Some call it breakfast, others call it a practical application of protein chemistry and controlled heat transfer. Can science help you better fry an egg? Find out below.

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For many of us our cooking careers start with frying an egg, usually alongside a piece of bacon, potentially as a hangover cure.

Unfortunately, this familiarity with the humble egg has bred a certain level of contempt. We forget that the egg is one of the most complex and useful ingredients we have. They are used to stabilise foams in fancy restaurants, make grandma’s custard, emulsify oil in your mayonnaise and without egg whites souffles, cakes and mousses would all be a bit flat. Eggs aren’t just ingredients they are tools.

To use any tool, you need to understand how it works if you are going to use it properly and for an egg that means understanding it’s chemistry. Successfully frying an egg is a good way to achieve this understanding.

In fact, learning the science behind frying an egg can teach you more about cooking in general than a whole battalion of Food Network celebrity chefs. And, unlike an omelette or a boiled egg, when you fry an egg, you can actually see the science unfold right there in the frying pan as you prepare your breakfast.

Science and breakfast, two of my favourite things.

The Anatomy of an Egg

We tend to think of an egg is a simple thing. There’s a shell, a yellow bit and a white bit. The yellow bit is the yolk and the white is the albumin. Simple right? Well, yes, but, in reality, underlying an eggs apparent simplicity is a great deal of complexity. For now, though, when thinking about how to fry one, this simple way of looking at an egg is almost all we need.

What we need to add to our model is the idea that, when frying an egg, we need to be thinking about more than just the white and the yolk. We also need to think about two other structures called the chalazae (there are two chalaza). The chalazae are actually sub-components of the white, they anchor the yolk in the centre of the egg, but, for reasons that will soon become apparent, they need to be considered separately from the other parts of the egg white when we are frying an egg.

If we have an idea of there being three different parts of the egg that are important when frying, not just two, we have everything we need for this post.

How to test if your egg is fresh

Not necessarily knowledge needed to fry an egg but maybe important when deciding which egg to fry, an egg also has an air bubble at its wide end. Because the shell is porous and lets out moisture, the egg will dry out slightly over time and this bubble will grow. We can use the fact to test an eggs freshness.

As the egg ages it loses water and the air bubble expands. So, if you put an egg in water and it sinks you have a fresh egg. As the egg ages the wide end will become more and more buoyant as the air bubble expands. The more the wide end floats up off the bottom the older the egg.

If the whole thing floats it is no good, it’s rotten. If your going to egg someones house these are the eggs to use as they will stink, just don’t eat them.

Understanding the Role of Egg Proteins

The next important concept we need to understand is that we can think of the yolk, the white and the chalazae as separate bags of water with a large number of proteins suspended in them (this is a gross simplification, but we can sort that out in a later post). Proteins are long chains made up of smaller molecules called amino acids. Under normal biological conditions proteins usually fold up about themselves, kind of like a ball of wool.

What the hell are proteins?

If you don’t know what proteins are, they are one of the basic building blocks of life. Skin, hair, muscle, many of the things in our bodies and in the food we eat, vegetable and animal, is made of proteins (you can read more about proteins in the protein quick bite).

To stay folded proteins rely on a few different types of chemical interactions that link different parts of the protein and these interactions stop working as the temperature increases. So if we take our bags of protein, i.e. the egg, and start heating them the proteins start to unfold.

As the proteins unfold into long strands they start getting tangled and a network of intertwined proteins begins to form. As the network develops water becomes trapped in in the compartments formed by the interacting proteins. As more and more proteins unwind and join the network more free water is trapped and the egg starts to ‘set’, it becomes solid.

This process is called coagulation and there is a lot of protein chemistry underlying this transition. For now, though, we can get a long way just by understanding that proteins will unfold, get ‘tangled up’ and trap water as you apply heat to them.

Coagulation and Cooking

One way of looking at cooking an egg is that you are simply using heat to coagulate egg proteins to the degree that produces an egg of the consistency you like to eat.

If there are only a few proteins forming the network there will still be plenty of free water in the egg and it will be runny and soft. As you increase the temperature more proteins unfold, the protein network grows and traps more water. Less free water means a firmer egg.

But beware, you can go too far. If you keep heating the egg the protein network becomes so tangled that there is less room between the proteins and water starts getting squeezed out. This not only dries out your egg, but you’ll also get some extra chemistry involving sulphur. Sulphur is the element responsible for a rotten egg smell, so you end up with a hard, dry and over-cooked egg that smells, well, rotten.

If you are interested in cooking, coagulation is something worth remembering because it doesn’t just apply to eggs. The exact same process occurs when you cook meat and, to a lesser extent, vegetables. The difference between a nice juicy steak and a dry, over-cooked piece of meat is the same story of water being forced out of an over-coagulated protein network (and some other factors).

The Complication: Coagulation Points

So pretty simple, right, to fry our egg all we need to do is apply heat until our proteins coagulate just the way we like them?

Well, no. There is a complication. Different proteins denature (the scientific word for a protein unfolding) at different temperatures so exactly when coagulation starts depends on what type of proteins you are heating.

Making our lives difficult is that the whites, the yolks and the chalazae all contain different kinds of proteins which means they all start coagulating at different temperatures. For the white it’s around 65$\degree$C and for the yolk it’s 70$\degree$C.

The chalazae are a real pain. The proteins in the chalazae are heat resistant and don’t start coagulating till they reach 80$\degree$C, a good ten degrees more than the yolk. Because of this, the chalazae often seem to be raw bits of egg white on top of a fried egg that otherwise looks done.

Egg Structure Component	Coagulation Temperature	Key Characteristics
Egg White (Albumin)	65°C	Sets first; traps water to form a solid matrix
Egg Yolk	70°C	Richer texture; sets firmer at higher temperatures
Chalazae	80°C	Anchors the yolk; heat-resistant protein strands

How Temperature Gradients can Ruin Your Breakfast

Given that we have three different coagulation points we need to worry about, the challenge with frying eggs is getting the white, the heat resistant chalazae and the yolk all cooked to the temperature you are aiming for without over- or under-cooking the other parts of the egg.

This is even more complicated when you consider that a frying egg, as it sits on the pan, experiences a temperature gradient. That is the bottom of the egg experiences a higher temperature than the top.

If you are going for a soft yolk you want all the white, including those heat resistant chalazae, that are experiencing the coolest temperatures on top of the frying egg, cooked without setting the yolk too much. If you want a hard yolk you need to get the yolk cooked to at least 70$\degree$C without letting the white get too much above 65$\degree$C, all while getting the chalazae up to 80$\degree$C.

This can be quite the challenge.

Tips for Cooking the Top Without Over-cooking the Bottom

I doubt that anyone has ever cooked a set yolk fried egg with each compartment at exactly the appropriate temperature, if you like a set yolk in your fried egg then you probably have to accept a bit of rubber in your white. But you can certainly minimise the damage.

For both runny and set yolks there are a few ways of getting the top of egg quickly up to temperature without overcooking the bottom. Most of these methods involving getting heat onto the top of the egg to cook it quicker and reduce the amount of time the bottom is exposed to high heat. So you can:

• Cover with a lid and steam the top: Pop a lid on your pan and start reflecting some heat back down onto the top of the egg. If you haven’t got too much oil in the pan you could also throw some water in before to generate steam (do not do this if you have a lot of oil as it will explode sending hot oil everywhere).
• Baste with hot fat: You can also baste your egg with your cooking fat, spooning hot oil or butter onto the top of your egg, if your worried about keeping your yolk runny just aim for the uncooked white. Tilting the frying pan while basting can also help keep the bottom of the egg away from the heat while your paying attention to the top of the egg.
• Disperse chalazae proteins: I’ve also read you can poke some holes in your egg white which I guess would disperse the chalazae proteins through the rest of the white minimising their effect on the final product, though I’m speculating and I have never tried it.
• The Over-Easy flip: Another method is the ‘easy-over’, just flip your egg and cook the top like the bottom. This certainly works but I find it risky as I inevitably stuff it up and end up with a broken yolk that makes me feel sad. Though if you want a hard cooked yolk flipping seems as good a way as any.
• Separate the white and the yolk: Some extremists separate the white and the yolk and cook them separately before recombining. Kudos for sure but I’m not doing that with a hangover.

Differing Opinions

Most of the advice online suggests frying eggs over medium heat using one of the methods outlined above to ensure the top is cooked. This gives a reasonably gentle temperature gradient between the bottom of the egg and the top and avoids the browning and crisping you get at higher temperatures. But there are other ways of looking at the problem.

The Low and Slow Method

Marco-Pierre White, famous for being the first British chef, and also the youngest chef in history, to get three Michelin stars, doesn’t agree with medium heat. He suggests that eggs should be fried over very low temperatures, in butter, with constant basting of the top of the egg.

Marco likes this method so much that he calls anything done to the contrary “disgusting food” (it’s in his book The Devil in the Kitchen). This method gets you a perfectly white egg, with a consistency more poached than fried, but certainly with a great deal of aesthetic appeal.

Marco might have changed his mind

I remember reading Marco’s “disgusting food” quote years ago when I had the book in physical copy. I’ve since lost the book but the full quote is repeated widely online (for example here, here, and here). Even so I thought I’d better check and in my Kindle version the “disgusting food” part of the quote is gone. Either the internet and I are misremembering or Marco must have had second thoughts about the strength of this conviction and removed it in later editions. I’m going for removal so I’m assuming he said it at some stage.

The original quote:
Visualize that fried egg on the plate. Do you want it to be burned around the edges? Do you want to see craters on the egg white?… The answer to [these] questions should be no. Yet the majority of people still crack an egg and drop it into searingly hot oil or fat and continue to cook it on high heat…. And the result…is an inedible destruction of that great ingredient—the egg. Maybe that’s how you like it, in which case carry on serving your disgusting food.

The High-Heat Crispy Method (Thai Kai Dao Style)

In parts of Asia, however, eggs are often fried very differently. In these parts they embrace the extra browning and crisping that comes from frying eggs at very high temperatures.

Particularly in Thailand and China, using a wok, eggs are fried incredibly quickly in smoking hot oil that is also used to baste the top of the egg. The result a perfectly fried crispy egg with a runny yolk, often with the white blistered by the extreme heat.

In Thailand, where they are called Kai Dao, eggs cooked this way are served on top of a wide variety of street foods from fried rice to pad kra pao.

Our first, but certainly not last, encounter with the Maillard reactions

The browning and crisping of the egg, sometimes incorrectly called caramelisation, is an example of the Maillard reactions that occur at high temperatures between proteins and sugars (we’ll become more acquainted with Maillard reactions in later posts, especially in steak cookery).

Which Method is Best?

Which way is better? Fast, slow, or really slow? I guess that comes down to personal preference, but I know that I prefer the Asian way 1) because I like the crispy bits and 2) because it takes about 30 seconds to fry an egg this way and I’m generally pretty hungry at breakfast time.

There may be times when I want a perfectly white fried egg, I can’t think of any off-hand but knowing it’s there is comforting in case I get around to opening a Michelin-starred restaurant.

On the other hand, for the home cook, the margin for error is a lot greater when using Marco’s method as the cooking is happening over a much longer period. You can see what is going on and take appropriate steps if things start going wrong.

The Asian method probably needs a few ruined eggs before you are proficient as the egg will overcook in the blink of the eye. You also can’t cook an egg the Asian way in butter as the butter would burn long before the egg got anywhere near the pan or wok.

So, if you’re unsure of your abilities or like the taste of butter with your eggs go medium or slow. Otherwise if, on a particular day, you feel like some crispy bits on your egg go Asian, if not go Marco, or go with the medium heat. The choice is yours, it’s your hangover after all.