The Curious Chemistry Of Custard - BBC Future

Combine milk, eggs, sugar and gentle heat and one of our most treasured comfort food appears. But how is this creamy concoction conjured? Veronique Greenwood peers into the pot.

This year was the year of pies. In a particularly long and snowy winter, I began to work my way through the Four and Twenty Blackbirds Pie Book, starting with a salt pork-apple pie. Now I've made 17 of the recipes, some of them multiple times, for a total pie count of 37 this year. Many recipes were leaps of faith. Really? I pour pork fat over the apples? Mix ground juniper berries with pears? Add roasted beets? – but none quite so much as my very first custard pie.

It was a Meyer lemon custard with a layer of chocolate ganache painstakingly spread in the bottom of the pie shell. I popped it into the oven, and waited. And waited. After 50 minutes, when it should have possessed a self-confident jiggle, my custard simply sloshed.

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The chemistry of custards is a delicate business, though the ingredients are simple: eggs, milk, sugar. The eggs, especially the yolks, are the chemical stars – it's their actions that matter most, generating the thick gel that's the key for a custard pie, says Guy Crosby, food scientist and science editor at America's Test Kitchen. The rest of the gang are there to help ease the eggs in the right direction. And in On Food and Cooking, Harold McGee emphasises that even the milk isn't strictly speaking required – just any substance with dissolved minerals. “Mix an egg with a cup of plain water,” he writes, “and you get curdled egg floating in water; include a pinch of salt and you get a coherent gel.”

So what is going on? (Or, in my custard, failing to go on?) As the liquid heats up, the yolk proteins, previously tightly packed in small granules, start to unfurl. Left to themselves, they'll make a few bonds with each other and create a tough, grainy substance at around 150 F (65C) – essentially, a hardboiled yolk. But the water in the milk and the sugar slow that process down, keeping them from binding so quickly. “The sugar is coating and physically blocking the proteins from getting together” until the temperature rises further, Crosby explains.