mucilage compounds (from the seed coat) that act as additional emulsifiers and increase the aqueous phase viscosity, slowing droplet collision rates.

Why Mayo Breaks When You Add Oil Too Fast

The classic mistake: adding oil too quickly when making mayo by hand. The mechanism: lecithin molecules need time to adsorb to the new oil-water interface created when an oil droplet forms. If oil is added faster than lecithin can migrate to the interface and adsorb, the new droplets form with inadequate emulsifier coverage. These underprotected droplets immediately coalesce with neighbors, forming large drops that are visible as grease pools — the broken emulsion.

Adding oil drop-by-drop at first creates a high lecithin-to-oil ratio for the initial droplets, giving plenty of coverage. Once a stable emulsion is established (the mixture turns thick and opaque), oil can be added faster because the existing droplets' lecithin can't easily desorb — the emulsion is self-reinforcing once the droplet size distribution is properly established.

Hollandaise: Warm Emulsification with Protein Complications

Hollandaise is similar to mayonnaise in its fundamental chemistry — oil-in-water emulsion, egg yolk lecithin as surfactant — but operates near the edge of egg yolk protein thermal stability. This is where the breaking problem originates.

Egg yolk contains two classes of emulsifying agent:

• Phospholipids (lecithin): Small-molecule surfactants, thermally stable up to the decomposition temperature of the lipid (~200°C+). These do not denature.
• Lipoprotein complexes (low-density lipoprotein, LDL, and high-density lipoprotein, HDL): Large protein-lipid complexes that are highly effective emulsifiers but denature with heat. LDL from egg yolk denatures around 65–70°C, and HDL around 70–75°C.

In hollandaise made below 65°C, both lecithin and the lipoprotein complexes contribute to emulsion stability — creating a very robust, thick sauce. Above 70°C, the lipoproteins denature and aggregate. This has two effects: (1) the denatured lipoprotein aggregates no longer coat the oil-water interface effectively, and (2) they can cause the aqueous protein phase to coagulate, producing the characteristic scrambled-egg appearance of overheated hollandaise.

The correct hollandaise temperature is 60–65°C (140–149°F) — hot enough to pasteurize the yolk (Salmonella is inactivated at 60°C for 3.5 minutes) but below the lipoprotein denaturation threshold.

Why Vinaigrette Is a Temporary Emulsion

A simple vinaigrette — oil plus acid, nothing else — is a temporary emulsion. When you shake it vigorously, you create small oil droplets dispersed in the vinegar/water phase. But there's no surfactant. Within minutes, the droplets coalesce and phase separation is complete.

Adding mustard creates a semi-permanent emulsion that lasts hours. Adding a small amount of egg yolk creates an emulsion that can last days. The difference is purely the surface coverage provided by the emulsifier and its ability to maintain an energetic barrier against coalescence.

For a vinaigrette that stays emulsified: 1 teaspoon Dijon mustard per 60mL (4 tablespoons) of vinaigrette provides enough mucilage emulsifier for a stable sauce lasting 2–4 hours. The mustard also increases continuous-phase viscosity, slowing droplet-droplet encounter rates.

Sauce Rescue: The Colloid Science Behind Recovery Techniques

A broken emulsion has undergone coalescence — small droplets merged into large ones, which then creamed (floated to the top) or sedimented. Recovery is possible if not all the emulsifier has been denatured or displaced:

Broken mayo: Start a new yolk in a clean bowl. Whisk it with a small amount of mustard and acid. Then very slowly whisk the broken mayo into the new yolk — effectively using the fresh emulsifier to re-coat the already-separated oil, treating the broken sauce as your "oil" to be incorporated drop by drop. Works because the lecithin from the original yolks is still present in the oil phase and will be re-activated by the mechanical process with fresh emulsifier assistance.

Broken hollandaise: If overheated (protein coagulation), it cannot be rescued — the denatured proteins cannot re-emulsify and the texture is permanently altered. If broken by temperature drop (fat solidified), warm gently while whisking — the phospholipid surfactant is thermally reversible. If broken by excess butter, remove 2 tablespoons of sauce to a clean bowl, whisk in a splash of warm water to re-establish the aqueous continuous phase, then slowly whisk in the remaining broken sauce.

Recommended Equipment (Amazon)