FAQ

I have a "pressing question" as a curious physics teacher: You have said pushing harder forces the coffee grains more into the filter which slows the flow of coffee into the cup. I have a hard time with this explanation. Pressing harder does not push downward on the grains. It increases the ambient pressure in the liquid which would be felt by all sides of each grain. A grain floating in the liquid would not be forced to the bottom by this process. It will go to the bottom due to gravity, but not increased ambient pressure. I can concede that there may be other reasons to prefer one pressure over another, but this explanation does not work for me. Have you, for instance, done objective testing of pressing time under different pressures? If there is a turn-around point in flow rate as a function of pressure, that would be interesting to know, but the explanation would have to be a bit more subtle. (None of this affects my appreciation for your invention, by the way. I love Aeropress coffee.) I am genuinely interested to know why coffee brewed at higher ambient pressure tastes richer. That would apply to high steam pressure in an espresso maker as well. I tentatively think of the Aeropress as an espresso maker simulator…pressure by different means. – From David C.

Hi David, I love physics. There are dynamic effects. The drag on each particle of grind is proportional to the flow velocity squared. So the dynamic drag on particles pushes them downward, compressing grind below. Then phenomenon below begin. I think that we can visualize any strata of grind as being sufficiently dense to behave approximately like a porous semi-solid. Pressing hard causes that strata become more densely packed and to also compress the grind below it. As we press, the more we compress the slurry, the greater this effect. There is no doubt that the pressing hard blocks flow and pressing gently gets the job done.