On Food and Cooking–Beating Egg Whites, for example
Wednesday, April 16th, 2008[Warning: this is a post from Nerd World.]
[The columnist John Carroll from the San Francisco Chronicle always warns his readers of impending columns about cats–some people love them, some people hate them. I’m borrowing the trick for my posts from Nerd World.]
Ever since I was a kid, I’ve found the language of science, and particularly chemistry and physics, to be a great tool for understanding the universe around me. I have an almost mysical relationship with the biochemistry of energy production. Just contemplating the citric acid cycle makes my heart beat faster.
So, some years ago I found the ideal book [for me] about food and cooking. It’s called On Food and Cooking: the science and lore of the kitchen. The second edition came out in 2004; it’s about three times larger than the first edition. It’s an encyclopedia of food. Ingredients are described in terms of biology, chemistry and physics.
One of the things that makes cooking so interesting to the practical chemist is that the raw materials are so much more complicated than the usual laboratory experiment. On top of that, the basic solvent for all [or almost all] food is water–but there are significant admixtures of oils, proteins, starches, and other solids that make our lives more interesting.
Let me take a moment to illustrate all this with an example from On Food and Cooking. The author, Harold McGee [also author of a column The Curious Cook], clarifies that a foam is a mixture of a gas–in most cases air–in a liquid matrix. The properties of the foam are determined by how small the bubbles are and by how easily the liquid phase regroups into a puddle. If you try to whip water, nothing much happens because water has a very high surface tension, so it comes back together readily and because the water molecues [H20] are so small that they slide over each other readily.
Here’s a mental image that might help. Imagine a packing carton with a layer of BBs on the bottom that’s about 4” [10 cm] deep. We’re pretending that we can see the water molecules [BBs] in our mixing bowl [the carton]. Now, take your hand and agitate the BBs furiously. When you stop, it looks pretty much the same, because the BBs have slide one over another, and come to rest pretty much where they started. –We’ll come back to this example shortly.
In order to whip something, the surface tension, the viscosity [how easily the liquid flows] and the size of the molecures need to be adjusted. A spectacularly good example of a whippable liquid is egg white. Egg white is a mixture of some fairly good sized proteins, particularly albumin, in water. The protein molecules are large, “gooey” and “stringy”, so egg white has it’s familiar texture.
Now let’s go back to our carton. Instead of BBs, fill it with shoelaces–or lengths of clothesline or even fettuccini alfredo. Now, as we stir them around, they begin to get tangled together. Before we know it, the whole contents of the box is moving around as one big lump–and if we want to get it untangled, we’re in for a long job.
Now we can translate our mental picture back to egg whites. The albumin molecules, in addition to being long and “gooey”, have the useful property that they unfold and start to tangle just by being agitated at room temperature. [The proteins in egg yolk don’t–so the technique is different: check out zabaglione.] As we continue to whip the egg whites, first we get a froth of large bubbles. But the egg white is sufficiently gooey that the bubbles don’t burst right away. Soon the froth is becoming a foam–the bubbles are much smaller, and the resulting mass starts to stick to the bowl when we turn it. At the same time, the albumin molecules are getting more and more tangled, and keeping those tiny bubbles from rejoining into big bubbles. We’ve reached the soft peak stage.
As we continue to whip the egg whites, we come to a time where the albumin molecules are about as tangled as they can get, and still allow the water molecules to be mixed in. This is the hard peak stage. But if we keep whipping, the albumin molecules will start to group so tightly that the water molecules are squeezed out, and the egg white mass will start to weep, and to collapse.
If we just leave our bowl of whipped egg whites on the counter, after a while it will collapse again. So we need to stabilize the foam. How? The two basic answers are: heat and cold.
We can take our egg white foam–suitably seasoned with sugar–and bake it into meringues. In this process, two important things happen. First, the sugar absorbs some of the water, so there is less an issue with weeping. Second, the baking changes the chemical properties of the albumin, so it’s no longer able to rearrange itself. Further, the whole structure is dried out–it becomes much harder.
I just mentioned that the sugar absorbs some of the water. That’s the reason that the sugar isn’t added to the egg whites until the soft peak stage. If the sugar is added at the beginning, the resulting mixture is so thick and sticky that it’s very difficult to get the foaming to start.
A particularly impressive use of our meringue relies on the excellent insulating properties of foams. That would be baked Alaska–where we seal ice cream inside a shell of meringue that can then be baked without melting the ice cream. Anyone who lives in a house with foam insulation can attest to the efficiency of foam as insulation–it’s the fact that the little bubbles don’t connect to each other that keeps the heat from flowing easily.
Alternatively, we can fold a flavoring into our eggwhite foam and then chill it into mousse. Charateristically, the flavoring includes lots of fat–think chocolate–that hardens as it cools. This hardening allows the egg white foam to retain it’s lightness, to the point that our mousse almost dissolves on the tongue.
This is perhaps a bit on the long side, but I thought it would give a bit of the flavor of On Food and Cooking. If you want even more detail, check out pp. 100-113 of the second edition.
One last trick that I can’t resist sharing. In my experience, the best way to separate eggs is to crack them into my hand and let the egg white slip through my fingers into another bowl while I hold the egg yolk in my palm. I was amazed at how much control over the process I had, and how tough the membrane around the yolk is.
Bob