Does starch thickened custard get thinner after a few days?

Question

A few days ago, I did a crème pâtissière (thick vanilla custard), using

• milk (300ml/1.25 cup)
• egg yolks (2)
• sugar (15g/1tbsp)
• potato starch (10g/1tbsp).

The end result had a relatively thick consistency, fit for my purpose. I had a bit left over, so I put it in the fridge after it cooled down. After a few days in the fridge, the custard got thinner, like a crème anglaise (thin vanilla custard, picture an eggnog-like consistency).

I wonder what happened. One of my guess is that the amount of starch was too small, but I can't explain the change of consistency.

Answer 1

It is not the starch alone, but the combination of starch and egg yolks. Yolks contain an enzyme which digests starch after some time, making the whole custard thinner.

If you absolutely have to use a custard with both starch and yolks, either consume it quickly, or boil it well (at least of minute of visible bubbling) to deactivate the yolk enzymes.