Eggs: An Introduction

On the Curious Kitchen, we’ve now covered the three macronutrients: proteins, carbohydrates (which include both sugars and starch), and fats (more broadly, lipids). We’ve explored their chemical structures, learned how they interact with each other, and seen how they affect our bakes.

Macronutrients are so named because we need to consume them in large quantities. They are crucial our composition, function, and survival. In fact, all living things need them, from the yeast that raise our bread to the wheat that forms our flour. The only other macromolecule necessary for life that we haven’t yet explored is genetic material, like DNA. (Future post on GMOs?) But with our foundation of proteins, carbohydrates, and lipids, we’re now ready to explore more chemically complex ingredients. First up? Let’s start with eggs.

Eggs house and feed developing chicks.

Biologically speaking, when a species replicates by combining two cells, the larger, more stationary of the two cells is called the egg. By this definition, all animals that reproduce sexually, including mammals, reptiles, birds, and fish, have eggs. But since this is a baking blog, we’ll focus on chicken eggs. Hens lay eggs, fertilized or not, until they have a certain number in their nest. If a human removes the egg(s), the chicken will lay more. Commercial hens typically lay 250 to 290 eggs a year!

The shell is designed to support the embryo’s growth.

Chicken eggs, of course, have a shell to hold the developing embryo. The shell is mostly made of a mineral called calcium carbonate, and it’s riddled with pores that allow oxygen to enter the egg. These pores are also permeable to carbon dioxide, water, and bacteria. When the egg is first laid, the pores are covered by a thin cuticle that prevents water loss and bacterial infection. The cuticle also gives the egg color. Shell color is merely a function of hen breed, and it does not reflect the nutritional value or taste of the egg. Over time, the cuticle wears down to allow more oxygen into the egg. The open pores also cause the egg to lose moisture.

As pictured in the diagram below, inside the shell are two antimicrobial membranes, which you might notice when you peel a hard-boiled egg. They separate at the blunt end of the egg to form an air pocket, which helps the chick breathe when it hatches. As the egg loses moisture from its open pores over time, the liquid part of the egg shrinks and the air pocket enlarges. If you crack an old egg, you can see this air pocket in the shell. You might have also noticed that older eggs, hard-boiled and peeled, have a concave bottom due to the egg’s large air pocket. The air pocket is also the mechanism behind the float test to determine egg freshness: if you submerge eggs in water, older eggs start to float at the blunt end while fresher eggs sink.

Structure of an egg. Note the air pocket formed by the two membranes at the blunt end of the egg.

In the kitchen, though, we’re more interested in what’s inside the shell. Eggs have two distinct parts: the yolk and the white, separated by the yolk membrane. Both contain water and nutrients for the growing embryo, but they are physically and chemically distinct, so we’ll discuss them separately.

Egg yolks are rich in fat and proteins.

The egg yolk makes up about one-third of a shelled chicken egg’s total weight. It’s a common misconception that the yolk is the unfertilized chick—it’s actually a food source for the developing embryo. The part of the egg that actually gets fertilized is the germ cell, shown in the diagram above. If you crack an egg, you’ll see the germ cell as a tiny white dot on top of the yolk. Because the yolk is primarily responsible for the developing embryo’s nutrition, it’s also a rich source of nutrients for us.

The germ cell is a small white dot on top of the egg yolk.

Egg yolks are yellow from carotenoids in hen feed. Carotenoids are molecules that give plants such as carrots, tomatoes, and watermelons their color. Egg yolks specifically are colored by a type of carotenoid called xanthophylls, which come from plants such as alfalfa and yellow corn. Most commercial hens have a standard feed, but depending on a hen’s diet, the color and taste of their egg yolk will change.

Egg yolks are half water, and they take on more water from the white over time.

Fresh egg yolks are about 50% water. Over time, however, the yolk takes on more water from the egg white, which enlarges the yolk and stretches the yolk membrane. This is the result of a ubiquitous process called diffusion, which we explored in detail in the post on poached and macerated fruits and my apple pie recipe. Essentially, in a freshly laid egg, the yolk contains a lower concentration of water than the egg white. (Egg whites are about 90% water.) To equalize the water concentration throughout the entire egg, water naturally migrates from the white into the yolk.

If you crack an old egg onto a flat surface, you can see the effects of the increased water content. Because the stretched membrane is weak, it can’t hold up the egg yolk, and the yolk lies flatter, as shown below. The yolk also more likely to break if we try to separate it from the white. In addition, the weakened membrane makes the yolk more susceptible to bacterial contamination.

Over time, the yolk membrane weakens, which makes the egg yolk lie flatter when the egg is cracked onto a flat surface. The white of an old egg also spreads more.

Egg yolks are emulsions with HDLs and LDLs.

The other 50% of the egg yolk is mostly made of proteins and lipids. Some of the proteins are dissolved in the water of the yolk and float around, but lipids, such as fats, don’t dissolve in water. Since lipids and water don’t mix, we might expect an egg yolk to contain distinct layers of fat and water, the way water and oil separate into layers in a glass. But we know that this is absurd: egg yolks are uniform all the way through. How, then, do they contain both fat and water?

To answer this question, let’s think about some other substances that contain both fat and water yet are homogenous. Mayonnaise is one. So are commercial oil and vinegar dressings. These mixtures are called emulsions, and they’re homogenous because they contain emulsifiers. Emulsifiers, like the mono- and diglycerides shown below, coat tiny spheres of fat and enable them to dissolve in water. We covered the chemistry of emulsifiers in detail toward the end of this post.

Emulsifiers, like the mono- and diglycerides pictured here, surround fats (or air) and dissolve them into water.

Egg yolks are also emulsions because their lipids are organized into tiny spheres and dissolved into water by emulsifiers. These spheres are too small for us to see, but they’re large enough to scatter light—that’s why an egg yolk is opaque. In fact, if you’ve ever mixed an egg yolk with salt, you’ve seen these spheres break apart into small pieces that let light through, which is why the yolk turns clear. (The salt displaces calcium, which destroys the structure of the spheres.) Another way we can indirectly observe the spheres is in a hard-boiled egg yolk. You know that crumbly, grainy texture? Those crumbs are the small spheres of lipids emulsified within the yolk. In an egg, there are so many spheres packed full of nutrients for the developing embryo that they get smushed into flat discs. Scientists call these spheres lipoproteins, but you may know them better as HDLs and LDLs.

HDLs and LDLs carry cholesterol.

HDLs and LDLs, high-density and low-density lipoproteins, are two types of lipoproteins. You’ve most likely heard of them in the context of cholesterol. In our bodies, cholesterol gives strength and rigidity to cell membranes, which delineate every single cell, and it’s also a precursor to important molecules like hormones and Vitamin D. An excess of cholesterol, however, can clog our arteries and lead to increased risk of heart disease.

Both HDLs and LDLs carry cholesterol through our bodies, but HDLs move it to our liver, where it can be broken down, while LDLs bring cholesterol to our cells. LDLs are also the ones that deposit cholesterol in our arteries. Thus, monitoring HDL and LDL ratios in our blood can thus help us track our cholesterol and our cardiovascular health.

Egg yolks contain emulsifiers like lecithin.

Okay, so, quick summary: HDLs and LDLs are lipoproteins, which are spheres of lipids (like cholesterol) within an egg yolk. Since lipids can’t dissolve in the water of an egg yolk on their own, the lipoprotein spheres must also contain emulsifiers. Let’s talk about these emulsifiers.

In HDLs and LDLs, the cholesterol is packaged into spheres by emulsifiers called phospholipids. These are a type of lipid that resemble the mono- and diglycerides we described here, and they are found in all living organisms. Most notably, phospholipids form the bulk of the cell membranes that define the boundaries of every single cell of every living organism. (Cholesterol merely reinforces this membrane.)

As pictured below, phospholipids have two “tails” made of fatty acids. Remember that the chemical structure of fatty acids makes them repel water. In a lipoprotein, phospholipids line up with their tails facing inward, away from the water in the environment and toward the lipid center. This leaves the phospholipid “head” to interact with water, which works out well because the structure of the head makes it attract water. As you can see, if you line up a bunch of phospholipids, they form a sphere (or membrane) around the cholesterol to shield it from and dissolve it into the surrounding water. Because phospholipids have this structure and function, they are emulsifiers. In HDLs and LDLs, the membrane also has various proteins embedded within it, which is why these spheres are called lipoproteins. Most of the protein content of an egg yolk is found in these lipoproteins or dissolved in the water of the yolk.

Lipoproteins like HDLs and LDLs consist of a lipid core surrounded by a phospholipid membrane. The phospholipids emulsify the lipid into the surrounding water.

We take advantage of natural emulsifiers in the kitchen.

The phospholipids are a broad class of molecules. They are used by every cell in existence on Earth, but the exact chemical composition of each phospholipid varies depending on the cell, the organism, and the environment. As bakers, we might be more familiar with a class of phospholipids called lecithin. Lecithin is another broad category of emulsifying molecules found throughout nature. Soy lecithin, for example, which you might have read off an ingredient label or used in vegan baking, is derived from soybeans. Milk (and other dairy products) also naturally contains lecithin. In eggs, lecithin makes up 10 percent of the total yolk! Since phospholipids like lecithin are responsible for forming membranes around new cells and carrying essential lipids, they are crucial for a rapidly growing chick. As we’ll see in the next post, however, we take advantage of lecithin’s emulsifying properties in the kitchen to make mayonnaise, cakes, muffins, and other bakes.

Egg yolks contain amylase.

Another component of egg yolk that is important for the kitchen is the enzyme amylase. Remember that amylases are enzymes that break down starches into sugars that are readily metabolized, as shown below. (We’ve mentioned amylase in the context of yeast and these pumpkin spice custard buns.) Scientists aren’t quite sure why unfertilized egg yolks contain amylases, given that there are no large carbohydrates to break down (or even an embryo to metabolize them), but they might be there so the chick can hatch with plenty of amylases ready to digest new foods. As we’ll discuss, though, in the kitchen, amylases are the reason we need to boil egg custards that contain starch.

Amylase breaks down large starch molecules (left) into small sugars (right).

Egg whites are mostly water with proteins.

All right. Now that we’ve explored the egg yolk in depth, let’s take a look at the stuff that surrounds it. Egg white makes up the remaining two-thirds of a shelled egg’s weight. It cushions the yolk and prevents the developing embryo from bumping into the shell. Egg whites are 90 percent water and 10 percent protein, and they also have tiny amounts of minerals and glucose.

Egg white proteins have a wide range of biological functions.

Egg whites contain about a dozen different proteins. Scientists aren’t quite sure of all their biological functions, but they protect the embryo in several important ways. First, you’ve probably seen the chalazae in eggs, two cords connected to the yolk from either end of the egg. As shown below, chalazae are made of egg white proteins twisted into a rope that keep the yolk centered in the egg.

Chalazae suspend the yold in the center of the egg, and the egg white has alternating layers of thick and thin white.

Other egg white proteins protect the embryo from bacterial and viral infection; lysozyme, for example, is an enzyme that breaks down the cell walls of bacteria. Without a cell wall to contain it, the bacterial cell swells with water until it bursts and dies, like a balloon that’s filled with so much air it pops. (Remember how water diffuses into the egg yolk from the white to balance the water concentration? Water diffuses into bacteria for the same reason.)

Egg white proteins also discourage predators from eating the egg by making the egg less nutritious. Some proteins block the action of predators’ digestive enzymes or bind tightly to the egg’s vitamins and minerals. If the predator can’t access the energy or nutrients of the egg, it’s not worth the trouble to obtain it. Humans have a bit of a workaround for these proteins. Because heat denatures proteins, we can inactivate the protective egg proteins and reap the full nutritional contents of cooked eggs. Of course, as we’ll discuss in the next post, heating eggs also causes a dramatic change, from liquid to solid, that is the basis for many of eggs’ roles in baking.

Egg whites thin with age due to protein degradation and decreasing acidity.

When you crack an egg into a pan, you might have noticed thinner egg white that spreads across the pan, and thicker egg white that stays closer to the yolk. Conveniently, these distinct portions are called the thick white and the thin white. There are actually four layers of alternating thin and thick white starting with thin white at the shell, as shown in the diagram above. The protein ovomucin, which makes up less than two percent of the egg white proteins, is responsible for the thickness of the thick white. As an egg ages, ovomucin disintegrates. This makes it more readily available for the embryo to metabolize, but it also thins the thick white. This is bad news for a cook, since runny whites are harder to fry and poach.

Egg white contains both thin and thick portions.

Over time, the white thins for another reason. Remember how the eggshell has pores? As the cuticle wears down and the pores open up, carbon dioxide leaves the egg. This actually changes the acidity of the egg. Carbon dioxide is dissolved as carbonic acid within the egg, which gives the egg a lower, more acidic pH. Over time, the carbonic acid leaves the egg as carbon dioxide, which makes the both the egg yolk and the egg white less acidic, or more alkaline. The effects of this pH change aren’t really significant in the egg yolk, but in the egg white, it causes clumps of protein to disperse. (Review the effect of pH on protein structure here.) The small, separate proteins are less effective at holding the white together, so the egg white thins with time. The egg white also becomes clearer. Protein aggregates in a fresh white, much like the spheres of fat in the egg yolk, scatter light to make the egg white cloudy. But as the clumps disperse, the egg white lets light through and clears up.

Egg whites contain sulfur.

The most abundant protein in egg white, making up more than half the total egg white protein, is ovalbumin. It’s important for the flavor, texture, and color of cooked eggs because of its chemical structure. (Review basic protein structure here!) Ovalbumin is the only egg protein that contains reactive sulfur groups, which means that it’s the only source of sulfur in the egg that undergoes chemical reactions. The ovalbumin content (and, by extension, the sulfur content) of egg whites is the reason they’re often described as “sulfury.” Rotten eggs have a sulfury smell because the ovalbumin undergoes chemical reactions with time, and the taste and aroma of cooked egg white are sulfury because the ovalbumin undergoes chemical changes with heat.

Eggs are versatile ingredients.

Ultimately, an egg is mostly water—about 76%. But its unique assortment of enzymes, proteins, fats, and emulsifiers are crucial to its biological function as a nutrient source and protective structure for developing embryos, and to its many culinary functions. In the next post, we’ll start to explore the purpose of eggs in bakes like cookies, cakes, and breads. In this series, we’ll also explore egg-based bakes, such as meringues and custards, in more detail.



References

Anton, M. Egg yolk: structures, functionalities and processes. Journal of the Science of Food and Agriculture 2013, 93(12), 2871–2880.

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

Freeman, S.; Quillin, K.; Allison, L. Biological Sciences, 5th ed.; Pearson: New York, 2014.

Ikeno, T.; Ikeno, K. Amylase Activity Increases in the Yolk of Fertilized Eggs During Incubation in Chickens. Poultry Science 1991, 70, 2176–2179.

McGee, H. On Food and Cooking; Scriber: New York, 2004.

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

Slonczewski, J. L.; Foster, J. W.; Zinser, E. R. Microbiology: An Evolving Science, 5th ed.; W. W. Norton & Company: New York, 2020.

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