Eggs & Emulsions
An egg is a small collection of separate chemical systems -- coagulation, foaming, emulsification -- that happen to share a shell. Understanding which system you're actually relying on in a given recipe is what makes eggs predictable instead of finicky.
The kitchen answer
A soft-boiled egg is possible at all because the white and the yolk don't cook solid at the same temperature. Custards curdle when they're rushed past that same point too fast. Whipped whites and mayonnaise aren't heat-driven at all -- they're two different tricks that don't care about temperature in the same way.
What's actually happening
Egg proteins are long folded-up chains that unfold and link together permanently when heated -- that's coagulation, and it's what takes an egg from liquid to solid. Egg white protein (mostly ovalbumin) coagulates around 60–65°C. Egg yolk, protected somewhat by its fat content, needs slightly more heat -- around 65–70°C. That gap is small, but it's real and it's exactly wide enough to cook a white fully solid while the yolk stays runny.
In a custard, egg proteins are diluted into milk or cream, which pushes their effective set point higher (typically 80–85°C for a whole custard) -- but they'll still overshoot and curdle into scrambled curds if they're heated too fast. Tempering -- whisking a little hot liquid into the eggs first, gradually -- isn't a formality, it's spreading the temperature rise out so the proteins coagulate slowly and evenly instead of seizing all at once.
Whipping egg whites uses a completely different lever: mechanical force, not heat. The same proteins that later coagulate with heat first unfold under the shear of a whisk, stretching out and linking up around air bubbles to trap them in a stable foam network. Because it's a structural trick and not a heat one, it's also fragile in a structural way -- a single drop of yolk fat coats the protein strands before they can link up, and the foam simply refuses to build volume.
What does that mean for your recipe
Not every "egg dish" is relying on the same chemistry, which matters when something goes wrong. Mayonnaise and hollandaise don't rely on heat at all -- they rely on lecithin, a natural emulsifier in egg yolk, physically holding oil droplets suspended in water. That's a completely different mechanism from coagulation, which is why a broken (split) mayonnaise can often be rescued by whisking it into a fresh yolk, but a curdled custard cannot be un-curdled -- the proteins have already locked into a solid network, and there's no mechanical fix for that.
What can go wrong
- Curdled custard: heat applied faster than the proteins can coagulate evenly -- some of them overshoot and seize into curds before the rest catch up. Lower heat and real tempering fix this; whisking harder after it's already happened doesn't.
- Meringue that won't build volume: almost always trace fat -- a speck of yolk, an oily bowl, or a whisk that wasn't fully clean -- coating the proteins before they can form the foam network.
- Weeping or grainy meringue: over-whipped past the point where the protein network can still hold water, so it starts squeezing liquid back out -- structurally the opposite problem from under-whipping, not a matter of "whip it more." Full breakdown →
- Split mayonnaise: oil added faster than the lecithin can emulsify it, so droplets coalesce instead of staying suspended -- fixable, because no protein has coagulated yet.
Try it yourself
Two bowls, two batches of the same egg whites, one variable changed:
- Whip one batch of clean egg whites to soft peaks, timing how long it takes.
- Whip a second, identical batch -- but mix in a single drop of egg yolk first.
- Compare volume, time-to-peak, and how stable each foam is after 10 minutes sitting.
Watch for: the contaminated batch building far less volume, far more slowly, if it builds any real foam at all -- direct, visible proof that this is a structural failure, not a "you didn't whip long enough" one.
Go deeper
Sugar changes egg chemistry too, and not just by sweetening it. Dissolved sugar competes with egg proteins for water molecules, which slows how readily those proteins can unfold and link up -- the practical effect is that a custard with more sugar coagulates at a noticeably higher temperature than a plain one, which is part of why sweet custards are more forgiving to cook than savory egg dishes with little or no sugar in them.
More on the way
Next for this section: the full French/Italian/Swiss meringue methods and how their stiffness stages differ, and a full tempering-method crème anglaise recipe.