Making coffee is sort of like making Hollandaise sauce. I mean, there are only two ingredients in brewed coffee, how could it possibly go wrong?
Well, I've screwed up enough batches of Hollandaise to know not to be suckered into "deceptively simple" recipes. (Seriously, though: Why is blending eggs, butter, and cream so hard?) The truth is that any number of variables can affect your finished coffee product, including temperature, agitation, the size of the batch, and many more. Today, we'll look at demystifying one of the variables that can make or break a coffee extraction: Grind size.
Why is it such an integral part of coffee making? Let's find out.
To illustrate, let's look at two methods at "opposite" ends of the brew spectrum, and the role that grind size plays in them: French press and espresso.
We know that the grind for a French press is much coarser than that required for espresso, but why? Well, a press pot extracts coffee using a steeping method, meaning the water and coffee commingle completely for the duration of the brew (usually about four minutes). In contrast, in an espresso extraction water is forced under pressure through the coffee, and is only in contact with the grounds for a very short period of time (generally between 20 and 30 seconds).
When coffee and water spends a longer time together, the liquid has more of a chance to absorb whatever the bean has to give it, so it doesn't need to be exposed to as much of the caffeinated surface area. When the water has to speed through the grounds, however, it only has a few fleeting seconds to dissolve the same amount of solids in order to create a rich and flavorful cup. In the latter case, the way to make that possible is to make more of the bean available to the water by turning it into a finer powder, exposing more of its savory surface area.
Espresso, then, is kind of the caffeinated equivalent of speed dating: You have to put a fine point on your most fascinating and appealing characteristics because you have less than ten minutes to sell yourself. French press is more wine-and-dine: You can take it a bit easier, revealing all the juicy details in due time.
So how do you find your magic grind size? Time is one way to check yourself: Most drip-style brews should take from three to four minutes to fully extract, while as I mentioned before espresso should clock in between 20 and 30 seconds. If you're finishing up before then, you'll likely want a slightly finer grind; any slower and you'll want to coarsen things up. (Think of the coarser grounds as being like rocks, and the finer ones like sand: Water passes slower through more-absorbent sand, right?)
Of course, the only test that really matters is taste: Does your cup have grassy or pencil-like flavors? That could be a sign of underextraction, which might be caused by too coarse a grind. Bitter, astringent aftertaste? It's possible you need to back off the fine-grinding a little bit. In the end, go with whatever tastes best to you, and don't be afraid to experiment.