Coffee extraction
 SCAA brew chart (American) |
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SCAE brew chart (European) |
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NCA brew chart (Norwegian) |
Coffee extraction is the process of dissolving soluble flavors from coffee grounds in water. Proper brewing of coffee requires using the quantity of coffee, ground precisely, extracted to the correct degree, controlled by the correct time and correct temperature.
Specialized vocabulary and guidelines exist to discuss coffee extraction, primarily various ratios, which are used to optimally brew coffee. The key concepts are:
These are related as follows:
which can be analyzed as the following identity:
A subtler issue is which solubles are dissolved – this depends on solubility of different substances at different temperatures, changes over the course of extraction (different substances are extracted during the first 1% of extraction than in the period from 19% extraction to 20% extraction), and is primarily affected by temperature.
Brewing guidelines are summarized in Brewing Control Charts, which graph these elements, and center around an "ideal" rectangle indicating the target brewing range. The yield is the horizontal (x-axis), the strength is the vertical (y-axis), and a given brewing ratio determines a radial line, since for a given brewing ratio the strength is directly proportional to the yield.
In brief, ideal yield is widely agreed to be 20%±2% (18%–22%), while ideal strength (for brewed coffee) varies between 1.25%±.10% (1.15%–1.35%) in American standards, to 1.40%±.10% (1.30%–1.50%) in Norwegian standards, with European standards falling in the middle at 1.20%–1.45%). This is most easily achieved with a brewing ratio of 55 g/L (55 grams of coffee per 1 L of water) in American standards, to 63 g/L in Norwegian standards, yielding approximately 14–16 grams of coffee for a standard 240 ml (8 oz) cup.
These guidelines apply regardless of brewing method, with the following exceptions: espresso is significantly different (much stronger, and more varied extraction), and dark roasts taste subjectively stronger than medium roasts (standards are based on medium roasts; equivalent strength requires using a lower brewing ratio for darker roasts).
These components are addressed in turn.
Ideal yield is widely agreed to be 20%±2% (18%–22%). This was originally computed by the Coffee Brewing Institute, under the direction of Professor E. E. Lockhart at MIT in the 1950s, and has been verified by subsequent tests by the SCAA.
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