As you may have already noticed when making any type of immersion or percolation coffee, wet coffee grinds have a tendency to form a crust at the top of the slurry. The crust is a layer of air bubbles, coffee grinds and water that acts like an insulation layer. The crust is usually left undisturbed for 3–5 minutes, until a cupper comes along and ‘breaks the crust’. The action of breaking the crust usually involves a cupper pushing a cupping spoon through the top of the crust, like you see in the video above.
Many cuppers prefer to stir right to the bottom the cupping bowl as they break the crust. This is the approach we prefer. We advise cuppers to apply at least 4 precise stirs with a cupping spoon, reaching to the very bottom of the cupping bowl.
The crust of coffee will be thicker in darker-roasted samples. Dark roast coffee produces around four times as much CO2 content — around 2% of the dry weight of whole beans — whereas a light roast will form as little as 0.5% CO2 (SCA). Larger coffee particles contain more roasting gas inside them than do fine particles, and thus they are more likely to float up into the crust. Most cupping protocols ask cuppers to break the crust 3–5 minutes after the water is added. This can be problematic if one cup is left for 3 minutes and another cup is left for 5 minutes: the cup that is left undisturbed for longer can reach a far higher extraction yield.
In this experiment we compare the extraction yields of cupping bowls that have been agitated multiple times compared with bowls that have had no agitation at all, compared with our customary four stirs. Because this topic is so controversial and you will encounter many different approaches to this part of the cupping method, we have put together a white paper on this subject. In this research our mission was to work out what the impact is of different timings and different methods of breaking the crust when cupping.