The incredible edible egg

Is it time for another science post? So be it. Today's Iron Blogger ingredient is... EGGS!!!!

First, I have to admit that I have something of an aversion to straight-up eggs in any form -- when I make eggs, they have to be heavily adulterated, with vegetables and cheese and meat and all kinds of other goodies. So I won't go into great detail about them... after this short play-along-at-home demo of scrambled eggs.

See, we're talking about egg proteins specifically here, and we're going to start with a couple of eggs, scrambled, with some salt, pepper, and a couple of tablespoons of milk if you wish. (If you're me, you'll probably want a handful of shredded sharp cheddar or gruyere to throw on at the end as well.) Get a small, preferably nonstick frying pan (always nonstick for eggs, unless you're an old hand at omelet making). Scramble up the eggs and heat the pan on medium high, then drop in a pat of butter or a splash of olive oil. Pour the eggs in, and begin stirring with a wooden spoon or a silicone spatula. As you stir and scrape the cooked egg off the bottom, they'll form curds of egg protein. Stir just until the egg is completely coagulated, then toss some cheese on top if you like and serve. Or, for the sake of the experiment, you could let it go just a bit longer, at which point liquid will start gushing out of the scrambled eggs and you'll have a swamp of cooked eggs in vaguely sulfurous dishwater. So what's happening?

Well, one of the things that makes eggs so useful as a culinary ingredient (apart from their high-powered energy/nutrition punch, which is just fine as long as you don't overindulge) is their proteins. Egg proteins are rather easy to denature, but readily recoagulate into various other shapes. In the scrambled egg case, you're taking a protein suspension, denaturing the natural crosslinks in the proteins therein, and applying heat to cause them to reassemble into a more-or-less solid gel. Overcooking the eggs causes the gel to break down and cause the water in the egg (and milk, if you added that) to squeeze out into the pan, causing the egg proteins to coagulate further into a dry and rubbery mass. (At this point, you're going to need the cheese anyway.) In fact, eggs are so useful, both as dishes unto themselves and as ingredients in other dishes, that at least one cookbook, Delia Smith's How To Cook, starts off with an entire chapter on cooking eggs; Shirley Corriher's CookWise and Julia Child's first great work, Mastering the Art of French Cooking, both include chapters on eggs as well. That was the opening act. Now the headliner: sponge cake.

You will find in many cookbooks a recipe for a simple sponge cake called a Génoise. This is one of the most basic recipes in the baker's repertoire -- it is a whole egg sponge cake, which depends for its lift and structure entirely on eggs and air, with no assistance from baking powder or yeast. I'm not going to get into the details of the recipe; instead I'll link you to Jacques Torres' recipe on foodnetwork.com and explain a little about what's going on. First, the ingredients:

• Pastry flour: low-gluten flour; provides starch and bulk to the recipe without the gluten crosslinks that would make it tough and bready. All-purpose flour can be used as well, though it needs a bit more care to avoid gluten development.
  
• Sugar: for flavor and mouthfeel, and may also aid the foaming process during the egg-beating step.
  
• Eggs: Structure. Not only does the foam lighten the mix and allow it to be cakey, it also literally holds the entire cake together, holding the starch particles from the flour in a matrix of egg proteins.
  
• Egg yolks or butter: Mostly for mouthfeel. You won't find this in every génoise, and in fact some French recipes are rather leaner than Torres'.
  
• Honey: This is an unusual ingredient, and I'm not 100% sure why it's here. My best guess: génoise makes for a rather dry cake, and is usually soaked in a syrup. My feeling is that the honey, being hygroscopic, is meant to moisten the cake a bit so it's a little more agreeable to American tastes in a dry state.
  

The first stage here is the key (and incidentally in my limited cake-making experience I find the double boiler overkill; the egg-sugar mix comes up to temperature in a perfectly timely fashion over a bowl of hot water, and can be done right in the mixer bowl that way) -- the heat aids in the denaturing of the proteins (other sponge cakes usually separate the eggs and beat the whites alone) and allows them to set ever so slightly into a foam stable enough to accept the flour being folded in. Essentially, the egg has reached a point not entirely unlike a plastic foam -- strings of proteins crosslinked into a net, creating a very high surface tension within the egg fluids and allowing them to trap large quantities of air. It is often said that the egg acts as the leavening in this case; this is not strictly true. The air (and to a lesser extent water vapor from the eggs) is really what provides the leavening, by expanding (and boiling) in the oven and causing the bubbles trapped in the egg foam to expand. But the cake won't rise without the egg protein bubbles.

At this point, the folding step comes in -- the flour is carefully sprinkled onto the egg foam and combined with the foam in such a way as to break as few of these bubbles as possible. There is a third step that Torres' recipe bypasses -- if you use melted butter, you may want to temper it first. Essentially, this constitutes mixing a part of the cake batter into the melted butter and combining them before folding that mixture back into the main body, the same way you did the flour. The key here is that the tempering step both emulsifies the butter (so it won't clump up as it cools and leave big greasy holes in the cake) and cools it so it doesn't scramble the eggs and ruin the structure of the cake.

Finally, the baking step. At this point we've come around to the same point as we did with our finished scrambled eggs -- we want the egg proteins to coagulate within the batter, solidifying the batter and trapping the air that we worked so hard to incorporate. The trapped gases within the batter will expand as the batter heats in the oven, causing the cake to increase in volume and lighten. The end result is perhaps a bit bland, but that's no thang -- soak it with some kind of flavored syrup, sandwich some favorite fruit preserves between the layers, and then just cover the whole thing in whipped cream or buttercream frosting, and you've got Cake 101.

After all this, it's worth pointing out that if you have baking powder or some other chemical leavening, you don't have to go through all the trouble of foaming the eggs; you can just as easily use a quickbread-type recipe that will create the bubbles for you. (In fact, a quickbread method is a good way to make breads out of grains such as barley or oats that don't have significant amounts of gluten.) And what if you can't or don't eat eggs? Well, xanthan gum, a goopy polysaccharide derived from a sugar fermentation process by a bacterium called Xanthomonas campestris, is used rather often for creating gluten-free breads, as it's a reasonably good substitute for gluten with some other grains. Tofu, agar, cocoa butter, and a number of other ideas have also been floated as substitutes; check this BBC article on vegan baking for more details and recipes. The methods for dealing with mouthfeel and structure issues are many and varied.

So there's a taste of what eggs do in kitchens. There's much, much more written on the subject, far more than can be covered in one (already rather overlong) blog post. I think, though, that this ought to cover the basics.

Further reading:

• Child, Julia and Simone Beck, Mastering the Art of French Cooking, Volume 2. New York: Knopf, 1972. Child and Beck's second collaboration, Mastering 2 contains much information on the practicalities of French baking, and possibly the definitive English-language recipe for Génoise cake, as well as much wisdom on bread making learned directly from France's master of bread, Raymond Calvel.
  
• Cook's Illustrated Magazine, Baking Illustrated. Brookline, MA: America's Test Kitchen, 2003. In which CI, the magazine of record for kitchen nerdistry, turns its attention to the ins and outs of baking. While not as scientific as Corriher or McGee, it is absolutely loaded with rubber-hits-the-road practical information.
  
• Corriher, Shirley, CookWise. New York: William Morrow & Co, 1997, ISBN 0688102298. The masterwork of a food scientist who specializes in practical applications for home cooks. Has an entire chapter on eggs, after going into great detail about baking. Her long-delayed sequel, BakeWise, might ship this year. Maybe.
  
• McGee, Harold, On Food and Cooking, 2ed. New York: Scribner, 2004, ISBN 0684800012. The microphotographs of various food structures, as well as many diagrams, make this an invaluable visual aid.