Why do espresso grains have to be finely ground and French press grains coursely ground?

Question

Why do espresso grains have to be finely ground?
_{(I've seen some moka pot instructions say medium is good, too.)}

I thought espresso machines extracted the most out of the grains, so it would seem they wouldn't need to be finely ground—that fine-ground would be suited for French press (lower temps & pressure, no?) and course ground for espresso (the exact opposite of what's recommended!).

Answer 1

Coffee brewing is simple physics and chemistry. In short, soluble compounds and oils migrate from the beans to the water. The extraction will depend on temperature, pressure and contact area between beans and water. There’s also a balance between desired and less desired flavor compounds (e.g. acidic and bitter) that a specific brewing method needs to manage, so you tweak the given parameters.

If you have a coarse grind, the total surface area is smaller, so to get a comparable extraction you need more time (minutes for hot water as in French press, hours for cold water as in cold brew). For finer and fine grinds, you brew shorter (pour over) and probably increase the pressure (espresso).

Of course you can also look at the problem in reverse, like in the question (“how much can method X extract”), and match the grind size to hit the optimum. However, there your assumption that e.g. an espresso machine can extract more is incorrect. The very short time plus the compact grounds means that comparatively little coffee flavor is extracted - but some would say that it’s the best parts ;-)

Answer 2

There are two obvious reasons why course grind is better for a French press:

1. All the water is added at once, so the brewing process is relatively fast. With fine-grind, your perfect time before plunging might be 1 minute, and with course grind it might be 5 minutes. If your timing is out by 15 seconds, it will cause a very noticeable difference with the fine-grind (±25%), while the course-grind will be far more forgiving (±5%).
2. Cleanup is so much easier, as the fine grounds are much more apt to become caught in the mesh and other parts.

Answer 3

Key to understanding this is the flow rate of the hot water: Fine grains = slow flow rate; coarse grains = fast flow rate.

Grain size determines how quickly the hot water flows, and thus how much time it is contact with the grains. Faster flow means the water has less time to be in contact with the grains and thus less time the water can extract the oils and other nutrients. (Certainly, surface area factors in, too; cf. Stephie's answer.)

Methods where the grains are soaked (cold-brew, French press, pour-over) don't require fine grains. One could use fine grains in such methods (as in the Turkish method), but it's not necessary, and they would be harder to filter out.

Methods where water quickly flows by the grains also extract more quickly, because of higher temperature and pressure.

Grain Size     Flow Rate     Extraction Time     Methods
⬤             •             ⬤                  Cold brew, French press, etc.
•              ⬤            •                   Espresso, moka, etc.

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Kingston's How to Make Coffee: The Science Behind the Bean:

When grinding for espresso at home, as a rule of thumb if the coffee is coming out too fast, use a finer grind to slow down the extraction; or, if the coffee is coming out too slowly, make the grind coarser.

If your coffee tastes a little sour, this is often due to underextraction. With methods of brewing other than espresso, using a finer grind will allow more coffee to be extracted during the brewing time. The same is true of bitter coffee—this could be due to your beans, equipment, or brewing method, but it may also be caused by overextraction. Make the grounds a little coarser and to slow down the extraction rate.