The canonical model of brew control since Lockhart’s 1957 work: a two-axis chart plotting strength (TDS, the fraction of the cup that is dissolved coffee) against extraction yield (the percentage of the grounds that dissolved), with the two linked by brew ratio. The SCA’s “Golden Cup” ideal sits at 1.15-1.35% TDS and 18-22% extraction; the surrounding regions read weak/strong on one axis and underdeveloped/bitter on the other. Extraction yield can proxy for taste at all because Coffee Compounds Extract in Order.
The classic chart, reproduced from Batali et al. 2020 (Scientific Reports, CC BY 4.0).
The chart’s power is its claim of sufficiency: a brew is characterized by where it lands on these two axes, which is why brewing frameworks like the 4-6 Brewing Method ultimately resolve to one knob per axis (see Most Cup Character Is Fixed Before Brewing). Its weak point is the target, not the axes: a 2023 revision found consumer liking does not map neatly onto the classic ideal box.