The Science of Food

For many of us our cooking careers start with frying an egg, usually alongside a piece of bacon, potentially as a hangover cure. For some that career goes no further, content with the ability to put a fried egg on the table, but for others that egg is the first step in a lifetime of exploring the art of cooking. As a gateway dish, and because of its ubiquity in our kitchens, we sometimes forget that the egg is one of the most complex and useful ingredients we have at our disposable. Even if you don’t like eating eggs you can’t escape them; 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.

All this is to say that eggs are not just ingredients they are tools. Eggs are emulsifiers, binders, leavening agents and enrichers of other foods, they provide structure, lightness and stability. Like any other tool you need to understand how it works if you are going to use it effectively and for an egg that means understanding it’s chemistry. Frying an egg is a good way to achieve this understanding. Learning what happens when you fry an egg teaches you more about the effect of heat on food than almost anything else. And unlike, say 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.

At first glance an egg seems pretty simple, a shell, a yellow bit and a white bit. The yellow bit is the yolk and the white is the albumin. Though the egg has a great deal of structure and sub-structure, for our purposes this is almost enough. For now we just want to note that the white has a sub-component called the chalazae (there are two chalaza) that anchor the yolk in the centre of the white. An egg also has an air bubble at it’s wide end and, because the shell is porous and lets out moisture, the egg will dry out slightly over time and this bubble will grow¹.

The next thing we need to realise is that you can think of the yolk and the white as bags of water with proteins dissolved in them². Proteins are long linear chains of amino acids but under normal biological conditions they fold up onto themselves, like a ball of wool. 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 water, 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 they form a network of intertwined proteins. This network traps water, immobilising it in the compartments formed by the protein network. As more and more free water is trapped the egg starts to ‘set’, to become solid. This process is called coagulation and there is a lot of protein chemistry underlying this transition which we’ll learn more about in future posts. But for now we can get a long way just by understanding that proteins will unfold and get ‘tangled up’ when you apply heat to them.

On a chemical level cooking an egg is basically using heat to unfold and tangle egg proteins to an extent that produces an egg of the texture you like to eat. If they are only a little bit tangled the egg will be runny and soft, there will still be plenty of free water in the egg. As the temperature increases the proteins will unfold more and become more tangled, the water compartments will become smaller and water will start to be squeezed out of the egg. If you take it too far the proteins will become so tangled and the compartments for the water so small that almost all the water will be forced out of the egg and you’ll end up with a very hard, dry egg. If you’ve gone this far there will also be some other chemistry going on involving sulphur, the element that gives off the rotten egg smell, so you’ll end up with a rock hard, dry egg that smells like, well, rotten eggs.

So pretty simple, right, well not quite. Different proteins denature (the scientific word for a protein unfolding) at different temperatures so it matters what type of proteins you have in your bag of protein. Egg whites and egg yolks contain different kinds of proteins which means the coagulation point of the yolk and the white is different. For egg whites it’s around 65C/150F and for egg yolks it’s 70C/158F. Further complicating things are the chalazae, the slightly thicker part of the egg white that anchors the yolk. The proteins that make up this substructure are heat resistant and don’t coagulate till they reach 80C/180F. You often see the chalazae as the slimy bit of white on top of the fried egg that is still uncooked even though the rest of the white looks done.

The challenge with frying eggs, and most egg cookery for that matter, is getting the white, including the heat resistant chalazae, and the yolk cooked to the temperature you are aiming for without over- or under-cooking the other parts of the egg. This is much more of a challenge when boiling eggs but it is also a factor when frying as the egg sitting on the surface of the frying pan will be experiencing a temperature gradient i.e. the bottom will be hotter than the top and you want the top of the egg (the yolk and those chalazae) to be cooked to your preference without over-cooking the bottom. If you are going for a soft yolk you want all the white cooked without setting the yolk too much which isn’t too difficult as the white will set sooner than the yolk. A set yolk is harder, in this case you need to get the yolk cooked to 70C without letting the white get too much above 65C while getting the chalazae up to 80C, quite the challenge.

To be honest 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 the white and the chalazae quickly up to temperature without overcooking the bottom.

Firstly you can put 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 as well and let the steam get at the top of the egg (do not do this if you have a lot of oil as it will explode sending hot oil everywhere). 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 from over-cooking as it is removed from the heat while your paying attention to the top of the egg.

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. 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. 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.

Most of the advice online suggests frying eggs over medium heat. 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. This browning, sometimes called caramelisation, is actually an example of the Maillard reactions that occur at high temperature between proteins and sugars (more on that in later posts). Marco-Pierre White goes even further suggesting that eggs should be fried over very low temperatures in butter with constant basting of the top of the egg. Marco likes this so much that he calls anything done to the contrary “disgusting food” (it’s in his book The Devil in the Kitchen³). This method gets you a perfectly white egg, with a consistency more poached than fried, but certainly with a great deal of aesthetic appeal.

In parts of Asia, however, eggs are often fried very differently. 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.

Which way is better? Fast, slow, or really slow? I don’t know, but I know that I prefer the Asian way 1) because I like the crispy bits and 2) it takes about 30 seconds to fry an egg this way and I’m generally pretty hungry at breakfast time. Of course there may be times that 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.

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 so you can see what is going on and take appropriate steps during the cooking process. 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 can’t cook an egg the Asian way in butter as the butter would burn long before the egg got near the pan or wok. If you like the taste of butter with your eggs you’ll have to use the Marco method. 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.

Footnotes

1. So to test an eggs freshness put it in some water. If the egg sinks you’re all good but as it gets older the wide end will become more and more buoyant. The more the wide end floats up, the older the egg is. If the whole thing floats it is no good, rotten. If your going to egg someones house these are the eggs to use as they will stink, just don’t eat them. ↩︎
2. If you don’t know what proteins are, they are the basic building blocks of life. Skin, hair, muscle, everything in our bodies and everything in the food we eat, vegetable and animal, is made of proteins (you can read more about proteins in the protein quick bite). ↩︎
3. 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. Either way it is still very clear that he thinks the slow cook method is the best way to fry an egg. The 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.“
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