So then what are the consequences for a custard when you overshoot the mark, as it were? Well you remember the earlier discussion of how proteins work, the way in which heat unfolds those long string-like molecules, making it possible for their free bonding sites to adhere to one another to create a liquid-trapping lattice (coagulation). Carry that idea too far and those protein molecules begin to bond too tightly to one another. The lattice tightens, squeezing water out like a fist tightening around a sponge. It’s this process that turns scrambled eggs to rubber, lemon meringue pies into soppy curds in pools of syrup, and flourless chocolate cakes into chalky chocolate hockey pucks.
Where “stirred” custards are concerned this isn’t necessarily the end of the world. A few lumps in your crème anglaise (and there are usually a few, in even the most carefully prepared batches) can simple be strained out. In the case of a still custard however, you’re done, finito, S.O.L.. Nothing to do but chalk it up to experience and start again.