If you have a home espresso machine, you already know this is no easy task: Noncommercial equipment just doesn't have the same oomph as what your local barista's working on. But that doesn't mean you can't at least improve what you've been making lately. Here are a couple of tricks to help you up your cappuccino ante.
For pros and novices alike, steaming milk is often harder than making espresso because it not only requires engaging with different and typically less-developed senses (hearing, touch), but also because it happens so fast, so loudly—if you're doing it wrong, anyway—and with a lot of very intimidating heat.
These secrets to perfect microfoam will help no matter what type of machine you've got—be it a cheapo home espresso machine or a higher quality "prosumer," or semicommercial-quality unit like a Rancilio Silvia. They're not going to turn you into a latte-art champion overnight, though they might help you achieve a more pleasant morning latte in the meantime. Take it from here and practice, practice, practice.
Get Steam Power
For starters, make sure your machine is capable of creating somewhere between 1 and 1.5 bars of steam pressure, which is the ideal range for reaching microfoam nirvana. (Consult the manual or packaging before you buy a new unit to make sure the machine is able to produce this much. For existing equipment, there should be a gauge somewhere on the machine's face that indicates the amount of pressure built up inside your steam boiler. Note that the steam pressure on most home espresso machines will be pretty significantly lessened after you make a shot of coffee since the water temperature inside will drop, which stalls the buildup of steam.)
If your steam wands have more than one hole on the bottom, you're in luck. More holes means more potential for controlled turbulence in your milk pitcher as you steam, which can help you keep the bubble texture nice and tight.
Understand the Milk
Before we get into the how of milk steaming, I have to first hammer home the idea that milk is a food, and adding heat to it changes its properties considerably. Milk exists as a protein-rich nutrient source for newborn mammals, in addition to being something smooth, creamy, and delicious when mixed with coffee. The proteins that make it functional as that food source are primarily what reacts and changes when steam heat is added to the liquid, and understanding that is key to mastering the perfect texture.
The way that these proteins react with air helps the creation of foam—preferably microfoam, or a collection of incredibly tight, small bubbles that form a creamy sort of matrix across the top of the finished liquid. The smooth, silky froth essential if you're going to try to create any sort of latte art.
That said, the fact that milk is a food is the number one reason you should never steam milk twice, nor should you top steamed milk off with cold milk and re-steam it. (No cheating!) The process of steaming milk is somewhat akin to scrambling eggs: You know that when you crack a raw egg into a hot skillet, it changes texture and becomes cooked. You could no more put that scrambled egg back into the fridge and have it cool back down to a "raw" state any more than you can un-steam milk once it's been exposed to heat and air from your machine's wand.
Start with fresh, cold milk every time, and only steam what you need when you need it.
Okay, let's get down and dirty. Rule number one is: Don't be afraid. Espresso machines can smell fear. Always approach the steam wand with confidence, and try not to freak out when things start to heat up.
Rule number two is: It's not going to be perfect the first time. Keep trying, you'll get there!
Step 1: Get started
Hold the handle of your milk pitcher in your less-dominant hand. Take your pitcher full of cold milk and insert the steam wand tip into it so that the holes are completely submerged in the liquid (but not touching the bottom or side of the pitcher), and turn the steam wand on full blast using your dominant hand.
Step 2: Add air
As soon as the wand is on all the way, remove your dominant hand from the steam-wand knob or lever, and place it on the side of the milk pitcher so that you can feel the temperature of the milk at it heats.
As soon as your hand is on the side of the pitcher, lower the pitcher until you hear an assertive, repetitive tssssk sound: This is the sound of air being introduced into the milk. You also want to see the milk spinning around in a fast circle, almost like a whirlpool.
Step 3: Stop aerating at "blood warm"
Hold the pitcher at the spot where you hear that tssssk sound—it should be reminiscent of paper tearing, very controlled and tight—until the pitcher is no longer cold, but not yet warm. It should feel the temperature of your hand, or what bakers call "blood warm."
This is the golden rule of making perfect steamed milk: As soon as the milk is no longer cold (roughly 100°F), stop adding air to it by gently lifting the milk pitcher again until the steam-wand tip is re-submerged into the milk and the tssssk sound ceases.
Step 4: Heat until done
Although you're done adding air for foam at this point, you're not yet through heating the milk. (And no one wants to drink a tepid cappuccino.) Hold the pitcher so that the (still-on!) steam wand is submerged just under the surface of the milk, until the side of the pitcher becomes uncomfortable to touch. The metal should feel just hot enough that you don't want to keep your hand against it for more than a second or two. When the milk pitcher reaches that just-too-hot temperature, turn the steam wand off.
(Don't pull the steam wand out of the milk until you've heard it power completely down, or you risk spraying hot foam all up in your face. Not fun.)
Step 5: Keep it clean
Always—always!—wipe and "purge," or release a little steam from your steam wand to make sure it's clear of milk gunk. Don't even look at your milk before you do this. Clean that thing!
Step 6: Groom your microfoam
If there are any large, visible bubbles on the surface of your steamed milk, lift and drop the pitcher on the counter firmly to pop them. "Groom" the milk to keep it one consistent texture by swirling the pitcher around and re-wetting the foam with liquid milk: The finished product should look something like a just-opened can of paint with a glossy sheen.
Step 7: Strong finish
Pour that deliciousness into your espresso and enjoy.
Have you tried steaming milk at home? Got any other milk-steaming tips for your fellow home baristas? Do tell: I'd love to hear success (and failure) stories.