Using Solubility Testing to Establish Pre-ground Coffee Standards and Your Cupping Grind
An advanced use of refractometry can be seen in the practice of cupping. For example, most roasteries will establish a standard cupping grind. The traditional practice of establishing standard grind settings can be quite hit and miss. Usually, from finest grind to coarsest, the standard settings will follow this approximate gradation:
The principle behind this practice is to ensure your customers make all their brews in an 18–22% range with each method. We think best practice here requires you to brew a version of each of these and measure extraction yields to test your typical grind settings. (A strictly scientific approach would look for three repetitions of each brew method).
The stovetop method, for instance, while having one of the shortest contact times of all these methods — around 60 seconds — brews at a very high temperature and usually delivers unexpectedly high extraction yields even on coarser grind settings. (This is also promoted by a pressurised erosion of solids which encourages a steeper concentration gradient between the grinds and the brew water and therefore more efficient extraction rates.) Even on a V60 grind setting, you can achieve 24% extraction yields.
When establishing the correct grind setting, we would recommend you target a higher extraction yield e.g. 21% extraction yield, for each test brew method — then re-adjust these settings every 250kg of burr life on a large shop grinder. As the burrs on a grinder wear down, fines production will increase and flavours associated with over-extraction will become more detectable at lower and lower extraction yields over time. Based on the leading shop grinder manufacturer’s recommendations, you should be able to produce 1500kg of ground coffee before over-extracted tastes enter at lower yields.