Imagine a small group of kids hanging out in a hallway. If a few other kids come walking down the hallway, some bumping into each other may occur but, generally, the stationary kids aren’t going to be touched and they aren’t going to go anywhere. If a large group of kids comes pouring down the hallway, nearly everybody is going to be bumped and some of them will get dragged down the hall with the flow. In this analogy, the kids just milling about are solutes and the other group coming down the hall is the solvent. In the first case, the solvent is not under pressure and in the second case it is. In the high-pressure scenario, the force of the solvent is so high that it is going to extract solutes faster and it is likely to snag solutes that wouldn’t likely be dislodged.

When gas is under pressure, it becomes much more soluble in liquid. So, for a given volume of liquid, you can put more gas into it when it is pressurized. Of course, when the pressure is released, the gas leaves the liquid. This is what happens with carbonated beverages. They are saturated with gas and sealed under pressure. When the container is opened, the gas leaves the liquid as bubbles, creating the carbonation that we so enjoy.

The most familiar coffee brew method that uses elevated pressure is espresso. The pressurized water (approximately nine times the pressure of air at sea level) is forced through a bed of coffee, yanking out a greater amount of solutes than would emerge without the pressure. The water also picks up a great deal of gas from the coffee. When the brew leaves the bed of coffee, the gas is released. However, whereas with carbonated beverages the gas escapes to the air, oils extracted from the coffee capture the gas, creating bubbles. We call these bubbles “crema!”

Did you know?

Milk curdles in coffee because the coffee’s pH is low enough to denature and precipitate the proteins in the milk. 

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