Extraction yield (EY)
also: extraction yield, EY
The percent of ground coffee mass dissolved into the cup; the ideal window is roughly 18 to 22 percent.
Extraction yield is the percentage of your ground coffee that actually dissolves into the finished drink. If you brew with 20 g of coffee and 4 g ends up dissolved in the cup, that is a 20 percent yield. The rest stays behind in the spent grounds.
It matters because coffee solubles do not all taste the same. Acids and fruity, bright compounds dissolve first; sugars and balancing flavors next; bitter and astringent compounds last. Pull too little and you stop in the sour, underdeveloped zone. Pull too much and you drag out the harsh, bitter compounds.
The conventional target window is roughly 18 to 22 percent, where most coffees taste sweet and balanced, though great light roasts can sit a touch higher and the window is a guideline, not a law. Note that yield is separate from TDS: yield is how much you extracted, TDS is how concentrated the result is. To know your yield precisely you combine a refractometer reading with your brew weights. See extraction-theory for the underlying chemistry.