Milk steaming has a distinct auditory quality, and the right pitch is one indicator of how the milk is transforming. An initial quiet screeching indicates that you are introducing the right amount of air. A loud, aggressive, and screeching sound is a sign that the steam wand is positioned too high or too deep into the milk, or is at a poor angle. Before you even begin to consider latte art, it is wise to view your milk pitcher as an opportunity to analyze the sound, movement, heat, and foam texture of the steaming milk.
Large dry bubbles are often created by over-steaming the milk, introducing air for too long, leading to the rapid expansion of milk, rough foam texture on the surface, and the finished foam remaining on top of the drink. These issues can lead to latte milk that is too airy for a smooth mouthfeel, or a cappuccino which is foamy with milk that seems unevenly distributed in the cup. The objective of your early latte art milk practice should not be to create perfect microfoam, but to create milk that appears more glossy, pours in a smoother stream, and does not separate into hot liquid at the bottom of the cup and stiff foam on top.
You want to begin with less milk than a full milk pitcher, because the smaller volume of milk will make it easier to observe. After you purge the steam wand, place the tip just under the milk surface and angle the milk pitcher slightly to help the milk start to swirl. As you add air for the first seconds, drop the tip of the steam wand into the milk slightly to continue swirling the milk without adding larger bubbles. When the steam wand is positioned too high, the milk surface begins to jump or spit. When the wand is too low in the milk, it is not swirling at all.
The temperature of the milk plays just as much of a role as the foam texture. Milk that is too hot can taste flat, or have a boiled flavor that overpowers the coffee. You want to keep an attentive hand on the side of the pitcher, in which the milk is heating, as the heat rises so that it becomes impossible to hold the pitcher. When you can only hold it for a few seconds, the milk has reached the correct temperature, so end the steam process. While using a thermometer to track milk temperature is helpful as you develop this skill, learning to feel the milk temperature will help you to sense how the temperature rises and to end the steam before the milk becomes too hot.
You can tap your steamed milk gently on your kitchen counter to bring the steamed milk texture back together; this will not solve every foam problem, but tapping and swirling can work some smaller dry bubbles back into the milk stream and smooth out the top surface. Pour quickly once the milk has been steamed, as textured milk does change with time, and if you wait too long, the foam and milk begin to separate, making it difficult to pour a uniform drink.
If you steam milk without pulling an espresso, you can focus completely on the wand position, sound, swirling milk movement, and final texture of the milk, and then pour the steamed milk into a clear glass or cup to see where the milk settles in your glass. This will tell you if the milk has a dry, bubbly surface, has a thin surface with foamy milk that floats on the top, or has become a smooth milk texture throughout, without the extra pressure of trying to pull your espresso.
For your next round of steaming milk, try focusing on one aspect to improve, such as lowering the sound of your steam, creating a more even milk swirl, having smaller bubbles, or stopping steam earlier to avoid overheating the milk. Steamed milk will always feel easier to read if you stop trying to focus on the pour, and try paying attention to the sounds and movements your steam wand is creating.