Immersion-brewed coffee extracts more quickly and to a higher extraction yield on a finer ajustes de molienda than a coarse one. Research looking at maximum extraction of the same coffee on a fine ajustes de molienda compared with a coarse ajustes de molienda 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 ajustes de molienda that is not excessively coarse.
However, we don’t use an ultra-fine ajustes de molienda 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 Preparación de Café Avanzado course.) When establishing the cupping ajustes de molienda 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 Refractometría protocols) this means one of two things.