Since the early 1990s, the latte art movement has introduced a visual and textural luxury to the coffee world in the form of microfoam. In most cafes, achieving this silky foam quality rests on the barista’s ability to entrain air into the milk in just the right amount. By ‘entraining air, we mean introducing air into your milk, thereby forming bubbles. We employ this technique to texturise milk so it is suitable for making latte art whilst creating a voluptuous mouthfeel across the whole range of milk-based drinks, from cortados to flat whites. This is a two-part process:
1. Entrain the air into the milk before it gets hot.
2. In doing so, increase the milk volume by one-third.
33% Aeration Target
Contrary to popular belief, and based on an exhaustive search through the scientific literature, it makes no difference where you position the steam wand in your milk in terms of its horizontal placement. It can go close to the edge of a milk pitcher, or it can go in the middle. Nor does the angle of your steam wand matter.
The vertical position of your steam wand does matter, however. If the wand is deeply submerged, you won’t entrain air into the milk; you’ll only add heat and some water as the steam condenses. If the steam wand is more than 1 mm above the surface of the milk, the extreme turbulence created in the milk by the injection of steam will make it spray all over you and the benchtop.
You can’t afford to lose all that latent heat from your steam into the atmosphere — you want it to go into the milk to heat it more efficiently. For this reason, when operating the steam wand, the tip should never be drawn away from the surface of the milk. Position the steam wand so it is ‘kissing’ the surface of the milk as you add the air. The incoming steam pressure must be sufficient to tear pockets of air into the milk. You will hear a sharp hissing noise as the air is entrained into the milk.
Part 1: How to steam milk.
Part 2: How to steam milk.
Aeration Targets
As soon as you have achieved a one-third increase in milk volume, all you need to do is submerge the wand beneath the surface and continue heating the milk without adding any more air. We tell you to aim for a one-third increase in milk volume, but there is some flexibility here. A volume increase in the 20–50% range produces milk that is suitable for latte art purposes. The turbulence in the pitcher caused by all the swirling of the liquid will ensure that the large bubbles will be burst by water pressure and only the smaller bubbles will remain. Many baristas like to make a rapid whirlpool effect in the pitcher. This can be convenient in bursting the macrobubbles, but is not an essential step. The turbulence is not responsible for the formation of microfoam — rather, the generous amount of flexible whey protein is what permits the elastic microscopic bubbles to form (Rouimi et al 2005).
When an espresso machine is performing at its optimum and your milk is in great condition, there are two keys to correctly entraining air into milk: aerating the milk before it is warm and adding the right amount of air. In Chapter 1, we will explore what we mean when we say milk is in ‘great condition’.
In this example we indicate our suggested lower and upper limits for aerating milk for latte art purposes, using 100 ml of fresh milk. Any volume gains inside the pink region are insufficient; anything in the gold region should work well. A volume at a level above the gold region level will make it difficult for your to pour complex designs.