Eggs and heat: the narrow range that decides everything
Almost everything that goes wrong with eggs is a temperature problem, and the window you are working in is narrower than it feels.
The numbers
Egg proteins unfold and bond to each other as they heat. Roughly:
- Egg white starts to set around 62°C and is firm by about 65°C.
- Yolk starts around 65°C and is firm by about 70°C.
- Whole egg mixed with milk or cream, as in a custard, sets somewhere around 80 to 85°C.
Past that upper figure the proteins keep bonding, squeeze out the water they were holding, and the mixture curdles. Smooth custard and scrambled egg are the same reaction, stopped at different points.
The gap between a set custard and a broken one is a few degrees.
Why dilution helps
Adding milk or cream raises the setting temperature, because the egg proteins are further apart and take more energy to find each other. More dairy means a higher and more forgiving set point, and a softer final texture.
More egg means it sets sooner and firmer. That is the whole dial on a custard.
Why starch changes the rules
Pastry cream contains flour or cornflour, and you boil pastry cream. That should curdle it, and it does not.
Starch granules get in the way of the egg proteins bonding tightly, and they also carry an enzyme problem in the other direction — raw egg yolk contains amylase, which breaks down starch and will thin your pastry cream if you never bring it to a proper boil. So pastry cream has to boil for a minute or two, both to thicken and to kill that enzyme.
That is why one custard must never boil and another one must.
Tempering
Pouring hot liquid straight onto eggs cooks them on contact. Pour a little of the hot liquid into the eggs while whisking, bring them up gently, then return the warmed mixture to the pan.
It is not a ritual. It is keeping the egg below its set point until it is diluted enough to survive the rest.
Carryover
A pan holds heat. A custard pulled off the heat at exactly the right point will keep climbing for a while in a hot pan.
Pull it slightly early, and if you are nervous, strain it into a cold bowl straight away. Straining also catches the small amount of chalazae and any bits that did set, which is why even a good custard usually goes through a sieve.
