Why do different brewing methods require different grind settings?
There are a lot of factors affecting the quality of a cup of coffee, and the grind setting is only one variable in the equation. However, it plays a large role in determining how much of the coffee is extracted while brewing.
When talking about coffee, extraction refers to the percentage of soluble solids that are pulled from the ground coffee beans and into your cup. Regardless of brewing method, the best concentration of flavors and coffee oils occurs when 18% of the coffee is extracted. It is helpful to remember that this number refers to a percentage of the coffee, not to the ratio of coffee grounds to water.
It is no coincidence that a faster brewing method like espresso requires a finer grind. Because a fine grind exposes more of the bean’s surface area to the water during brewing, it takes less time to extract the ideal 18% soluble solids from the coffee. This is why baristas adjust the grind to achieve the proper length of brew-time for espresso (20 to 30 seconds).
If the shot pulls too slowly it is likely that the grind is too fine. This also means that more soluble solids will be extracted because more of the bean is exposed to water for a longer amount of time. This leads to over-extraction, meaning bitter flavors will find their way into your drink. Assuming everything else remains the same, adjusting to a coarser grind will fix the problem.
Press pot coffee represents the other end of the spectrum. A much coarser grind is used because of the longer desired brew-time. It takes approximately 8 times as long to brew coffee in a press pot, yet we achieve the same level of extraction because the water remains in contact with the grounds for a longer time.
The pictures below illustrate fine to coarse grind settings. Using a burr grinder is the best way to achieve a consistent grind.