Learning Espresso - Frothing Milk

Steaming milk is tricky business. From the hands of a skilled barista will produce luxuriously frothed milk. Let’s take a look into the techniques and science.

Overview

Why do we bothering steaming milk? We’re doing two things, heating and aeration.

Heating

There are a myriad of ways to heat milk:

• microwave
• boil
• mixing with another hot beverage, such as coffee
• steaming

We heat milk because we enjoy hot beverages hot. We don’t heat milk if we are going for a cold beverage.

Steaming is not unique here, when it comes to raising the temperature. At high temperatures above 65C (150F), the milk will denature and it’s considered undesirable.

Aeration

Aeration is the addition of air, which is trapped in bubbles. Milk proteins form a network, which can capture these air bubbles. This is what allows whipped cream to work. And like whipped cream, we enjoy the resulting change in texture.

Like heating, there are a myriad of ways to aerate milk:

• whipping
• steaming
• shaken

By mixing the milk, we encourage the milk proteins to bind and form a network, allowing bubbles to be trapped. So steaming milk is not unique here.

Steaming Milk

What’s unique about steaming milk is that it’s a combination of heating and aeration. Milk proteins bind better at colder temperatures. They become less stable at higher temperatures. A steam wand is able to quickly get this going, so the protein matrix is more stable.

The steam wand injects air at a fast rate, producing tiny bubbles that are trapped. The texture of this is microfoam and lends to a luxurious mouthfeel. Manual frothers are not able to replicate the microfoam, they only produce larger bubbles.

How to Steam Milk

There are 3 aspects while steaming milk:

• aeration
• heating
• mixing

The first step is aeration or stretching phase. While the milk is cold, we quickly introduce all the air within a 3-5 second span. The milk will quickly grow in volume. Too little and the milk will be thin and too much it will be thick.

After stretching, we heat. The milk is brought up to serving temperature, around 65C (150F). Too hot and the foam will become too stiff and separate from the microfoam.

And the entire time, we are focused on mixing. We want a homogenous mixture so the whole drink should be stirred.

Vortex

Getting a vortex going is crucial to microfoam. Establishing this right out of the gate is crucial, there is not much opportunity to adjust.

The steam wand should be off-center and angled slightly, 20-30 degrees from vertical. This positioning will set the vortex to spin within the pitcher and go to bottom.

After aeration, the vortex will suck in the foam, break it down, and distribute it evenly. This is the key to microfoam. The more time we have swirling the foam into the vortex, the more big bubbles we’ll be able to breakdown.