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Grind size has two main functions in coffee brewing.
First, It increases the surface area of the coffee and allows extraction to happen.
Have you ever been so tired you forgot to grind your coffee before you threw it into your brewer of choice, it doesn’t brew so well does it? That’s because the surface area of the bean is just the outside portion of the coffee. When you grind coffee you are essentially cutting it up into smaller pieces and exposing more and more of the coffee creating higher potential for extraction. Grinding is arguably the most important factor in determining the final quality of the coffee for this reason, it is in charge of releasing all of the potential coffee flavor from the bean. But you can under grind and over grind your coffee which leads us to…
The second thing grind size does. Grind size regulates the flavor intensity of your coffee. There’s a lot of potential flavors inside coffee, some of that is good, some of that is not so good. You can often grind so coarse (think of the unground coffee from earlier) that you don't get enough of the potential flavor in the coffee, this leads to a watery and under extracted cup of coffee. You can also grind too finely (like dust) and get too much of the potential flavor, we call this over extraction. Over extraction is when you have pulled all of the flavor out of the coffee grounds, this sound like a good thing but not all of the flavors in coffee are desirable, those bitter and astringent flavors that you sometimes get from your cup of coffee are often created by over extraction, and that over extraction usually comes from grinding a bit too finely.
And finally what does this all mean practically? When we are brewing coffee we are looking to brew a balanced cup of coffee, this means marrying the right grind size for the brew you are intending to create. The contact time between hot water and coffee grounds has a very direct correlation to the overall strength of the coffee’s flavor. For us in the cafe this means we time our coffee, if we are making a pour over we are looking for it to come out between 3-4 minutes from the first time water touches the coffee grounds.. We can manipulate the contact time by either making the grind size bigger or smaller. If the coffee is done brewing in less than 3 minutes we would make the grind size smaller (finer) this will slow the flow of water through the coffee and allow for more of the coffee’s flavor to find its way into the cup. Conversely, if the coffee is done brewing in more than 4 minutes or is tasting more bitter than you want, we would make the grind size bigger (coarser) to speed up the flow of water through the coffee and allow for less of the bitter compounds the chance to make it into your cup!
As a general guide these are the general grind sizes/textures we aim for in our different brew methods:
These are all starting points but will vary depending on the coffee and equipment you are using so feel free to experiment with these grind settings until you get the flavors you are looking for!