D: I tried making my grandmother's cream pie recipe last night. I cooked milk, eggs, flour and sugar in a saucepan until thick. Then I cooled the mixture and spooned it into a crust. The pie looked perfect last night, but this morning the filling had turned from nice thick custard into a soupy liquid! What happened?!
Y: Custards or pudding thickened with both eggs and starch, can end up watery or soupy from either undercooking OR overcooking. But the biochemical causes for the runny filling differ in each case.
D: Really? How can either mistake result in the same problem?
Y: It's all in the eggs, Don. Yolks and whites both contain enzymes that break down carbohydrates, like starches. In fertilized eggs, the enzymes amylase (found in yolks) and lysozyme (in whites) have an antibacterial function. They protect the growing chick from infection by breaking down carbohydrates on the outer layer of bacteria cells. But for cooks, these enzymes are a potential source of woe. When heated to boiling, the starch-digesting enzymes are inactivated, but if undercooked, the amylase and lysozyme start to break down any starches present.
D: Like the flour I used to thicken the pie filling?!
Y: Exactly!
D: But then why would OVERcooking also give me watery custard?
Y: Egg whites contain long string-like, protein molecules. Heating too quickly or too long causes these protein molecules to change shape, toughen and shrink. When overcooked, custard looks curdled and spongy rather than creamy, and the proteins' shape change squeezes out water molecules from the whites as well as preventing the milk from thickening.
D: Sounds like I'll need more practice to master grandma's pie recipe—luckily the failed attempts are still delicious!
Y: This moment of science comes from Indiana University. I'm Yael Ksander
D: and I'm Don Glass.
