Learn the science behind:
Custard ice creams, creme brulée, lemon meringue pies, they all can’t go without: egg yolks. Without them, custards melt too quickly, creme brulées are runny, and the pie filling wouldn’t set.
Egg yolks can thicken liquids. But, it’s a fine balance. Handled incorrectly and they may curdle or scramble. It’s truly a matter of knowing your eggs and how best to treat them.
Splitting an egg, to optimize functionality
Eggs are special ingredients, made up of two very distinct phases: the white, and the yolk. Both have a clearly defined role in the egg itself, in case it would have grown out into a chicken. And as a result, both have a very different composition and their own optimal uses in cooking and food manufacturing.
Egg whites foam
Egg whites, which only turn white when heated, are great at forming foams. Use a whisk and within a matter of seconds the formerly translucent liquid ingredient becomes white, light and fluffy!
Egg whites consist of mostly water and proteins. As such, they don’t provide a lot of richness to a dish. But, their unique foaming capacity makes them great for a wide range of meringues.
Egg yolks enrich
Egg yolks on the other hand still contain a good amount of proteins, though a different type than egg whites, but they also contain fat, as well as an emulsifier called lecithin. This combination of ingredients makes egg yolks suited for a range of different applications.
Why do egg yolks cause thickening?
On this blog we’ve discussed the thickening power of flour and corn starch in great detail before. In those cases large carbohydrates in the flour and starch will absorb water and thickening a mixture. It’s pretty hard to overcook a custard made with these thickening agents, the carbohydrates will keep doing their work.
When making an egg yolk custard though things change slightly. Here it’s not the carbohydrates that cause thickening of the custard, insead, proteins do the job. That’s also the reason these custards are more prone to ending up in a mess. If you overheat the proteins, they will form too tight networks, resulting in a curdling mixture with solid like lumps.
So how to these proteins do there job? When heating the proteins they will slowly unfold. This unfolding allows them to bind water and form a network of proteins. This network prevents any liquid in the custard to flow freely and thus we end up with a thicker gel like consistency, the custard.
Managing those egg proteins
The trick to making a good egg yolk custard is to heat the custard slowly while constantly stirring. Stirring is important to keep the temperature the same throughout the entire bowl. If you don’t stir the bottom might heat up a lot more than the top which causes the proteins to curdle at the bottom, while not yet doing their job at the top. The slow heat is important to prevent overcooking the mix. Once you’ve become more comfortable with making the custard there’s no reason why you wouldn’t speed up a little at the start and only slow down more towards the end. Another trick when making custards with larger quantities milk or cream is to scald (pre-heat) these liquids and pour them into the egg yolk slowly, causing more careful thickening.
Lemon bars/pie using egg yolk custard
Now that you know what happens, try to use that knowledge when making the recipe described below. The recipe I used has been slightly adapted from The Kitchn. I mostly reduced the amount of lemon, it was lemony enough as is for sure. Also, I tend to leave out a straining step and just leave the lemon zest inside the pie.
Regarding the use of zest, there’s a good reason for not only using juice, but also zest.
And maybe you’re wondering why you can’t add the butter at the start. Theoretically you could (I accidentally already added some), but the butter is very helpful here. Once the thickening process of a custard has started it will take a while again for it to slow down. Just taking the pan from the heat isn’t enough, it will keep on thickening for a while. By adding some colder butter you will speed up cooling and thus stop and further thickening!