Egg Coagulation in the Kitchen: Custards

One of the most important functions of eggs is to provide structure and determine texture. We saw this with breads, cakes, cookies, and muffins, with brownies, and also with meringue. But there’s one more category of baked good that depends on eggs: custards and creams. Eggs set and thicken crème brûlée, quiche, cheesecake, pastry cream, and crème anglaise. In this post, we’ll first review how an egg cooks, then explore how different ingredients and techniques affect this process to create smooth custards and creams.

Egg proteins create structure and change texture when they cook.

As we discussed in this post, eggs provide structure because they contain proteins. Proteins are responsible for an egg white’s transformation from clear liquid to white solid as it cooks. (Egg yolks also have proteins, but because they contain fats and yellow color as well, the direct effects of the proteins are harder to picture.) Let’s review what happens to the proteins when we cook an egg white.

Remember that proteins are like long strings of amino acid beads. Amino acid beads have different chemical properties that dictate how they interact with each other and their environment. As you can see in the example below, these properties are like rules that fold the protein string into a three-dimensional shape. For this discussion, the most pertinent types of amino acids are the hydrophilic, “water-loving” ones, which interact with water, and the hydrophobic, “water-fearing” ones, which avoid water.

We can think of proteins as strings of beads with rules about which beads attract or repel each other. These rules fold the protein string into a three-dimensional shape.

Egg proteins coagulate when heated.

As we discussed in the context of egg foams, in a raw egg, individual protein strings are folded into wads with water-loving amino acids on the outside and water-avoiding amino acids tucked inside. These proteins float freely in the watery egg white, and the white is clear because light easily passes through the space between the proteins.

When we heat the egg white to a certain temperature, the protein strings unravel and denature, exposing the amino acid beads inside. Remember, these are the water-fearing amino acids. Now that water has infiltrated their hiding place, they need to find another way to avoid it. As the protein drifts through the egg white, the exposed water-avoiding amino acids find other hydrophobic amino acids from neighboring protein strings. Perfect! They stick together and the proteins coagulate.

Now imagine this process happening with every protein. The result is a strong mesh of protein strings that extends throughout the entire egg white and solidifies it into a food we bite through. The protein network also blocks light, so the egg becomes opaque.

At this point, the protein mesh is holding the egg’s water. But if we continue to cook the egg white and heat the proteins, the proteins draw closer together and squeeze the water out. Overcooked egg whites have a rubbery texture because the proteins are densely packed, and they’re dry because there’s not much water left. In a frying pan, the excess water evaporates so we don’t see it, but in an overcooked, curdled custard, this squeezed-out water is the liquid that surrounds the curds.

The proteins in a raw egg are folded in their native state. As they cook, they denature, then coagulate. If the proteins are heated further, they will overcook.

Egg protein structure determines custard and cream texture.

When we cook custards and creams, the egg proteins undergo the exact same process. And just like a fried egg, the shape and density of the protein mesh determine the texture of our bake, whether that be delicate, thick, and smooth; or tough, cracked, and curdled. In fact, the difference between a custard and a cream comes down to the proteins. Custards, like cheesecake and flan, are solid. They’re baked in a container, and the proteins form a single unbroken network through the entire mass. Creams, on the other hand, are meant to be poured or piped. They’re usually stirred as they cook or cool, which breaks the egg protein mesh into smaller pieces. The fragments still thicken the cream, but they won’t hold it together into a solid.

However, our manipulation of protein structure begins before the custard is even heated. The ingredients we add to the eggs affect the density of the protein mesh that will form, and by extension, the texture of the final product.

Some ingredients increase tenderness by preventing coagulation.

Most custard ingredients limit coagulation simply by diluting the egg proteins. If the proteins are further apart, they’re less likely to find each other and coagulate when they’re cooked, so they form a sparser, weaker protein network and a more fragile, soft custard. However, dilution is only part of the story. Let’s take a closer look at how these ingredients work. (And if you’d like to see some of these ingredients in action, check out the post on Pumpkin Custard Buns!)

Liquids

Most custards and creams contain a liquid like milk, cream, or even water. If you like to add a dash of milk to your scrambled eggs, you’ve tasted the difference yourself: eggs made with additional liquid are softer and moister.

Liquids tenderize by diluting egg proteins. The more liquid we add, the sparser the protein network and the more delicate the custard. Since dairy products like milk and cream are predominantly made of water, they also increase the water content of the custard. When the egg proteins cook, they hold the extra liquid within their coagulated mesh for greater moistness.

Diluted egg proteins coagulate less and trap more water.

Fats and emulsifiers

Milk and cream also tenderize custards because they contain fats. Other sources of fats include egg yolks, oil, cream cheese, and butter. Fats dilute egg proteins, but they also interfere with coagulation.

Remember how coagulation occurs when hydrophobic amino acid beads stick to each other to avoid water? Well, fats are also hydrophobic, which means they’re just as good for avoiding water. If exposed hydrophobic amino acids bump into fat first, they’ll stick to it, and they no longer need to find another protein’s amino acids. Because of this, fats reduce coagulation. Since emulsifiers also have a hydrophobic end, they have the same effect. We can see the effect of fats and emulsifiers just by comparing the egg yolk and egg white in an over-easy egg. The white is solid and the yolk runny because the fats and emulsifiers in the yolk prevent it from coagulating as quickly as the white.

Fats and emulsifiers coat hydrophobic amino acids, so egg proteins don’t coagulate.

Fats from cream and egg yolks also add creaminess to custards. In these ingredients, the fat is packaged into tiny spheres that give us the perception of creaminess when they roll across our tongues. These droplets of fat add their creaminess to custards as well.

Sugar

Another tenderizing ingredient in custards is sugar. Sugar dilutes egg proteins, but it also hoards water for itself, which limits the amount of water available to help egg proteins unfold. If the proteins can’t unfold, they can’t coagulate. (Sugar’s strong attraction to water also defines its roles in egg foams and its interactions with gluten and starch.) An excessive amount of sugar can actually prevent a custard from setting. Conversely, savory quiches, which contain minimal sugar, are more likely to overcook and curdle than sweet custards.

Starch

Starch isn’t a requisite component of custards and creams, but it’s often used for thickening. For example, flour stiffens pastry cream so that it can hold its shape in a cream puff.

Remember that starches are long chains of sugar molecules packaged into compact granules. When they’re heated, starch granules absorb water, swell, and burst, releasing chains of starch into the surrounding liquid. Because the starch chains and the burst granules are large, they impede the flow of the liquid around them and thicken it.

Starch granules absorb water, swell, and burst as they cook.

When a starch custard cooks, the starch granules use a lot of heat to swell. This leaves the egg proteins with less heat to unravel, so coagulation is delayed. Furthermore, when the starch granules burst, the starch chains create physical obstacles that block egg proteins from each other. If the proteins can’t find each other, they can’t coagulate.

Starch chains prevent egg proteins from coagulating.

Starch makes custard so resistant to heat that it can be boiled directly on the stove. In fact, it’s crucial to bring starch custards to a full boil. They rely on starch as much as eggs for thickening, but eggs contain an enzyme (called alpha-amylase) that cuts up the starch chains. Given several hours, these enzymes would leave only tiny fragments of starch in the custard, and the custard would thin. Boiling the custard brings it to a high enough temperature to inactivate amylase and prevent the custard from thinning.

Given its own roles in texture, it’s not surprising that starch changes the texture of a custard. Without starch, custards are delicate and fragile. Starch coarsens and stiffens that texture, and it also weakens the custard’s flavor.

Raw pineapple

Pineapples contain another type of enzyme, proteases, that snips protein strings into tiny pieces. These enzymes are what cause the stinging in your mouth when you eat raw pineapple, and they’ll break egg proteins into fragments that can’t coagulate. To add pineapple to a custard, it’s better to cook it first or use canned pineapple. Both of these methods expose the pineapple to high heat, which inactivates the proteases.

Some ingredients increase tenderness by preventing the proteins from tightening.

Other ingredients add tenderness to custards by encouraging the egg proteins to coagulate earlier. The proteins coagulate so fast that they don’t fully unwind before they start sticking together. Since portions of the protein strings are still wadded up, they don’t participate in the coagulated network, so the protein mesh is sparser and the custard more tender.

Salts and minerals

Salts and minerals, which can be added to custards via salt, dairy, or hard water, encourage egg proteins to coagulate quickly. Without them, negative charges on egg proteins repel each other, so when the proteins unravel, they need to look for a neighbor to coagulate with. When salts and minerals dissolve in water, they form charged ions. Positive ions hide the negative charges on the egg proteins, which allows the proteins to float closer together and coagulate as they unwind. (Remember, salt masks repelling charges on neighboring gluten strands, too!) Because the proteins don’t fully unravel by the time they coagulate, the mesh is sparser and the resulting custard more tender.

Salts and minerals encourage egg proteins to bond quickly, before they’ve fully unraveled.

Acids

Acids come from fruits and cultured dairy products such as yogurt, buttermilk, and sour cream. Like salts and minerals, they mask repelling forces to bring proteins closer together. And as in egg foams, they prevent super-strong disulfide bonds from gluing proteins together. If they can’t form these types of bonds, the protein strings can’t coagulate as tightly together, so the custard is more tender.

Acids like lemon juice tenderize custards.

Successful custards and creams are cooked slowly.

After we combine our ingredients, it’s time to cook the proteins and set them into shape. Remember that each stage of this process is entirely dependent on temperature. It’s as if the proteins are traveling down a road with a series of gates. First, they must reach the specific temperature at which they unwind. Then, they need to reach a higher temperature to stick together and coagulate. And finally, if the proteins get even hotter, they overcook. The difference between coagulation and overcooking can be as little as 5–10°F (2–5°C).

Overcooking creates curds and cracks.

Why is overcooking so bad? Remember that overcooked proteins draw together and squeeze water out. In creams, this means curds: clumps of dense protein floating in their own separated liquid. Curdled creams can be saved by straining out the curds or by blending the sauce smooth, but interestingly, if we blend a curdled cream, the result is thicker than a properly cooked cream. Despite being blitzed into miniscule pieces, the protein mesh is still too tight.

In custards, overcooked proteins can also curdle, or they might crack. As clumps of proteins overcook, tighten, and rigidify, they break apart, leaving a fissure down the center. Though a cracked custard can still taste delicious, a smooth surface indicates that the custard was cooked just right.

If overcooked, custards like this pumpkin pie filling can crack.

Slow cooking creates the best texture.

For the smoothest texture and the most insurance against curds and cracks, custards and creams should be cooked slowly. This gives us more time to remove them from heat before they overcook.

Slow cooking also ensures that the egg proteins coagulate as much as possible. If the proteins are heated too quickly, they can start to stick together before they’ve fully unraveled. If this happens, not all of the protein strings join the coagulated mesh, and the eggs lose some of their thickening power. As we discussed above, salt and acid do exactly this in a custard to make it tender. However, once they’re added, we need the egg proteins to coagulate as much as possible in order to get the right texture. Otherwise, we might end up with creams that are too thin or custards that don’t set.

Finally, cooking slowly means that our proteins cook evenly. We don’t want some proteins coagulate and tighten into overcooked curds before the colder proteins even have a chance to unravel. Not only could this create a curdled custard, but it also limits the thickening power of the egg proteins because not all of them join the coagulated network.

Water enforces low heat for slow cooking.

One way to cook custards slowly is to use water. Water keeps the temperature down because it cannot exceed its boiling point of 212°F (100°C). If it reaches that point, it becomes steam, but the steam will not get any hotter if there’s still water to heat.

Ramekins of custard and cheesecakes are often baked within a larger pan of water, called a water bath, to ensure even cooking. Without a water bath, the outsides of the custard would quickly become as hot as the oven, over 325°F (165°C), while the center was still cold. The edges of the custard would quickly overcook and crack. With a water bath, the outsides of the custard are insulated from the high heat of the oven. In practice, the temperature of a water bath doesn’t even reach the boiling point of water and instead hovers around 180–190°F (82–88°C). By surrounding our custards with water, we keep the temperature of the edges of the custard low so that the entire custard can heat and cook at a similar rate. Even with a water bath, it’s important not to overcook the custard, which often means turning off the heat before the custard has completely set. Residual heat will continue to cook it, and it will firm up as it cools.

Double boilers, where the eggs are cooked in a bowl on top of a pot of simmering water, use the same idea for creams. If we heated the eggs directly on the stove, the proteins at the bottom could quickly scorch. In a double boiler, on the other hand, the custard is heated by steam, which maintains a steady temperature despite fluctuations in the strength of the heat source. Since the temperature of the steam is low, around 212°F (100°C), the cream cooks slowly.

Other ingredients change coagulation temperature.

Another way to give ourselves more control over protein coagulation is to increase the temperature at which coagulation occurs. If we think back to the series of gates a protein must pass as it cooks, increasing the coagulation temperature is like taking the second gate, coagulation (and with it, the third gate, overcooking) and moving them further down the path so the proteins have a longer distance to go before reaching them. Conversely, decreasing the coagulation temperature would move the gates closer up.

Ingredients that make it harder for egg proteins to coagulate (liquids, fats, sugar, and starches) necessarily raise the temperature at which coagulation occurs. For example, in an over-easy egg, which has a solid white and a runny yolk, the entire egg reaches the same temperature as it cooks. That temperature is hot enough for the white to set, but not for the fat-rich yolk. Since fat prevents the egg yolk proteins from coagulating, we would need to cook them longer and make them hotter in order for them to solidify.

The opposite is also true: ingredients that make it easier for egg proteins to bond (i.e., salt and acid) decrease their coagulation temperature. Custards usually have a coagulation temperature 10–20°F (5–11°C) higher than plain eggs.

Tempering takes advantage of an increase in coagulation temperature.

We take advantage of the change in coagulation temperature when we temper eggs. To temper, we heat up milk, whisk a little into our eggs, then pour the mixture into the rest of the hot milk. If we skipped the tempering, the eggs would coagulate as soon as they hit the hot liquid and make egg drop soup. But by diluting the eggs first, we raise the proteins’ coagulation temperature so that the milk isn’t hot enough to cook them. As extra insurance, we can mix the eggs with sugar, which also increases their coagulation temperature, before adding hot liquid to them.

These pumpkin buns are filled with a custard that includes a tempering step.

We could also combine the eggs with the entire volume of milk first, then heat them together. However, since we need to raise the temperature slowly, it takes a long time to heat that much liquid. Tempering quickly raises the temperature of the eggs to somewhere close to their coagulation temperature, so once they start cooking, we don’t have to wait very long.

Custards and creams are a delicate balance of structure and tenderness.

Like many things in baking, custards and creams require a delicate balance. The egg proteins need enough create enough structure for the custard to set thicken, but the structure must also be delicate enough for the custard to be tender and smooth. Through the addition of different ingredients and the use of various techniques, we can manipulate the egg proteins into the right balance for custards with the best textures.



References

Corriher, S. O. Bakewise; Scribner: New York, 2008.

Corriher, S. O. CookWise; William Morror and Company, Inc.: New York, 1997.

Crosby, G. The Science of Good Cooking; America’s Test Kitchen: Brookline, 2012.

Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.

McGee, H. On Food and Cooking, 2nd ed; Scribner: New York, 2004.

Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry, 6th ed.; Freeman, W. H. & Company: New York, 2012.

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