Choosing the right milk for coffee can be daunting. From a cow, goat, or sheep? Organic or conventional? Whole, reduced fat, or skim? Pasteurized or sterilized (UHT, shelf-stable, more common in Europe than the United States)? Goat's milk and sheep's milk share a stronger taste that easily masks a great espresso's aroma, so I don't recommend experimenting with them.
Fat content is critical. Whole milk with 3.5 to 4 percent fat blends ideally with coffee. The fats carry espresso's aromas nicely, and when the milk is steamed, they help produce a lasting, creamy foam with more flavor. As logic suggests, less fat means less creaminess, a lesser-bodied taste, and less richness.
I prefer whole, organic milk, making for a wonderfully creamy, full, pure taste. Whatever you prefer, be sure to use pasteurized milk. When heated, sterilized (UHT) milk produces a "cooked" taste that covers an espresso's good aroma. Homogenized milk, by contrast, distributes fats well throughout the heated liquid.
Okay, let's steam things up. (Advanced baristas: much of the following will be very familiar to you.)
First, make sure you have the right equipment, starting with a steel steaming pitcher. You needn't spend a lot to get a good pitcher; just make sure it has a pronounced spout. Twelve-ounce pitchers are good for one cup, 20-ounce for two cups. Critical: be sure to fill the pitcher between 40 and 60 percent of capacity, leaving room for the resulting foam. Cup size is also critical. Authentic Italian cappuccino uses a six- or seven-ounce cup, one shot of espresso (one ounce), and five or six ounces of steamed milk (resulting in of three to four ounces of milk after steaming). Increase the formula in proportion for larger drinks.
It's critical to use fresh, chilled milk every time, never milk that's already been heated. Steam is water, so too much water makes milk less creamy. Keep your milk cold, from 35 to 40 F, to ensure enough steaming time; overheating denatures proteins, ruining milk's taste and smell. Optimal, final temperature is 140 to 150 F. Carefully monitor the milk by using a kitchen thermometer or, like the pros, by resting your hand on the side of the pitcher. When it's too hot to touch, steaming is complete.
To optimize the amount of air introduced, keep the nozzle about a half-centimeter below the milk's surface, near the pitcher's side but not against it. Don't move the pitcher. The steam's power will move the milk in a circular motion, creating air bubbles (emulsion), and reducing them to microbubbles (stabilization). The resulting, smaller bubbles make for more stable, longer-lasting foam.
Using that steel pitcher with a pronounced spout, and with a little good training and practice, you'll create patterns like the popular heart, apple, leaf, or tulip, just by pouring the milk into the cup over the espresso and "drawing" each picture as you go. Start slow, and increase speed while pouring, using a side-to-side "sketching" motion. Barista's secret: sprinkling a little cocoa powder on the foam makes the patterns appear with more definition. (The authentic cappuccino recipe does not include cocoa powder.)