In the 1950s, coffee science really started to get rolling. The National Coffee Association commissioned Professor E.E. Lockhart to complete a food technology Ph.D. from the Massachusetts Institute of Technology (MIT). This was a first for the coffee industry.
Sensory science as we know it today was not a recognised practice in the 1950s. Nonetheless, with the help of a sensory panel, Lockhart established the target for optimum extraction rates, which is still relevant today.
If you extract between 18–22% and have decent quality coffee, you’ll certainly produce acceptable quality. But if you want optimum flavour, you need to find out specifically what extraction yield suits the combination of your roast profile and condition of your equipment.
Note: To put Lockhart’s 18–22% extraction yield target in perspective, take the example of instant coffee. Maximum extraction of above 28% is standard practice in the production of instant coffee. They achieve this through a combination of multiple extractions and by brewing in a pressurised chamber up to 150°C. The sensory characteristics of an instant coffee quickly convey why maximum extraction doesn’t work from a sensory standpoint — you end up with an extremely bitter, mouth-drying taste, reminiscent of cardboard.
Here are some charts from 1959