Milk Foaming

A multitude of foam consistencies are used in the preparation of foods and beverages. Chefs use foams in cooking because they make taste sensations last longer; the flavour sits between the bubbles in a foam and makes its way to your taste buds as if through a maze. In filter coffee (served black, no milk) flavour takes a direct route to the palate. But with a latte, flavour compounds must slide over a complex matrix of thousands of bubbles until they reach taste receptors on the tongue. This means microfoams produce flavour sensations that last long and are light in taste and texture.


Chocolate mousse features large, closed-cell foam bubbles (each cell occupies a distinctly separate pocket, surrounded by solid material), as does yeasted bread dough. After the dough is baked into bread, its bubbles form an open-cell foam. You know this by the way water immediately soaks into the bread. Similarly, if you dunk a biscuit or cookie into your coffee, liquid almost immediately penetrates the biscuit. The foam we like to use for coffee has a structure that is finer and more complex than that of these large-cell foams.


If you manipulate the components in milk and rearrange the concentrations of its protein, fat, sugar, and water, it’s possible to make a foam that’s so compact, with such tiny bubbles, that it could take 24 hours for the liquid to separate from the bubbles. The tinier the bubbles, the longer it takes for this separation process to finish. This process is known as drainage. For latte art purposes, we don’t necessarily want absolutely microscopic bubbles, but we also don’t want bubbles so large we can count them with the naked eye.


Adding just the right amount of air into milk is a very difficult task to execute consistently. It’s invariably the motor skill that takes the most time to train with rookie Baristas. Even the slightest misstep such as adding bubbles after the milk is warm can completely ruin the mouthfeel of the drink. 


Luckily, there’s no innate human touch that affects this process. We’re just making tiny bubbles, and trying to do it consistently. A machine is very capable of injecting air into a liquid in a precise and repeatable manner. Imagine a tiny air compressor and electronic valve. We can control the pressure of the air and the time the valve is open with incredible accuracy. With a fixed volume of milk we can add a fixed volume of air, creating a consistent increase in the volume of the liquid, and a homogenous size of bubble. 


Another advantage of precision such as this is the ability to modify.

On this image, you can see how the milk foam expands 60 times under the microscope. The white line is 1mm thick in actual size.
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