I confess I’ve been saving this question since the weekend, as it dovetails beautifully with today’s discussion on the subject of egg yolks. Reader Peet writes (wrote):
Hi Joe! I’m having a problem with my pastry creams, i.e. they’re too thin. I cook them but have never been comfortable with bringing them to a boil since I’m afraid of curdling, and anyway egg proteins thicken at a much lower temperature. Which means there’s no reason the creams shouldn’t be thick, but they start out thick and eventually turn to soup in the refrigerator. Is boiling really the key to a thick pastry cream? And if so, why?
I think I see the problem, Peet. The heart of it is that pastry cream isn’t a protein-thickened custard (at least not primarily), but rather a starch-thickened custard. But this poses an excellent question: why must starch-thickened custards be boiled? The answer has to do with an enzyme called amylase that’s present in egg yolks. If you remember how flour thickens watery mixtures, it does so when it warms enough that the individual starch molecules — long, string-like amylose molecules and branch-like amylopectin molecules — start to separate from the flour granules. They float off and get tangled up with one another, restricting the flow of the water around them.
That’s all well and good, unless something interferes with that starch tangle. This is where the amylase comes in. As it’s name implies, amylase is an enzyme specifically designed to disassemble amylose molecules. It’s very effective at the job, and since it’s not actually alive, it can perform the function at very high heat. Indeed amylase works best between the temperatures of about 155 and 185 degrees Fahrenheit — right about your maximum cooking temperature — and the presence of calcium (from the cream) actually makes it even more temperature resistant.
Beyond 190 amylase starts to run into something of a headwind, and it’s deactivated completely around boiling temperature. This is why we boil starch-thickened custards, but only for about a minute or so (beyond that, something else that’s undesirable can happen).