Grinders Modify Surface Area
When we turn our grinder’s dial, we are adjusting the fineness or coarseness of our coffee. The two burrs will move closer (for fine grind) or farther apart (for coarse grind). It is here that we are exposing more of less surface area. Now for a little extraction crash course.
We can only brew coffee that is ground. It accelerates the rate in which we can get to its solubles: dissolvable solids that make up the sugars, oils, and acids in coffee. The faster your brewing method is, the finer your coffee grind may need to be. It is all about balancing the time your coffee is in contact with brewing water and how much surface area you are exposing to the water.
Coarser grind sizes expose less surface area. Cold brew uses a coarse grind size because the coffee is in contact with water anywhere from 14 to 48 hours! A fine grind would expose too much surface area to water thus extracting too many solubles. This leaves us with a bitter that tastes more like molasses than iced coffee.
Fine grind is used for things like espresso and Aeropress, which uses pressure to help extract coffee. Since these brewing methods involve pressure and a shorter amount of time than cold brew or the 4-minute Mr. Coffee, we need a finer grind to get all those solubles in time!
If you were to use a coarse grind size in an espresso machine, your coffee would be flying out and hitting the wall behind you. There is not enough restriction between a coarse grind and 9 bars of pressure, thus water flows out fast without grabbing the sugars, oils, and acids we need for something delicious.