Immersion-brewed coffee extracts more quickly and to a higher extraction yield on a finer grind setting than a coarse one. Research looking at maximum extraction of the same coffee on a fine grind setting compared with a coarse grind setting found that the finer grind reached higher extraction yields. This was the case even after the grinds had been immersed in 90° C water (194° F) with constant agitation for 5 hours (Moroney et al, 2015). These results suggest that the centre of some very coarsely ground coffee particles may be almost impenetrable to water. For this reason we tend to use a cupping grind setting that is not excessively coarse.
However, we don’t use an ultra-fine grind setting either. This is because the cupping method does not involve any filtration. As we demonstrate with the experiment below, extremely fine grinding creates an increase in the amount of colloids in the cup. Gravity can do a more efficient job at allowing the fine particles to come out of suspension if they are less numerous and slightly larger.
If You Have a Coffee Refractometer
Some baristas have access to a technology such as a refractometer which allows them to calculate extraction yields in coffee. (We discuss best-practice in refractometer application the Advanced Coffee Making course.) When establishing the cupping grind setting using a refractometer, we recommend you target a relatively high extraction yield — e.g., 21%. Then cupping becomes a case of looking for ‘the odd one out’. For example, imaging all of the coffees on your cupping table have reached approximately 21% extraction yield after they have been steeping for eight minutes. if one coffee on the cupping table amongst a group of other samples has only reached 17% after 8 minutes, it is a strong signal that the coffee has been underdeveloped by the roaster.
If none of the coffees you are cupping with are reaching at least 21% extraction yield when you follow the BH Refractometry protocols) this means one of two things.