Don Iszatt of Finca Coffee Roasters describes a study of coffee solids extraction in espresso brew. He says “We wanted to take an empirical look at extraction, and found a mathematical function that describes the rate of extraction”.
The amount of dissolved solids in the liquid-form coffee is measured as a percentage of the mass of the dry coffee and is called “yield”. The yield is higher during the initial stages of the brew, and as the extraction progresses, it becomes more difficult to dissolve these solids.
In the experiment, they ran separate espresso shots, stopping them at different points, measuring the mass of the espresso collected, calculating the resulting yield and plotting espresso yield against mass. They analyzed the data and came up with a power function y = 3.9x^0.45, where y is the yield and x is the mass. Don called this function an Espresso Power Curve (EPC).