Why is my pastry cream (custard) grainy?

Question

I made a simple pastry cream from a recipe that I've used once before uneventfully, and this time it turned out with a grainy texture.

The procedure was basically to: whisk together 3 egg yolks and an egg together with sugar and cornstarch in one bowl, separately bring milk with butter to a boil, temper the egg mixture with the hot milk, and then add the warm egg mixture to the milk and boiling for three minutes.

What might I have done wrong that would cause a grainy temperature? Did I not get the eggs warm enough before adding to the boiling milk? Did I boil for too long or too short a time? Are there any other potential culprits I'm not considering.

Answer 1

I think it all got too hot, but there might be other problems.

1. Did you mix the cornstarch in completely? Leaving undissolved cornstarch is a sure way to obtain 'blobs'.
2. Did you pour the hot milk onto the egg-mixture? You should do that very slowly and whisk vigorously (while trying to splash everything). If the eggs get too hot, they will coagulate.
3. Did you use a double boiler? Putting the pan on an open flame will make the bottom too hot and the eggs will coagulate.
4. Even if you did use a double boiler, did you whisk regularly while heating? The eggs at the bottom will become far warmer than the eggs in the center...

As rumtscho mentioned, a thermometer will be useful for monitoring the done-ness of the custard.

Generally, a fully cooked custard should not exceed 80 °C; it begins setting at 70 °C.

Source: Wikipedia