When I make a sauce by deglazing the pan with wine or stock after sauteing meat, the result is usually too thin for my taste, even after reducing it. So in accordance with French culinary custom, I "finish" the sauce by adding a "nut" of butter and whisking it in lightly, whereupon the sauce magically thickens. Adding any other fat, such as olive oil, doesn't do that. Why does butter do it?
Calling butter a fat is like calling the Queen Mary 2 a transatlantic ferry. Butter's magic arises from its uniqueness, not only in its history and renowned flavor but in its composition. Butter contains a relatively large amount of water, and it's the water that gives butter its unfatlike properties, such as being able to bring a sauce together and frothing up when heated in a saute pan. I'll get to these two phenomena later, but, first, a little background.
Butter is a complex blend of fat (by law, at least 80 percent in the United States and 82 percent in the European Union) and water (16 percent to 18 percent), plus 1 percent or 2 percent protein (mostly casein) and perhaps 1.5 percent to 3 percent salt, which both kicks up the flavor and wards off rancidity. A touch of annatto extract, a fat-soluble natural coloring, is often added, especially in the winter when most breeds of cow produce paler fat because their diets are devoid of carotene-rich, new-growth vegetation.
Fats and water, of course, won't ordinarily mix. But in butter, the milk's fatty part (frequently called butterfat) and the milk's watery part (the buttermilk) are combined in what appears to be a hom*ogeneous mass. On a microscopic scale, we would see that the water is in the form of tiny (less than 2 ten-thousandths of an inch) globules, dispersed uniformly throughout the sea of fat like so many grains of sand in a gelatin mold. Such a stable configuration of two liquids that won't ordinarily mix is called an emulsion. Butter is an emulsion of water in oil.
Seemingly paradoxical is the fact that butter is made from cream, an emulsion with precisely the opposite structure. It consists of microscopic fat globules dispersed throughout a watery liquid: an oil-in-water emulsion. When cream is churned into butter, the mechanical action breaks the membranes surrounding the tiny fat globules so that they can coalesce; first into rice-size grains and ultimately, after being squeezed and kneaded, into a continuous mass incorporating microscopic globules of water.
But, you say, butter is mostly a solid fat, not a liquid oil. Actually, it is both. For one thing, in chemistry, the word "fat" means a triglyceride, whether it happens to be solid or liquid at room temperature. Moreover, the milk fat in butter is partially in the form of a sea of soft, almost-liquid "free fat" and partially in the form of solid crystals. Butters that have been churned at different temperatures and then cooled and tempered differently (much as a pastry chef tempers chocolate to control its fat crystals) wind up with different ratios of free fat to crystals, and hence with different degrees of firmness, ranging from soft spreaders to bread shredders.
Advertisem*nt
Wee Beasties
Various breeds of bacteria, some good and some bad, view milk sugar (lactose) as yummy victuals and will thrive in cream if we let them. The bad guys can be knocked off by pasteurization, while the good guys can be encouraged by warm temperatures to go ahead and nosh, because as they do, they generate some wonderfully flavorful byproducts. Both of these measures have important consequences for the butter.
In the United States, all cream used to make commercial butter must first be pasteurized by being held at 165 degrees for 30 minutes, a process that connoisseurs insist imparts a slightly cooked off-flavor compared with down-on-the-farm, unpasteurized butter. In the best of all possible worlds, though regrettably not very often in our part of the world, the cream will then be cultured (or "ripened," "matured" or "soured") by the addition of good-guy bacteria, usually a mixture of Lactococcus and Leuconostoc strains, which produce lactic acid and diacetyl. Lactic acid gives butter a pleasant tang, while diacetyl is the major characteristic flavor chemical in butter. Unfortunately, most American mass producers of butter (up to 8,000 pounds per batch) skip the time-consuming culturing step.
Advertisem*nt
Cooking With It
By this time, you have figured out that butter froths up in the saute pan because its water turns to steam. The steam then bubbles its way out noisily. But in spite of its high water content, hot butter doesn't spatter in the pan as other hot fats do in the presence of water. Instead, the butter merely foams up around the food. That's because butter's water is in the form of individual, microscopic globules. They don't join together into droplets that, in contact with hot fat, would explode into relatively large bursts of steam, carrying spatters of hot fat along with them.
How, then, does butter hom*ogenize and thicken a pan sauce? In two ways. First, butter's fat content can absorb the fat in the pan while its water content can absorb the wine or stock, thus bringing them into a sort of matrimonial harmony. But the marriage wouldn't last very long if the butter didn't contain a small amount (about 0.24 percent) of lecithin, an emulsifier. An emulsifier's molecules stabilize an emulsion by latching onto both fat molecules and water molecules at the same time, effectively tying them together. When the entire contents of the pan have thus become a fat-and-water emulsion, the contents will obviously be thicker, glossier and more unctuous than the watery wine or stock alone. Now you know why French chefs ever since Escoffier have been finishing their pan sauces with butter.
Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton). He can be reached at wolke@pitt.edu.
