Egg Science: How Heat, Acid, and Beating Change What Eggs Do

One ingredient. Four completely different jobs.

Add an egg to soup and it thickens. Whip egg whites and they turn into a foam that can stand stiff peaks. Mix egg yolk into oil and water and get a stable sauce that shouldn’t exist. Combine eggs with ground meat and they hold the mixture together through cooking.

Each of these is a different chemical process. The egg can do all of them because it’s a remarkably complex system — proteins, fats, emulsifiers, and water in specific proportions.

Function 1: Coagulation

This is the one most people know. Apply heat and eggs solidify.

The reason is protein denaturation and coagulation. Egg whites are about 90% water and 10% dissolved proteins, dominated by ovalbumin (about 54% of the protein). In raw form, ovalbumin is folded into a compact globe that doesn’t interact strongly with neighboring molecules. The egg white flows freely.

Heat adds energy. Above around 144°F (62°C), ovalbumin and other egg white proteins begin to unfold. As they unwind, they expose hydrophobic regions that were previously buried inside the fold. Those exposed hydrophobic regions don’t want to be near water, so they bond to other exposed hydrophobic regions on neighboring proteins. Proteins bond to proteins. The network solidifies.

Egg yolks start coagulating around 149°F (65°C). A whole egg mixture falls somewhere between the two temperatures.

That last stage is why overcooked scrambled eggs are simultaneously rubbery and wet. The expelled water isn’t from anywhere else in the pan. It’s water that was part of the protein structure, forced out when the network over-contracted.

The path to soft, creamy scrambled eggs is keeping temperature below 165°F and pulling off heat early — the residual heat in the pan continues cooking the eggs for another 30-60 seconds.

What acid does to coagulation. Acid lowers coagulation temperature and tightens the protein network slightly. Adding cream of tartar to scrambled eggs makes them set at a slightly lower temperature with marginally firmer texture. In custards (crème brûlée, pastry cream), acid makes the texture firmer and slightly tougher. Adding lemon juice to a delicate custard is usually a mistake.

What sugar does to coagulation. Sugar has the opposite effect: it raises the coagulation temperature and softens the final set. Sugar physically interferes with protein crosslinking, spreading the proteins out so they can’t bond as tightly. This is why sweet custards are smoother and more delicate than savory egg dishes cooked to the same temperature. It’s also why crème brûlée can be cooked to higher temperatures without curdling than plain scrambled eggs.

Function 2: Emulsification

Mayonnaise is oil suspended in water. Those two don’t mix on their own. Give them a vigorous shake and they blend briefly, then separate within seconds.

Egg yolk makes them stay mixed.

The key molecule is lecithin, a phospholipid naturally present in egg yolk. A phospholipid has a water-loving (hydrophilic) head group and a fat-loving (hydrophobic) tail. This dual nature allows lecithin to sit at the boundary between oil droplets and the surrounding water phase.

When you whisk egg yolk into oil while adding the oil slowly, the mechanical action breaks the oil into tiny droplets. Lecithin molecules orient themselves at each droplet’s surface: hydrophobic tail in the oil, hydrophilic head out in the water. Each lecithin molecule physically shields the oil droplet from merging with other droplets. The result is millions of tiny oil droplets suspended in water, each coated in a layer of lecithin.

That’s mayonnaise: an oil-in-water emulsion stabilized by egg yolk lecithin.

The same principle makes hollandaise, béarnaise, and aioli work. The ratio changes, but the mechanism is identical.

What goes wrong: if you add oil too fast, there aren’t enough lecithin molecules available to coat all the new droplets before they merge back together. The emulsion breaks — you see oil and a pool of liquid separating. To fix it, start fresh with a new yolk in a clean bowl and slowly whisk the broken mayo into it, giving the new lecithin time to stabilize each addition.

Cold temperature also helps. Cool oil has higher viscosity, which makes it easier to break into small droplets. Commercial mayo is produced under controlled temperatures. At home, room temperature eggs and oil are fine, but don’t use cold-straight-from-the-fridge oil.

See Emulsification for the broader picture on how emulsions work.

Function 3: Foaming

Egg whites are about 90% water and 10% protein — and almost no fat. This composition makes them ideal for creating foam.

When you beat egg whites, you force air into the liquid while mechanical action denatures proteins at the air-water interface. Those denatured proteins unfold and rearrange around each air bubble, forming a protein film that keeps the bubble stable.

The result at stiff peaks: a network of air bubbles each coated in a layer of protein. The foam can hold its shape because the protein network has enough structure to resist collapse.

Why fat kills foam. Fat (from yolk, from a greasy bowl, from oily hands) competes with proteins at the air-water interface. Fat molecules displace proteins, and the fat-based film is much weaker than a protein film. The foam collapses partially or completely.

This is why meringue recipes are strict about bowl cleanliness and why separating yolks from whites requires care. Even a small amount of yolk — a few drops — can ruin a meringue. If a yolk breaks in the bowl, it’s usually faster to discard and start over than to try to work around it.

What acid does. Adding cream of tartar (or a small amount of lemon juice) to egg whites before or during beating lowers the pH and changes the charge state of the proteins. Under these conditions, the protein film around each bubble is more stable. The foam holds stiff peaks longer, tolerates heat in the oven better, and is less prone to collapsing. Use about 1/8 teaspoon of cream of tartar per egg white.

Copper bowls. Traditional meringue recipes call for beating egg whites in an unlined copper bowl. Copper ions from the bowl’s surface react with conalbumin (an egg white protein) to form a stable copper-conalbumin complex that makes the foam even more resistant to breakdown. The effect is real, measurable, and replicated by adding a pinch of cream of tartar to a non-copper bowl.

Overbeating. Stop at stiff peaks. When you continue beating past stiff peaks, the protein network becomes over-stretched, loses its elasticity, and the foam structure begins to weep liquid and break apart. The foam looks dry, grainy, and lumpy. You can’t recover it. Start over.

Function 4: Binding

When eggs are cooked among other ingredients, the coagulating proteins form a matrix that physically links the other ingredients together.

In meatballs and burgers, raw egg is mixed with ground meat, breadcrumbs, and aromatics. During cooking, the egg proteins coagulate and weave between the meat proteins and breadcrumbs, holding everything together as a cohesive ball. Without egg (or a binding substitute), the mixture crumbles.

In batters and coatings, eggs bind the dry coating to food surfaces. Egg wash acts as a glue: the proteins coagulate during frying or baking, cementing the breadcrumb layer to the food. Standard breading procedure (flour, egg wash, breadcrumbs) uses egg specifically for this binding step.

In casseroles and quiches, eggs do double duty: they bind the filling ingredients and, as they coagulate, create the custard matrix that holds the dish together after baking.

The binding function depends on coagulation. Cook to an internal temperature where the eggs fully set — around 165°F (74°C) — and the binders are stable. Undercooked, and the matrix hasn’t fully formed. The dish may be soggy or fall apart.

Egg Size and Baking

Eggs are standardized by weight in the US. Large eggs weigh about 50g out of shell. Extra-large eggs weigh about 56g. Jumbo eggs about 63g.

In savory cooking, the size difference rarely matters. In baking, it matters more. Standard US baking recipes assume large eggs. Using extra-large eggs adds roughly 12% more egg protein and fat per egg. In large batches with many eggs, this shifts the liquid-to-dry ratio noticeably. For recipes that use 4+ eggs, matching the specified size produces more consistent results.