by Hannah Neuschwander

The silky musculature of milk froth: Look close enough and the stuff appears as a cloudy constellation of architecture, bewildering geodesic domes packed in random sequence, straining against gravity’s decisive pull with uncanny complexity, the states of matter (liquid, solid, gas) jumbled and mingling in an affront to easy distinctions, a floor plan tattered by strong winds.

Creating this artful substance is the mark of expert technique and thoughtful preparation. But what the hell is it? It’s nothing introduced into something. It’s punching a thousand little holes in your milk. It’s broken chemical bonds and denatured proteins and the release of sweet, sweet lactose. It’s harnessing the mysteries of physics and chemistry to make gastronomic alchemy in your thirsty little mouth.

To understand foam, you have to understand milk. Milk, for the most part, is water (about 86 percent). Throw in a little fat (depending on the kind of milk), some proteins, a little sugar in the form of lactose, and voila—cow juice. These ingredients exist in emulsion (little globules of liquid, the milk fat, all floating around in another liquid, water, without really mixing). All emulsions are unstable, meaning that the liquids will separate. Thus, when milk first exits a cow, the fat, which is less dense than water, rises to the top of the bucket, creating cream on top and skim milk underneath. Dairies separate the fat from the skim milk and then add it back in precise quantities to create two-percent, whole milk (usually about 3.5-percent fat), half-and-half (10–18 percent), whipping cream (36 percent), and the like.

Milk, like coffee, has to undergo some processing before it can be used in a café. Differences in processing can affect the taste of your milk, although won’t contribute significantly to the quality of foam you can produce. Milk must, by law, be pasteurized. (There’s a thriving black market in “raw” milk based on supposed nutritional qualities, but as anyone who grew up with a cow knows, it tastes like Guernsey ass.) Like many living substances, milk is swarming with bacteria. Pasteurization is the process of heating milk to lower the number of harmful bacteria. When milk goes bad, it’s because the useful life of the pasteurization process is up, and the bacteria that naturally live in milk have reclaimed their rightful territory.

There are two processes for pasteurization: UHT, ultra-high temperature or ultra-pasteurization, and HTST, high temperature, short time. The former process flash heats milk to about 190 degrees Fahrenheit, killing more bacteria than HTST and giving the milk a longer shelf life. HTST heats milk to around 165 degrees for up to 20 seconds—just a bit higher than the temperature used to steam milk in a café. The lower temperatures mean that the milk has a shorter shelf life (two to three weeks), but generally tastes sweeter and doesn’t carry the faint aroma of scald that you can detect in ultra-pasteurized milk. (If your milk is not labeled as one or the other, it is HTST processed.)