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How Eggs Yolks Thicken Custards, Pies, Desserts, and More
Custard ice cream, creme brulée, lemon meringue pie. They all rely on 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.
- Egg yolks need heat to thicken
- How to thicken a liquid with egg yolks
- Why use egg yolks to thicken?
- Recipes using egg yolks to thicken
Egg yolks need heat to thicken
All of the applications mentioned in the start have something crucial in common. They’ve all undergone some sort of heat treatment. The custard was cooked, the creme brulee and pie were baked. It’s only after they were heated that they thickened up. This is because heat is crucial for egg yolks to showcase their thickening power.
Heat unfolds proteins
Egg yolks contain a special set of proteins. Proteins themselves are very common in food, but every type behaves slightly differently. The proteins in egg yolks can’t handle too much heat that well. When you heat up egg yolk proteins sufficiently, they will start to unfold and uncurl, scientists would say they denature.
While doing so though, the proteins bind water and form a soft network with each other. They prevent water from moving freely and this is what causes a liquid to thicken. The product behaves more like a solid than it does a liquid.
Too much heat curdles the egg
But, too much heat is not a good thing either! At this point, they’ll curl up even more. The network tightens and instead of holding onto water, will start to expel water. As a result, you end up with an unwanted scramble in which tight curds of protein float within the remainder of the liquid.
The exact temperature depends…
If you’d boil or fry an egg, just as is, most of the proteins in the egg yolk will denature within a temperature range of 65-70°C (149-158°F). How fast this happens depends on the method you use to cook the eggs. If you cook an egg at exactly 70°C (158°F) it may still take a long time for it to heat through completely and for all the proteins to denature.
Things get more complicated when you mix your egg yolks with other ingredients, such as sugar, milk, cream, etc. The temperature at which your egg yolks have fully thickened the resulting mixture may well be closer to or even over 80°C (176°F).
And may require precision
There may be a tight line between getting all of that thickening power out of a yolk, and ruining it by scrambling the mixture. It may only be a degree in temperature. So better be careful when handling those eggs!
How to thicken a liquid with egg yolks
Now that we know what happens within, let’s have a look at how we can make it happen. How can we thicken a dessert, without causing any curdling? There are a few factors to keep in mind.
Even heating – Stirring & Water baths
It is crucial to evenly heat a mixture that you’d like to thicken with yolks, especially if you’re doing so in a pot on a stove. The bottom of the pot will be hotter than the top, so might cause the egg to curdle way before the top has even started to curdle.
The same goes if you’re baking in a hot oven. The temperature of that oven will be a lot higher than the temperature your eggs need to be. This can cause the outside to overcook, while the inside hasn’t even started to thicken!
So, if you use a stovetop, stir continuously. If you’re baking something in the oven, use a water bath. The water bath will never get above 100°C (212°F), greatly reducing the chances for curdled eggs.
Take it easy
Since the transformation between a silky smooth texture and a clumpy water one is quite sudden, it’s important to take it easy. Once you get close to the perfect texture, don’t hurry things along.
Pre-heat other liquids for a quick start
It’s a commonly used method for making custards and the like. Bring your milk, or other liquid, to a boil, often referred to as scalding. Then, pour (part of) that liquid into your prepared egg yolks. This way, you’re pre-heating the eggs, without risking overcooking them. Often, you’ll end up with a mix of about 60-70°C (140-145°F). The big advantage here is that you can heat up those liquids fast. No need to do it slowly, or stir all the time. Instead, you can heat those considerably faster. This way, you’ll only have to take it slow at the end, greatly cutting down on time.
Why use egg yolks to thicken?
Egg yolks aren’t your only choice when it comes to thickening sauces, soups, desserts, and more. But, they do result in some unique characteristics. First, egg yolks don’t just contain proteins. They also contain a considerable amount of fat. This provides some creaminess to whatever it is you’re trying to thicken.
Another extra benefit of egg yolks can be the lecithin that hides within them. Lecithin is an emulsifier, it helps fat and water to remain mixed, preventing separation. In desserts with a lot of fat this can be very helpful, as it stabilizes the mix, even before it has started to thicken.
Mayonnaise is generally made using egg yolks. But it’s not the egg yolks that cause the thickening here. That is done by the fat droplets. However, the lecithin helps a lot in emulsifying and keeping the mayo together!
Cornstarch can thicken, but it’s different
If you do not want to use egg yolks, there are several alternatives, depending on what it is exactly you’re looking for. We’ve got several examples on this website on replacing egg yolks with corn starch in both ice cream and cakes.
However, corn starch does work slightly differently. The gel it forms is slightly less light and fluffy as one made with egg yolks. As a result, the final product will turn out different, but whether it meets your needs depends all on your criteria.
Corn starch thickens liquids through gelatinization of the starch, a very different mechanism than that of egg yolk thickening. Other starches such as potato and arrowroot, as well as the starch in flour work in more or less the same way.
Why not the whites?
So far we’ve only discussed egg yolks. But, every egg yolk comes with an egg white. Why not use that as well? It’s because despite being part of the same product, yolks and whites behave very differently.
Egg yolks contain a reasonable amount of fat, aside from the protein. Egg whites on the other hand do not. Furthermore, the protein in egg whites are quite different to those in the yolk. They set at slightly lower temperatures that the egg yolk proteins. As a result, adding egg white, though not being impossible, will impact the texture of the overall thickened liquid. It might be less creamy, becuase of the lower fat content, and the overall texture might be bit more bouncy.
Have egg whites left over from a recipe only requiring yolks? Don’t throw them away! There are plenty of other products you can make with it, such as meringues.
Recipes using egg yolks to thicken
Mariano Cardenas, How to confirm you’ve cooked perfect eggs, using scientific instruments, Sep-3, 2020, link
Cooking for Geeks, 141°F / 61°C: Eggs Begin to Set, link
mmcmillan1, The Perfect Egg, Aug-18, 2014, link