This is a reprinted excerpt from my new book The Food Lab: Better Home Cooking Through Science. Copyright © 2015 by J. Kenji López-Alt. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.

When I say eggs, I’m pretty much always referring to chicken eggs, by far the most prevalent type of avian egg in the world. But are all chicken eggs created equal? Do some taste better than others? What factors affect how they work in recipes, and how can I make sure to get the best out of them? Here are the answers to all those questions and more.

Identification

Q: What exactly is an egg?

An egg is a vessel for the developing embryo of an animal that reproduces through sexual reproduction. In the culinary sense, we’re usually referring to eggs from avian animals that are expelled from the body, like chicken eggs.

Eggs consist of a yolk, a tight white within a membrane, and a loose watery white.

Q: What’s inside the egg that makes it so culinarily useful?

There are two basic parts to an egg: yolk and white.

The yolk is the nutritive source for the developing embryo, and it accounts for about 75 percent of the calories in an egg. Yolks may appear rich and fatty, but, in fact, they are essentially sacks of water that contain dissolved proteins, along with larger masses of protein and fat linked together with lecithin, an emulsifying molecule that allows fat and water molecules get along together harmoniously. We’ll get back to that in a moment.

The white is also mostly water, along with a few proteins — the most important being ovalbumin, ovomucin, and ovotransferrin, which give it its unique capacity to both set when cooked and be whipped into stiff, shaving cream–like peaks.

Because the proteins in eggs are already dissolved and spread out in a liquid, it is very easy to incorporate them into other foods — much more than, say, meat proteins, which are relatively firmly set in place in relation to one another. (Have you ever tried whipping a steak? I have. It doesn’t work.) Additionally, the fact that eggs contain such a wide variety of proteins, each of which behaves in a slightly different way when heat or mechanical action is applied, means that as a cook, you have great control over the final texture of your finished dish. Eggs cooked to 140°F, for example, will be soft and custard-like, while those cooked to 180°F will be bouncy and firm.

Labeling: Size and Quality

Q: Eggs come in a few different sizes at the supermarket. Which ones should I be reaching for?

Any carton of eggs that displays the United States Department of Agriculture (USDA) shield on it was packed according to USDA weight standards, which define six different classes, as shown in the chart below.

Jumbo: 2.5 ounces

Extra large: 2.25 ounces

Large: 2 ounces

Medium: 1.75 ounces

Small: 1.5 ounces

Peewee: 1.25 ounces

In reality, you’re unlikely to see small or peewee eggs at the supermarket — chickens these days are bred to produce eggs medium-size and up. Large eggs are the standard in most recipes, including the ones in this book. I do like to have jumbo eggs on hand in my fridge, though, for those post-nightout mornings when I can really use that extra half ounce of fried egg to fill me up. You’re also more likely to find one of the coveted double yolks in a larger egg.

Q: What about those letter grades on the side of the carton? Are Grade A eggs better than Grade B?

Like sizing, grading of eggs is a voluntary action that most manufacturers choose to comply with in order to get the USDA stamp of approval on their boxes. USDA grading experts examine sample eggs from each batch to determine the grade based on the quality of the whites, yolks, and shells. Eggs with the firmest whites, tallest-standing yolks, and cleanest shells will get an AA stamp, while eggs with watery whites, flat yolks, and stained shells receive a B. Grade A lies in the middle and is what most retail stores carry for consumers. As far as cooking quality goes, a firm white and yolk are important for things like poached eggs and fried eggs where a nice, tight appearance is desired, but in most cooking or baking application, any grade’ll do — it’s a cosmetic difference alone.

Egg Freshness

Q: You mentioned that lower-graded eggs have watery whites and so will tend to spread out more than higher-graded ones. But doesn’t freshness play a role in this too?

Indeed it does. Very fresh eggs have tighter yolks and whites that will hold their shape much better during poaching or frying, as well as yolks that will remain better centered when boiled. Because of the way their proteins break down, eggs become looser and looser as they age. There’s another important change too: as eggs age, they become more and more alkaline. This is particularly important in meringue-based dishes, as the pH of egg whites can greatly affect their foaming power. Egg whites foam best in slightly acidic environments, which means that old eggs will produce looser, wetter foams. To counteract this, a pinch of acidic cream of tartar will help your meringues stay stiff and weep-free.

Loose whites in older eggs

Q: I’ve heard that older eggs are better for boiling because they are easier to peel. Is this true? Is there any culinary advantage to using older eggs?

I believed this for the longest time — until I actually tested it with a few cartons of eggs from different sources, comparing them with some eggs I got from my neighbor in Brooklyn’s backyard that were less than a week old. Guess what? Whether the eggs were a week old or two and a half months old, they were just as likely to have shells that stuck to them when peeling. On top of that, with older eggs, the yolks become uncentered, gravitating toward the egg wall, making for unattractive slices.

No matter how you plan on cooking them, fresh eggs are better than old ones.

Q: Is there a trick to getting the shell off a hard-boiled egg without mutilating the white?

I’ve tried every method known to man, ticking them off one at a time. Shocking the eggs in ice water? It makes no difference. Poking a hole in the shells before cooking them? Nope, sorry. Steaming or pressure-cooking them? Nuh-uh. Adding vinegar to the water? All that does is dissolve the outermost layer of shell.

Peeled hard boiled egg started in how water on left, cold water on right.

In fact, I discovered that the only thing that really seems to make a difference is the initial cooking phase. Drop the eggs into hot water, and they’ll peel pretty easily (though even this doesn’t work 100 percent of the time). Heat them up slowly, starting with cold water, and the egg proteins will end up fused to the inside of the shell.

As far as the actual peeling process, the easiest way is to peel the still-hot eggs under cool running water, starting from the fat end, where the air pocket is located. When the eggs are hot, the connection between the membrane and egg white is weaker, making it easier to remove the shell. The cool water not only helps gently dislodge stubborn bits of shell, it also prevents your fingers from getting burned. I put a fine-mesh strainer or colander in the sink to catch the shells, for easy cleanup.

Peel eggs under running water for best restults.

Q: How do I know how old an egg is?

You can try checking the carton label, which will give you a rough idea. On pretty much every package of eggs, you’ll see a sell-by date, as well as a pack date (also known as the Julian date), the date on which the eggs were inspected, cleaned, and placed in the carton. The pack date is the three-digit number immediately above the sell-by date, starting with 001 for January 1 and ending at 365 for December 31. Legally, the sell-by date can be no more than 45 days after the pack date, but when properly refrigerated, eggs will remain wholesome for well beyond this 45-day period — 60 to 70 days is reasonable.

While it’s possible that the eggs you’re buying were laid within a few days of their pack date, manufacturers have up to 30 days to clean and pack eggs, which means that, in theory, if you buy a carton of eggs on its expiration date, it may already be 75 days old! Clearly, checking the expiration date is not the most reliable way to guarantee fresh eggs. You’re much better off checking the pack date.

Q: What if I buy eggs without a pack date or I’ve transferred the eggs to the egg compartment in my refrigerator door and no longer know the date?

First off, everyone tells you that if you want to maximize shelf life, you should get those eggs out of the fridge door and into the coldest part of your fridge. True. But what they fail to tell you is that even on a shelf in the door, eggs will last for several weeks beyond their pack date. So unless you eat or cook with eggs only on very rare occasions, go ahead and keep them in the door. You’ll use ’em up long before they go bad.

Old eggs stand up when submerged in water.

That said, there’s a quick and easy test to gauge the freshness of an egg: just drop it into a bowl of water. Eggshells are porous: they can lose about 4 microliters of water a day to evaporation while simultaneously taking air into the space between the shell and the inner membrane near the fat end. In very fresh eggs, the air space is tiny and the egg will sink to the bottom of the bowl and lie on its side. As eggs get older, the air space will grow, so old eggs will sink and then stand on their points as the air in the larger end tries to rise. If you’ve got an egg that floats, it’s probably past its prime and should be discarded.

Q: My local farmers’ market sells unrefrigerated eggs, and I’ve seen some supermarkets in Europe where the eggs just sit out on shelves. Are they crazy, or is it me?

Most likely it’s you. When eggs are first laid,they are covered in a thin wax-like coating called the cuticle. This cuticle is the egg’s first barrier against bacterial infection and excessive moisture loss. In the United States, USDA-stamped eggs are all washed prior to packaging, a step that removes the cuticle. It may mean that our packaged eggs are cleaner to begin with, but it does mean that they have less protection against future bacterial infection as they sit in the supermarket — refrigeration is necessary to help prevent this. But many eggs sold at farmers’ markets or in European supermarkets have not been washed prior to packing. The cuticle remains intact, so refrigeration is unnecessary, but the eggs tend to have a shorter shelf life than refrigerated eggs.

Q: What about the “pasteurized eggs” I’m seeing on the market these days?

Pasteurized eggs are a relatively new product. They are sterilized by submerging the eggs in a water bath at around 130°F, a temperature that, given enough time, is hot enough to kill any harmful bacteria on or inside the egg but cool enough that the egg won’t cook. Pasteurized eggs are useful for people who like to eat their eggs runny or in raw preparations like mayonnaise but don’t want to run the (very minimal) risk of getting sick from them. For most cooking purposes, pasteurized eggs will work fine, though you’ll notice that the whites are runnier (making them difficult to poach or fry), and that they take about twice as long to whip into peaks. The yolks work just as well as those from regular eggs in mayonnaise or Caesar salad dressing.

Q: Is it true that brown eggs are healthier than white?

Absolutely not. The color of the eggshell has to do with the breed of chicken, and it is largely controlled by market demands. In most of New England, brown eggs are the norm, while the majority of the rest of the country prefers white eggs. They are completely interchangeable.

Egg Labeling

Q: I miss the old days, when I could walk into the supermarket and pick up a carton of eggs without feeling like I was making an important life decision. Thesedays, there are dozens of varieties to choose from. What do all the labels mean?

It is confusing, and it largely has to do with growing consumer awareness about the conditions in which egg-laying chickens are kept. Most spend their lives as little more than egg-producing machines, housed in batteries of individual cages, unable to spread their wings or even move, with little or no access to a space where a chicken could perform its natural behaviors. The label on the carton can be an indication of better welfare for the birds.

Natural indicates that the eggs are minimally processed, but since all eggs are sold minimally processed, the label effectively means nothing. Similarly, the term Farm-Fresh carries with it no guarantees, because presumably nobody is selling rotten eggs that don’t come from a farm.

indicates that the eggs are minimally processed, but since all eggs are sold minimally processed, the label effectively means nothing. Similarly, the term Farm-Fresh carries with it no guarantees, because presumably nobody is selling rotten eggs that don’t come from a farm. Free-Range, Free-Roaming, and Cage-Free eggs come from chickens that are not kept in battery cages, but instead in large open barns or warehouses. That is a major improvement in quality of life for the chickens, allowing them to engage in natural behaviors like pecking, dust-bathing, and spreading their wings. Free-Range and FreeRoaming chickens generally also have access to outdoor areas, but the labeling laws have no requirements as far as the size or quality of the area goes, nor for how long the chickens must be allowed out. Fact of the matter is, most of these chickens never set foot outside the barn. These labels are not audited — you’re going on the word of the producer alone.

eggs come from chickens that are not kept in battery cages, but instead in large open barns or warehouses. That is a major improvement in quality of life for the chickens, allowing them to engage in natural behaviors like pecking, dust-bathing, and spreading their wings. Free-Range and FreeRoaming chickens generally also have access to outdoor areas, but the labeling laws have no requirements as far as the size or quality of the area goes, nor for how long the chickens must be allowed out. Fact of the matter is, most of these chickens never set foot outside the barn. These labels are not audited — you’re going on the word of the producer alone. Certified Organic eggs come from chickens kept in open barns or warehouses with an unspecified degree of outdoor access (again, for all intents and purposes, probably none). They must be fed an organic, all-vegetarian diet free of animal byproducts, antibiotics, and pesticides, and farms are checked for compliance by the USDA.

eggs come from chickens kept in open barns or warehouses with an unspecified degree of outdoor access (again, for all intents and purposes, probably none). They must be fed an organic, all-vegetarian diet free of animal byproducts, antibiotics, and pesticides, and farms are checked for compliance by the USDA. Certified Humane eggs have been verified by third-party auditors, and this label requires stricter controls on stocking densities, giving the chickens more space and the ability to engage in natural behaviors like nesting and perching. Producers are not allowed to engage in forced molting, the practice of inducing hens into a laying cycle by starving them (this practice is allowed for all other types of eggs).

eggs have been verified by third-party auditors, and this label requires stricter controls on stocking densities, giving the chickens more space and the ability to engage in natural behaviors like nesting and perching. Producers are not allowed to engage in forced molting, the practice of inducing hens into a laying cycle by starving them (this practice is allowed for all other types of eggs). Omega-3–Enriched eggs come from chickens that have been fed supplements made from flaxseed or fish oil to increase the levels of omega-3 fatty acid — an essential fatty acid touted with several health benefits — in their yolks. While some people claim eggs high in omega-3s have a “fishy” aroma, in blind tastings, I’ve found no significant differences in the way these eggs taste.

If animal welfare is a concern, you are making a good step in the right direction by purchasing only Certified Organic or Certified Humane eggs. If you’ve got a local farmers’ market where you can actually talk to the farmer producing the eggs you’re purchasing, you’re making an even better decision. Of course, the very best thing you can do is to build your own coop (or, better yet, convince your neighbor to do so) and keep a couple chickens. It won’t save you much money in the long run, unless you keep a large flock and eat a lot of eggs, but you’ll have the freshest-possible eggs and probably make plenty of friends in the process.

Q: That’s all well and good for the chickens, but do Certified Organic or local eggs taste better, like the guys at the farmers’ market would like you to think?

That’s a good question, and one that I’ve wondered about often. It seems natural that a happier, healthier chicken roaming around a backyard poking, scratching, eating bugs and worms, clucking, and doing all the charming and funny things chickens do should produce tastier eggs, right? I mean, I know that some of the best-tasting eggs I’ve ever eaten have come fresh out of the coops or backyards of friends who keep their own flocks. The yolks were richer, the whites tighter and more flavorful, and it was just an all-around better experience. Or was it? What if all their greatness was simply in my head?

To test this, I organized a blind tasting in which I had tasters taste regular supermarket eggs, plain organic eggs, organic eggs with varying levels of omega-3, and eggs fresh from 100-percent freeroaming, pasture-raised chickens. All of the eggs were served scrambled. The results? Indeed the pastured eggs and omega-3–enriched eggs fared better than the standard supermarket eggs. But I also noticed another correlation: the color of the eggs varied quite a bit, with the pastured eggs on the more intensely orange end of the spectrum. And the more omega-3s the eggs contained, the deeper orange the yolk. The plain organic eggs and standard factory eggs were the palest of the lot.This difference in pigmentation can be attributed to the varying diets of the chickens. Pastured hens eat bugs and flowers, both of which contribute color to yolks. Chickens bred for eggs with high omega-3 acids are fed with a diet enriched with flaxseeds and sea kelp, which contribute color. Chickens that lay these more expensive eggs are also sometimes fed pigmented supplements, like marigold leaves, that make their yolks nice and bright. Could it be that the flavor differences tasters were reporting had more to do with their reaction to the color than to the actual flavor of the egg?



Eggs dyed green showed that there was no real difference in flavor between different types of eggs once the bias of color was eliminated.

In order to eliminate color as a variable, I cooked up the same kinds of eggs, this time dying them green with some food coloring. When I readministered the tasting with green eggs, there was absolutely no correlation between flavor and provenance. People liked the regular supermarket eggs just as much as the eggs that had come straight from the pasture.

Want to see the same effect for yourself? Take a look at these two (identical save for some Photoshop color tinkering) pans of eggs and tell me which one you’d rather eat: