When I make ice cream or some other sort of custard I would normally ‘cheat’, I’d use some corn starch or regular flour to thicken up my custard. It’s some sort of cheating since it should also be possible without any flours, but by using egg (yolks) only. Nevertheless, those often didn’t seem to work out. Until I made these lemon bars. The custard on top thickened perfect without using any ‘cheating’ thickening agent.
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!
- 60g butter
- 40g icing sugar
- 75g flour
- 40g almond flour (or take 40g of almonds and blitz them into a powder using a food processor)
- pinch of salt
- 2 eggs
- 2 egg yolks
- 100g granulated sugar
- zest from 1 lemon
- juice from 2 lemons
- pinch of salt
- 60g butter
- Mix the butter and icing sugar. Quickly mix in the flour, almond flour and salt, but don't mix for too long. It will be pretty crumbly at this point but stick together enough if you put it in a baking tray.
- Take a round 16cm baking tray, cover the bottom with baking paper and spread the dough around.
- Bake in the oven at 180C for about 20 minutes, the crust should start browning slightly.
- Mix all the ingredients (except for the butter) in a small sauce pan.
- Gently heat on a low fire while continuously whisking. The mixture should thicken into a curd.
- Once it's thickened up (do not overheat or it will scramble) take from the heat and mix in the butter until it has melted completely.
- Pour over the crust and put back into the oven at 180C and bake for another 10 minutes. It should have not set completely, still move slightly when shaken.