Egg yolks

The yellow of the egg. Weigh it, do not count it. A large yolk is 18 to 20 g, so egg size stops mattering once you work in grams. Yolk is about 16 percent protein and about 50 percent water. In a custard it is the only structural element. Its proteins start to unfold near 62 degrees C, aggregate through the seventies, and lock into a network near 80 degrees C. Above about 85 degrees C the network contracts and pushes out the water it holds, which is the weeping you see in an overbaked custard. Sugar raises the temperature at which yolk sets, by about 1 degree C for every 3 percent sugar. That is why a sweet custard is forgiving and a savoury one curdles quickly. Separate yolks straight from the fridge. The membrane is tougher when cold and ruptures less.