We often come across recipes where we need to mix egg yolks with caster sugar. This would appear to be a very ordinary and simple thing to do but, be warned, these two ingredients can behave oddly together.
Let’s take confectioner’s custard (crème pâtissière, or french pastry cream) as an example: there’s a stage at which the egg yolks are mixed with the sugar. Is there anything particularly difficult about this? Not really. We mix the yolks and sugar together (without whisking) and that’s all, but there’s an important little tip well worth knowing. You might be tempted to separate the egg yolks from the whites and simply tip them into the weighed-out sugar, as shown in the photo below. Then why not go off to do something else, such as putting the milk on to heat up, before coming back to the egg yolks and sugar mixture later?
Why not? Because you will regret it if you do! Something starts to happen as soon as the egg yolks come into contact with the sugar. The sugar sucks out the water from the egg yolks (we say that sugar is hygroscopic) and this effectively begins to “cook” the surface of the yolks. The result is patches of hard yolk (like in a hard-boiled egg) which will not mix in when you add the boiling milk. These will remain as small lumps in your custard and spoil its normal smooth, creamy texture.
This might not seem like a big deal, but it happens very rapidly. If the egg yolks and sugar are in contact for longer than a minute, the reaction will begin and little hard lumps will start to develop.
How can we avoid this? It’s really quite simple, and this goes for any recipe involving a mixture of egg yolks and sugar: put the egg yolks into bowl, then add the sugar required and mix immediately with a spoon or, better still, with a soft spatula. Once mixed like this, the mixture can be left until later without any problem.
As a precaution, you can do what professional pastry chefs do: 1) make sure you use this method of mixing immediately, 2) strain your mixture of egg yolks+sugar+milk+vanilla+cornflour through a fine sieve when you pour it back into the saucepan. This way, you can be sure that your custard will be perfectly smooth. You might well be surprised by what you see left in the sieve: lumps, milk skin, bits of cooked egg yolk, shreds of vanilla pod…
It is worth mentioning that professional pastry chefs will often strain their cold custard a second time through a very fine sieve before using, to make absolutely sure that the texture is smooth. But maybe that’s going a bit far for us amateurs…
To sum up: A recipe where egg yolks are mixed with sugar? Mix these two together immediately with a soft spatula, then you can safely leave them while you get on with the rest of the recipe