Custardy battle

Everybody likes custard, but who likes lumpy – or even chunky – custard? Today I’m presenting 4 simple techniques to keep the chunks out of your custard.

Before we start, I should clarify that by custard I mean the real deal: made with milk or cream, sweetened with sugar and thickened by either whole eggs or just the yolks.

This is not to disrespect the stuff you make using custard powder, which can be quite yummy and fascinating in its own right, being a non-Newtonian fluid that gets more viscous when you apply more pressure. But let’s be honest: real custard, or crème anglaise if you will, is a much better companion for plum pudding, and you can’t make ice cream out of custard powder.

Like I said, it’s thickened with eggs, but it’s very easy to go too far and end up with lots of lumpy, grainy curds. For this reason many people are intimidated and won’t go near it, despite the culinary temptations. But you can remove some of the mystery using science; and my main reference is as usual Harold McGee’s book On Food and Cooking, an invaluable resource for any kitchen.

Alright, we’ll get the primary but least interesting technique out of the way early: method 1 you will see in just about every recipe: strain out the lumps.

That may sound obvious, and if so, good on you. What you’ll find with most of the other methods is that you’ll reduce the lumps, but there’ll often still be a few. So it’s safest just to strain them.

Okay, method 2 is a bit more scientific: use a thermometer. Here’s where we need to understand how eggs work.

Eggs of course are wonderful devices, designed to protect and nourish baby chickens. If you take a 55g egg, most of it, about 41g, is water. There’s 6g of fat, only 0.6g of carbohydrate, 213mg of cholesterol, and what we’re interested in, 6.6g of protein.

There are many different kinds of protein in both the yolk and the white, all designed to do different jobs: obviously, they provide food for the chick, but they also protect against infection, prevent them from being nutritious for predators – laboratory animals fed on raw egg actually lose weight – and physically protect the embryo.

The proteins – and remember proteins are long chains of amino acids all joined together – are folded up tightly into little knots floating in the liquid. This is why the white starts off transparent.

Coagulation of egg proteins: on the left an uncooked egg, with tightly bound knots of proteins; on the right, the cooked egg with unravelled proteins forming new networks (click to embiggen))
Coagulation of egg proteins: on the left an uncooked egg, with tightly bound knots of proteins; on the right, the cooked egg with unravelled proteins forming new networks (picture adapted from Harold McGee's book, and is obviously not to scale - nor are eggs oblong)

But when you heat the egg, the proteins jostle around and break the bonds that fold them so tightly together – we say they denature, or lose their natural form.

They unfold into long chains which then bump into and bond to each other, forming a network or messy web of protein throughout the material. This is when the egg sets or coagulates, becoming solid and opaque.These proteins are actually joined together using disulfide bonds, that is, involving bonds between sulphur atoms. So you can add certain chemicals like sodium borohydride (NaBH4) which break those bonds and can “uncook” the egg.

Anyway, the different proteins coagulate at different temperatures, but overall a whole egg, that’s yolk and white combined, sets at about 73°C. But in custard it’s diluted – a basic recipe is one whole egg to 1 cup of milk and 1 tbsp sugar – so it thickens at a higher temperature, about 78-80°C.

You get the lumps when you overheat it: then the proteins bind too tightly together, forcing out the water from their little networks and becoming hard little lumps. That happens at only another 5 or so degrees, so you need to watch the temperature very closely. Try to keep it below 85°C.

Method 3 is actually the most reliable: add flour or cornflour, or even cocoa. The starch granules absorb water and also dissolve a bit, all getting in the way of the proteins bonding.

You can even bring the temperature up to the boil, in fact, as McGee says, if you want a thick, stiff custard like crème pâtissière you must boil it, because the egg yolks contain an enzyme called amylase that digests starch and will make it all runny again unless you kill the amylase first by boiling it.

So there you go. Making custard is just like doing a chemistry experiment: understand the basic science, use the right ingredients, and watch the conditions.

Wait, what about method 4? That’s easy: practice, practice and practice, and get really good at it. Sounds facetious, but after enough times heating it slowly – like by using a double boiler – and using a kitchen thermometer to keep it just a bit over  80°C, you’ll get used to how it thickens, becomes glossy, and coats the back of a wooden spoon, as the professionals say.

Really, the art of cooking is just science with practice.

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