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Updated for 2021
Ideally, steamed milk should be velvety and sweet. For a properly frothed milk, your milk relies on three things: lactose, proteins, and fats. Lactose is responsible for the sweet flavors in milk. Proteins create the stability for the formation of foam. Fats create richness in the milk, imbuing the froth with body and velvety texture for a superb mouthfeel. When milk is heated the sugars from lactose begin to caramelize. A good target range for milk froth is somewhere between 140-150 °F. Exceeding this causes the milk to lose its sweetness and flavor.
The ratio of proteins to fats is what determines the final texture of the milk. High fat milks, such as 2% or whole milk are typically more desirable. These milks have a higher fat to protein ratio. This means the foam isn't as stable, but it also means that the foam distributes more readily. The milk foam will not separate as easily, will taste more velvety and have a wetter mouthfeel. It will both taste and feel richer. Because higher fat milks mix more easily they are the easiest for beginners to work with.
Lower percentage milks such as skim milk or 1% milk will have stiffer foam that holds together longer. The foam also doesn't mix as well with the steamed milk because of the low fat content, causing quick separation of the froth from the steamed milk. These milks will tend to result in a foam with drier mouthfeel, and watery steamed milk. Milk alternatives can produce a comparable result. Soy, for instance, is more viscous than almond milk, and will froth quite nicely.
It’s good practice to purge your steam wand to heat it up and get any residue or water out of the way. Pour cold milk into a frothing pitcher. Cold milk lets you heat the milk for longer, allowing you more time to texture your milk, before hitting the optimal temperature.
For auto-frothing and manual/commercial-style wands, you want to start with the steam off and position the tip of the wand slightly below the surface of the milk. Then turn the steam on. For auto-frothing wands, you can stop there. The machine will do the rest of work for you.
For manual wands, lower the pitcher until the tip of the wand is just under the surface of the milk and you can hear air getting sucked in. A good rule of thumb is to use the spout to hold the wand steady and position it in such a way to cause the milk to whirlpool around the pitcher. This method aerates the milk with small, uniform bubbles called microfoam.
To stop adding air, raise the pitcher to lower your wand deeper into the milk. For lattes, you want to stop adding air when your frothing pitcher starts to feel warm. For fluffier cappuccino style froth, continue adding air for a little longer.
If your machine has a pannarello wand, raise the pitcher to lower the wand enough to submerge the air intake hole in the side of the wand. For manual wands, lower the steam tip just below the surface of the milk. You want to maintain a whirlpool effect in the pitcher when you submerge the steam wand, and continue heating the milk until the pitcher becomes uncomfortable to hold.
Close the steam valve with the wand still submerged. Remove the pitcher and position the steam wand over the drip tray to purge any milk inside. It’s good practice to wipe the wand with a wet cloth after each use.
If there are any large bubbles in your pitcher you can pop them by banging it on the counter like a gavel. Swirling the milk slows separation until you’re ready to pour it into an espresso shot.