Cooking for engineers

A new regular section in E&T starts with a look at one of the most amazing ingredients to be found in the kitchen

The egg is one of the most amazing ingredients to be found in the kitchen. It can play a dominant role in breakfast, lunch, dinner, and dessert or a supporting role in those same meals. It can provide flavour, texture and structure. Eggs can act as emulsifiers, clarifiers, insulators and stabilisers. You've probably eaten eggs all your life, cooked with them, made a cake or pastries with them, but might not have spent the time to think about them in detail. I'll help with that.

A simple look at the egg reveals that it is comprised mainly of three parts: a shell, the white (or albumin) and the yolk. While for most culinary applications this is as much as anyone needs to understand, there is much more to see if you take a closer look.

Underneath the shell (a remarkably strong structure of calcium carbonate and protein filled with thousands of tiny holes that let air enter and exit), is actually a pair of thin membranes. One membrane is flush against the shell and the other surrounds the white. You can easily see this membrane when peeling an older egg that has been hard boiled. Cracking the blunt end (or the big end for 'Gulliver's Travels' fans or computer scientists) of the egg and peeling some shell away from the air pocket that forms there usually enables you to separate the shell from its membrane and to see the latter stuck to the solidified white.

Below the shell is the white, but there's more to it than a homogenous collection of protein suspended in water. In fact, there are layers of thin and thick albumin. You can spot them when frying an egg. Crack an egg into a pan and you'll see that some of the white runs freer that the rest (which usually stays piled up around the yolk, like a protective plateau). The runny whites are the thin albumin. The whites also have one more component - the chalaza - which can be found on both ends of the yolk anchored to the shell (at the big-endian and little-endian regions). The chalaza keeps the yolk anchored to the centre (roughly) of the egg and retains as much albumin between the yolk and the egg shell as possible, preventing the former from getting squashed.

It is made out of the same material as the thick albumin, but is twisted giving it strength, structure, and elasticity. You can most easily see the chalaza when beating an egg for scrambled eggs. Crack an egg into a bowl and take a look, the white will have a couple of strands of a different colour that connect to the yolk. When you beat the egg with a fork, the yolk and whites mix fairly easily, but part of the white just won't incorporate and stays separate. This is the chalaza.

The yolk is separated from the white by a porous membrane that lets in water as the egg ages. As the yolk expands, the membrane weakens due to being stretched. That's why it is easier to fry a fresh egg than an older one without breaking the yolk. The yolk itself is composed of about six or seven layers of yellow and white. The yellow yolk is pretty much what you would expect - 16 per cent protein and 35 per cent fat - while the white yolk is mostly water and only 5 per cent protein and 4 per cent fat. The white yolk layers, created while the hen sleeps, are much thinner than the yellow yolks (the yellow yolk comprises about 95 per cent of the whole yolk) which are formed when the hen is active during the day. It is possible to see the layers for yourself if you cut into a hard boiled yolk with a new razor or surgical blade.

The material of the yolk is mostly comprised of tiny fat-filled structures of protein, cholesterol, and lecithin. Lecithin is an emulsifying phospholipid that binds to both fat and water. The proteins in the yolk (they form similar bonds) and the lecithin are utilised when using egg yolks to emulsify oils and aqueous solutions (such as vegetable oil and vinegar to form mayonnaise or butter and lemon juice to make Hollandaise sauce).

With enough beating, these same proteins begin to unravel and interweave to form an uneven three-dimensional lattice allowing air to be trapped. Unfortunately, the effect is short-lived when using only egg yolks, and the bubbles pop readily. Introducing a little water, some gentle heat, and slight acidity helps unravel more protein and results in a stable foam. A popular example of this is the use of egg yolks, sugar, and Marsala wine to make the light dessert/sauce known as zabaglione.

The proportions of protein and water in the yolk don't make it easy to form a stable foam without the additional liquid (such as the Marsala wine), but egg whites don't have this problem. Being 87 per cent water and 11 per cent protein, they constitute the perfect ratio for foaming. Whisking an egg white vigorously can easily produce an eight-fold increase in volume. Over-whisking it, on the other hand, can cause the proteins to interweave and bond too much, collapsing the very air bubbles we strived to create. Introducing an acid (like cream of tartar or tartaric acid, lemon juice, or, in the case of zabaglione, Marsala) can help reduce some of the bonding between proteins allowing the foam to stabilise easier. Foaming egg whites in a copper or silver bowl has much the same effect. This is most easily witnessed when beating egg whites for soufflés. (When sweetened, egg white foams are called meringues.)

We've only just begun to touch the surface of what eggs can be used for, but we can't help mentioning the recipes that focus solely on the egg. Eggs cooked in the shell, fried whole, omelette, and scrambled are some of the more common techniques, but pickled eggs, salt preserved eggs, and the Chinese pidan (a tasty egg preparation that uses an alkaline coating to transform the texture and flavour of the raw egg over the course of a couple months) are among the less known varieties.

Eaten whole, blended into a cheesecake or delicate custard, foamed into a topping for a meringue pie, or beaten and drizzled into egg drop soup, the egg adds much to our culinary repertoire.

For more fascinating cooking facts and recipes, see Michael Chu's immensely popular website (500,00 unique visitors a month) www.cookingforengineers.com

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