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The Science of Eggs

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By David Joachim and Andrew Schloss
From Fine Cooking #134, pp. 26-27

Cooking transforms no other food as dramatically as an egg. Whether you prefer them hard-cooked, poached, fried, or scrambled, knowing how eggs go from raw to cooked can help you perfect your technique.

What happens when eggs cook
The yolk and white (albumen) of raw eggs are essentially just sacks of water dispersed with proteins-about 1,000 water molecules to every one protein molecule. Protein molecules are relatively enormous, composed of hundreds of amino acids bound together into long chains. In a raw egg, the chains are folded into compact globs held together by fairly weak chemical bonds connecting the folds. Due to the chemistry of egg albumen, most of the protein globs in the white have a negative electrical charge and therefore repel each other, which keeps the white watery and loose. In the yolk, some of the proteins are bound up with fat, so although some yolk proteins repel each other, the electrical charge of others is neutralized by their fat coating, which makes yolk proteins less repulsive to one another. That’s why a raw yolk, though still liquid, is less runny than a raw egg white.

As an egg heats, all of its molecules move faster and collide. Gradually, the collisions become so intense that the weak bonds holding the amino acid chains into folds start breaking apart, and the egg proteins unfold. As the heat increases, these loose protein strings continue to move and become tangled into a three-dimensional web. The egg still contains more water than protein, but the water is now dispersed in the protein web so it can no longer flow together, turning the liquid egg into a semi-solid. This happens at around 145°F for egg white and 150°F for egg yolk.

Continued heating causes more bonds to form, leaving less space for the water. Eventually, much of the water is squeezed out (this is referred to as weeping) and evaporates, causing the egg protein to coagulate. When eggs are overcooked, the protein web becomes so tight and retains so little water that the egg white becomes rubbery and the yolk chalky, a textural difference due to fat interspersed with the protein web in the yolk.

Control texture with heat and stirring
To better understand how heat affects eggs, think of a fried egg. If cooked over a raging hot flame, it’ll become rubbery and overcooked. But use medium-low heat, and it turns out tender. If you like crispy, browned edges and a runny yolk, the trick is to use medium-high heat but remove the egg from the pan as soon as the white firms so it doesn’t overcook. To keep your fried egg whites from “feathering” (running) in the pan, use the freshest eggs possible-they have thicker whites and stronger yolk membranes, so the yolks are less likely to break, too. It also helps to cook the eggs in a nonstick or wellseasoned pan with butter, not oil. Butter’s saturated fat and natural emulsifiers prevent sticking better than oil.

Similar rules apply to scrambled eggs, but there’s an added variable: stirring. For soft, small curds, use low heat and stir almost constantly to heat the eggs evenly and keep them from shooting too far past 158°F, the temperature at which blended eggs start to coagulate. If you prefer large, firm curds, use medium-high heat and occasional stirring. You can also create a more silky, custard-like curd by adding about 1 Tbs. of cream, milk, or water for every large egg, which dilutes the proteins and raises the coagulation temperature. The eggs will take longer to cook and need low heat to prevent the extra liquid from weeping.

Omelets cook much like scrambled eggs, except they’re rolled or folded at the end. The key is to avoid overloading the pan, which increases cooking time and creates a rubbery omelet with an overcooked skin. For a two-egg omelet, use an 8-inch pan, and for a three-egg omelet, use a 10-inch pan. For a fluffy omelet, start with high heat and rapid stirring to moderate the heat transfer to the moving eggs, incorporate a bit of air, and avoid an overcooked brown skin. Then reduce the heat to low to finish cooking, filling, and folding. If you like a denser omelet, use low heat and slow stirring so as not to incorporate air.

Don’t boil “hard-boiled” eggs. A rapid boil can crack the delicate shell. Plus, boiling for more than 20 minutes causes hydrogen sulfide in the egg white to react with iron in the egg yolk, creating a harmless but foulsmelling green ring of ferrous sulfide around the yolk.

A better way to cook eggs in the shell is the cold-start method: Put a single layer of large eggs in a saucepan, and add enough cold water to cover by 1 inch. Cover the pan and bring the water just to a simmer, then remove from the heat and let the eggs stand, covered, in the hot water for 4 minutes (for soft-cooked eggs with liquid yolks) to 12 minutes (for hard-cooked eggs with firm yolks). Next, immediately chill the eggs in an ice-water bath for a few minutes to halt the cooking.

When poaching eggs, add salt and acid to the water to speed coagulation of the egg white and prevent it from feathering. Use about 1 tsp. kosher salt and 2 tsp. vinegar for every 3 cups of water. Boil the water, crack the eggs into heatproof cups or ramekins, slip the eggs into the water, and reduce the heat so the water is just under a simmer. Three to five minutes of gentle poaching creates a tender egg white and a very runny to slightly runny egg yolk. As with fried eggs, fresh eggs feather less than older eggs because the whites are thicker. To help create a more compact shape, you can use the professional chef’s vortex method: Boil the water in a deep pot, and then vigorously stir the center with a spoon to create a vortex. Slip the egg into the center, and the vortex helps spin the egg white neatly around the yolk into a teardrop-shaped pocket.

Tips for Peeling Cooked Eggs
There are few things as frustrating as an eggshell that refuses to be peeled without tearing away some of the cooked egg white beneath it. Fortunately, there are a few things you can do to minimize the chance of this happening.

First, use older eggs. As an egg ages, its albumen shrinks and the space between the inner and outer shell membrane enlarges, which facilitates separating the membranes and removing the shell. Supermarket eggs tend to be old enough for cooking inshell by the time you buy them, but if your eggs are from a farmers’ market (or direct from a hen), wait about two weeks before cooking them in the shell (and if you can’t wait, fry or poach them). Second, when chilling cooked eggs in ice water, gently crack the shells in a few spots to let water seep under them to ease peeling.

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  • Schwevy | 04/10/2016

    I'll just add that I also put them into an ice bath or very cold tap water (like canada in January) to stop the cooking immediately. No sulfur rings, no yucky smell.Easy to peel. Way less product loss.

  • Schwevy | 04/10/2016

    I disagree with the advice to cook eggs from cold. As a caterer, I have cut my peeling costs in half, also increasing my usability of whole eggs by half by doing the following:

    Heat water to a simmer. If you are doing large volumes, then the higher the temperature the better- even a rolling boil will not 'lose' you as many eggs as putting them into cooler water. If I am doing 3 dozen, let's say, then I put the three dozen into a strainer while the water is boiling and turn it off JUST before I put the three dozen eggs, gently into the water, all at once.

    Then, cook on a simmer for 10-12 minutes for hard boiled. DO NOT ever cook hard boiled eggs on a rolling boil, except right before you put them in.

    These eggs, EVEN BRAND NEW ONES, will peel immediately and most will be good enough to use for devilled eggs. Although, if you really want perfection in a devilled egg, shake the pan regularly for the first 5 minutes of cooking to centre the yolk and make sure the white is uniformly thick around the edges.

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