The Science of Food

The world of sauces can be pretty intimidating. The French, in particular, have elevated sauce making to such an extraordinary extent that the whole idea of making a sauce can seem overwhelming. They have their mother sauces, their daughter sauces and a whole culinary tradition based specifically around sauces. To the French a good sauce is the hallmark of culinary ability and in a French kitchen the saucier stands in rank below only the sous and executive chefs. Faced with this tradition, and that it can take up to 24 hours to make a demi-glace, the home cook may think it is just easier to leave sauces to the professionals. I certainly did.

After I’d been cooking for a while though I came to a couple of realisations: firstly, I wasn’t running a Michelin starred restaurant, and secondly, a sauce is just a thickened liquid containing some tasty stuff. Yeah, if a good restaurant expects us to part with our hard-earned money it needs to make sure its stocks and sauces stand out but us home cooks can lower our standards just a little and still make some pretty impressive and tasty sauces with a fraction of the effort. This is possible because the whole art of saucing rests on a few techniques that are actually pretty simple and we can use these techniques to make some simple and quick sauces that still taste pretty good indeed.

If you a regular reader of this blog we’ve already covered some of these techniques. We’ve looked at vinaigrettes; a water in oil emulsion that can evoke a whole world of flavours based on your choice of oil, vinegar and aromatics (chimichurri, for example, goes great on steak). We’ve learned how to make a mayonnaise in 15 seconds, which can then be used to make tartar sauce, remoulade, cocktail sauce, aioli, thousand island and ranch dressing. We’ve looked at making a roux, the technique behind the French mother sauces bechamel, velouté and espagnole. These sauces are, in turn, the base for things like cheese sauce, mornay sauce, demi-glace and bordelaise sauce, not to mention good old gravy and, for some Asian influence, curry sauce. Really, once you master vinaigrettes, mayonnaises and rouxs you can stick anything your fevered flavour dreams comes up with in them. More importantly, if you want to spend 24 hours making a stock from scratch and then reducing a demi glace you can, but these techniques can also be used to whip up something quick and easy for a mid-week dinner.

Today I want to add another weapon to our sauce making arsenal and have a look at using egg proteins to thicken a sauce¹. The most famous example of this type of sauce is custard, which, at its simplest, is just a mixture of milk (or cream), sugar and flavourings that has been turned into a gel using denatured egg yolk proteins². The gel can remain a thickened liquid, like in creme anglaise, or be set into a solid, like a custard tart. We’ve seen something very similar to this when we looked at rouxs³ that use the gelatinisation of starches to create a gel. Starches break down and form a network of “unwound” polysaccharides that trap water and thicken a liquid. The amount of thickening is dependent on the amount of starch you add and how many of the starch granules you break down at the temperature you are using.

When we make custard we are doing exactly the same thing, we’re just using denatured egg proteins instead of polysaccharides to build the network. In my second ever post, on how to fry an egg, I described the process of coagulation where heat denatures proteins that unwind and form a network that traps water and “sets” the egg. Making custard is exactly like frying an egg, or making scrambled eggs, in that the aim is to coagulate egg proteins to a certain consistency. Just like cooking with starch, the thickness of our custard will depend on how many egg proteins we’ve added and to what degree we’ve coagulated them. Using these dials, we can produce all the different types of custard, from a runny creme anglaise to a set custard like creme brûlée.

There is a problem though. If we overcook starch it just breaks down, and our gel gets thinner. If we overcook eggs, bad things happen. We already know that as the temperature increases the network of coagulated egg proteins will get tighter and tighter until it squeezes all the water out leaving a dried-up egg. If this happens in our custard we’ll end up with a lumpy custard containing little grains of coagulated egg yolk. If that’s not bad enough, we’ll also get the distinctive sulphuric flavours that an over-cooked egg develops when sulphur containing amino acids break down and form hydrogen sulphide, a gas that smells like rotten eggs. None of this is conducive to a delicate custard, so it’s crucial to keep a tight control over the coagulation of our egg proteins when we are making custard.

You may be asking why bother with egg yolks if they are so delicate? Can’t we just use a starch to thicken our custard? Well, the answer is, yes, we totally can, but then it wouldn’t be a custard. The reason why we bother with eggs is that they produce a more delicate gel and their fat content and emulsifiers bring a velvety texture and a creamy flavour that you don’t get with starches. For this reason, custard is defined as a gel made with some type of dairy that is thickened with egg proteins. Without the eggs it isn’t custard. It is only in the world of corporate marketing that custard powder makes custard. The majority of custard powders contain flavourings and cornstarch but no eggs, making it a starch thickened mixture and not technically a custard. Not to say that starches are completely absent from the custard world, they can be used in conjunction with egg proteins to make a more robust gel. We see this in something like creme patisserie where we want the flavour of custard in a more robust gel that can be piped and used as a filling.

So, if we want to make a proper custard there is no getting around the fact that we are going to be dealing with eggs. And because we want to coagulate our egg yolk proteins without over-cooking them, making custard is really an exercise in temperature control. The easiest way to do this is to have some specialised kit. If you were working in a laboratory, you’d head straight to a water bath, a device that provides a constant even temperature and, in the kitchen, you could use a sous vide machine, which is basically the same thing. If you are lucky enough to have a sous vide machine at home, you can make a pouring custard by simply vacuum sealing your custard mix and chucking it into the water at a temperature of 79-82C for an hour or so and hey presto custard. I’ve done this and perfect custard every time. Things get a little more complicated for a set custard, you need to find jars and make sure you only finger-tighten them to prevent pressure build up, but it’s doable and there’s plenty of recipes for that on the internet.

If you are not a kitchen nerd and don’t have a sous vide machine, it’s time for lower tech, but still time-tested, methods. A common technique is to use a double boiler; gently simmer water in a saucepan with a heat proof bowl placed on top. Use the mixing bowl to gently heat your custard mixture to 79-82C and wait for it to thicken to the consistency you are after. By using the steam from the simmering water instead of the direct heat of the hob you provide a gentler temperature gradient in your mixing bowl that reduces the risk of curdled proteins. You should also be constantly whisking so that none of the mixture stays in the hotter areas at the bottom of the bowl for too long. If you are feeling brave, you can certainly make pouring custard directly in a saucepan over a low heat, but it takes a lot more care and things can go wrong a lot quicker than if you use a double boiler because of the much steeper temperature gradient.

When making a custard a common technique is to steep your milk, or cream, with some flavour producing ingredients before you add the egg yolks. This provides some time for the flavours of other ingredients, particularly fat-soluble ones, to permeate the milk. When steeping you gently heat the milk and other ingredients until it is at a gentle simmer, take it off the heat and let it sit for a while and then strain. Depending on how long you’ve let your milk sit, it may still be well above the temperature of the egg protein’s coagulation point. So, if you just add eggs they’ll instantly curdle and ruin your custard. In this case you can “temper” your eggs by adding small amounts of the heated milk to them while vigorously whisking, much like when making mayonnaise, the aim here though is to control the temperature. When you’ve sufficiently diluted your eggs, you can add them to the bulk of the milk mixture. Alternatively, you can just let your steeped milk cool right down before adding the eggs and then reheat the whole thing in the double boiler.

Some of you may have picked up that the temperature range I’ve been talking about for custard is higher than the coagulation point of egg yolk proteins that I discussed in the frying an egg post. In that post we learned that the coagulation point of the yolk is about 70C, but we’re looking at taking our custard up to 79-83C. Why aren’t our proteins overcooking? The difference arises because when we mix egg yolk proteins with the other ingredients their coagulation point increases. In custard, for example, we have a lot of fats, from the milk, added sugar and the proteins themselves are more dilute than they are in an egg yolk. The fats and, in particular, the sugars interact with the egg proteins preventing them from forming a network and, because they are diluted, the proteins have a harder time finding each other. This means that we need a higher temperature than we did when frying our egg.

This also means that the coagulation point for your custard can vary from recipe to recipe, depending on the other ingredients⁴. This isn’t such a problem when making pouring custard as you can see how thick the custard is as you stir⁵. Things are a bit more difficult for a baked custard. If you aren’t using a sous vide then you are most likely using a bain-marie⁶, to make standalone custards like a flan or creme brûlée, or you have your custard in a pastry base, that has probably been blind baked. Because you can’t be sure of the exact temperature that your tart will be properly set it can be difficult to tell when it is done. One rule of thumb is that the middle of the tart should be a little bit wobbly when you gently shake the tart, remembering that it will stiffen up a bit when you let it cool, but sometimes it just takes trial and error to perfect a particular recipe. If it is taking longer than expected, just be patient, turning up the heat is rarely the way to deal with a slow setting custard⁷.

I should point out that I am not a good baker so take any baking advice from me under advisement. Baking custards is one of those areas in cooking where art meets science, we know the science, egg proteins properly coagulated, but the vagaries of the ingredients, the oven and the other equipment you are using is where the art, and the eye for knowing when a tart is done, comes in. My daughter has never forgiven me for telling her to give her chocolate lava cakes five more minutes in the oven, which, of course, led to them being overdone. So, learn from her mistake and disregard any specific baking advice I may dispense that isn’t directly linked to chemistry.

Custards are a great thing, but they have a reputation as being a bit tricky to make. I don’t really think that’s true. If you understand what’s going on, and you’re happy to have one or two failed attempts in the beginning, there is no reason you can’t be whipping up a custard like a pro in no time. It’s also well worth the time mastering the skill. It’s been said that custards are the mayonnaise of the pastry world; you can put whatever flavours you want into a custard, and chances are it will be delicious. French-style ice-cream is basically frozen custard and if you want an easy recipe for creme anglaise then just melt some ice cream⁸. Although we often think of custard as a sweet ingredient it’s also a useful savoury component. Quiche is basically a savoury custard tart, Chinese and Japanese cuisines both have steamed savoury custard dishes, like chawanmushi from Japan, royal custard is used in Germany as a garnish for stews and soups and various Balkan versions of moussaka use set custard as a topping.

Although dairy is one of the defining characteristics of custard, there is no reason why you can’t use egg proteins to thicken other mixtures. Carbonara, for example, uses egg yolks to thicken its sauce using the residual heat of the pasta to coagulate the egg proteins. The Greek Avgolemono is a warm broth thickened using eggs and lemon juice⁹ that can be used as a sauce or a soup. Sopa de Ajo is a cherished Spanish garlic soup thickened the same way. Indeed, just about any soup can be thickened using eggs, sometimes after mixing with cream to form a rich liaison, as it is called in France.

That’s about it for custard and other egg thickened dishes. In my egg frying post, I pointed out that eggs are one of the most versatile and useful tools that we have in the kitchen, and custard is just one more piece of evidence to back up that claim. It’s also a delicious piece of evidence and learning how to make custard is something you’ll never live to regret. Well, probably, you may come to regret an expanding waist line unless you use your custard superpower sparingly.

Footnotes

1. A lot of custard recipes only call for the egg yolks, while others use the whole egg and yet others use only the whites. Generally, I’ll assume we are using yolks only, but I’ll call out situations where the part of the egg used may make a difference. ↩︎
2. Now, hang on, you might be saying, we’ve already used eggs to thicken sauces, what’s mayonnaise but an egg thickened sauce? Well, the difference is that in mayonnaise we were using the emulsifiers from the egg yolk to create an oil in water emulsion, in custard we are denaturing the egg proteins so they will form a gel. ↩︎
3. This is a bit off topic but apparently the plural form of the word roux is also roux. Given it is a French word that English has stolen I guess that this is an expression of the French rule that words ending with -s, -x, or -z keep the same spelling in the plural. But French has plural definite and indefinite articles that reduce confusion while English relies on the -s. So, I’m going to keep to the English rule that plural nouns have an added -s. I know we have zero plural nouns like deer and sheep, but I don’t think roux is thought of that way. Basically, it just sounds weird to my ear without the -s. ↩︎
4. I’m also ignoring the fact that some custards can be made with the whole egg or with the whites only. We know that egg whites have a lower coagulation point than the yolks so if your recipe calls for whites not yolks, or whole eggs, the coagulation temperature will vary accordingly. ↩︎
5. In fact, I recommend just taking a custard all the way to scrambled eggs at least once. Once you have made the complete journey you are better able to judge exactly where you are the next time. ↩︎
6. A bain-marie is simply some container that is filled with water to provide a buffer from the direct heat of the oven. For example, if you take a baking tray and half fill it with water and then place ramekins containing a custard mix into the water for baking then you are using a bain-marie. ↩︎
7. An exception to this are things like Portuguese custard tarts that are cooked at very high temperatures for a short period of time. I’ve never tried making these, but I expect it takes some trial and error to get the cooking time right. ↩︎
8. OK, this is kind of true. Turns out that ice-cream is the kind of rabbit hole that leads to another post. In the States, at least, ice-cream can be a bunch of different things, and there is also something in the States called frozen custard that is made with a different churning process. So, it’s kind of true. If you are going to melt ice-cream to make creme anglaise than use a fancy, i.e. expensive, French-style ice-cream. ↩︎
9. The use of lemon juice in Avgolemono has a biochemical basis distinct from it’s role as a flavouring. In the acids post we looked at how pH alters the charges on proteins which can change how they interact with each other. In this case the acid of the lemon alters the protein charges such that they are less inclined to coagulate making them more resistant to the heat of the soup. ↩︎