Pastry cream is ruined if you overcook the eggs. How do you determine its doneness?
Pastry cream is an anomaly among custards. Although overheating a typical custard can lead to curdling, it’s vital to bring pastry cream almost to a boil. Doing so sets the eggs and activates the starch, thereby ensuring a proper consistency.
As a typical custard heats, the egg proteins unravel and intertwine, eventually forming cross-links that can result in coagulation, or curdling. Why is pastry cream different? Because it contains starch (flour or cornstarch), which affects texture in two ways. First, the starch interferes with the cross-linking of egg proteins, thus preventing coagulation. Second, pastry cream must be heated to a temperature high enough to destroy the amylase enzyme present in egg yolks, which would otherwise break down the starch and make the pastry cream runny.
The takeaway: For pastry cream that will be thick, not soupy, once it cools, make sure to heat it sufficiently. When it’s hot enough, three or four bubbles will burst on the surface, its temperature will read 200 degrees on an instant-read thermometer, and it will appear thick and glossy.