Calling butter a fat is like calling a truffle a mushroom. 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 unfat-like properties, such as being able to bring a sauce together and frothing up when heated in a sauté 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 to 18 percent), plus 1 or 2 percent protein (mostly casein) and, if salted, 1.5 to 3 percent salt, which both kicks up the flavor and wards off rancidity. A touch of a fat-soluble yellow-orange pigment is often added, especially in the winter, when cows of most breeds produce paler fat because their diets are devoid of carotene-rich, new-growth vegetation. The pigment, used also to color cheese, is annatto (achiote in Spanish), from the seeds of the South American tree Bixa orellana.
Fats and water won’t ordinarily mix. But in butter, the milk’s fatty part (the butterfat) and watery part (the buttermilk) are combined in what appears to be a homogeneous mass. On a microscopic scale, we would see that the water is in the form of tiny globules (measuring less than 0.0002 inch), dispersed uniformly throughout the sea of semi-liquid fat like so many poppy seeds in a Jell-O mold. Such a stable configuration of two liquids that won’t ordinarily mix is called an emulsion. Butter is a water-in-oil emulsion.
Seemingly paradoxical is the fact that butter is made from cream, an emulsion with precisely the opposite structure. Cream 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 open the surfaces of the tiny fat globules so that they can coalesce, first into rice, sized grains, and ultimately, after being squeezed and kneaded, into a continuous mass incorporating microscopic globules of water.
But, you say, butter is mostly solid fat, not a liquid oil. Actually, it is both. For one thing, in chemistry, the same word, fat, is used whether the substance 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.
Enter the wee little beasties. Several breeds of bacteria, some good for us and some bad, view milk sugar (lactose) as yummy victuals and will thrive in cream if we let them. The bad ones can be knocked off by pasteurization, while the good ones can be encouraged by warm temperatures to go ahead and nosh, generating some wonderfully flavorful by-products as they do. 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°F ( 74°C) 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 bacteria, usually a mixture of Lactococcus and Leuconostoc strains, which produce lactic acid and diacetyl. Lactic acid adds a pleasant tang to butter, while diacetyl is the chemical that gives butter its most prominent characteristic flavor. Unfortunately, most American mass producers of butter (at up to 8,000 pounds per batch) skip the time-consuming culturing step.
By this time you have figured out that butter froths up in the sauté pan because its water turns to steam, which 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 homogenize 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 keeping 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. French chefs ever since Escoffier have been finishing their pan sauces with une noisette of butter.