Take away the water in a cup of coffee and what’s left? You’ll find some brown powder made from the soluble parts of the coffee grinds you used to brew with. That’s the extracted mass. If you recorded exactly how much dry coffee you used to brew the coffee, you now have some very useful knowledge. By weighing the extracted mass and dividing it by the weight of the dry coffee, you know what fraction you extracted from the grinds — this is your extraction yield.
This is the formula: Extracted Mass / Dose = Extraction yield%
Green coffee beans contain insoluble protein, fibre, cellulose and fat. Though some of these components have an impact on roasting chemistry, they cannot be dissolved in hot water so when we calculate extraction yields, we discount the contribution of insoluble material. In roasted coffee, this insoluble material constitutes approximately 68–72% of the bean mass.
When E.E. Lockhart began work on his PhD this was one of the questions he put forward. In his seminal work “The Soluble Solids In Coffee As An Index To Cup Quality”, Lockhart states:
At this point it is pertinent to ask “how much soluble material is there in coffee?”. In one recent report (Mabrouk & Deatherage, 1956) repeated extraction of a Latin American blend gave about 32 percent of soluble material. In another (Thaler, 1955) the value for coffee from Angola, Colombia and Rio were about 36 percent, 30 percent, and 29 percent respectively. These values refer to the amounts obtainable with complete extraction and would rarely be attained in normal beverage preparation.
Recent experiments investigating maximum solubility by Moroney et al., 2015, obtained extraction yield measurements between 28–32% for both fine and coarse grind settings. Interestingly, their research reports slightly lower yields for the coarser grind, even after hours-long extraction times. The method involved continuous stirring of immersions of coffee held at 90°C for five hours. The same test was repeated over 10 hours but measurements were not found to increase beyond the levels reached after five hours.