“Extraction is a measure of the solubles yield. It is also expressed as a percentage, comparing the amount of coffee flavoring material in the beverage to the amount of coffee grounds used to prepare the beverage. Approximately 28% of the organic and inorganic matter matter contained in roasted coffee beans will readily dissolve in water. The remaining 72% is cellulose bean fiber that isn’t water soluble under normal brewing conditions.”
— Ted Lingle, The Coffee Brewing Handbook, 2011 Specialty Coffee Association of America.
When we talk about extraction, we are evaluating the process in which particles dissolve from the ground coffee into the water. Roasted coffee is a wonderfully complex thing. Though only 28% of every coffee bean can dissolve in water, that 28% represents 800-1000 different flavor compounds, half of them aromatic gasses, and half of them soluble solids. It’s much, much harder to track what gasses we’ve pulled from the ground coffee into the brewed coffee, so we’re usually more concerned about the coffee solids we’ve dissolved: what are they, and how much of them have we dissolved?
It’s pretty easy to break up the soluble material in coffee into four basic categories: fruit acids, which come from the fruit itself; fruit sugars, which are also derived from the coffee fruit; caramelized sugars, which develop during the roasting process; and bitter, dry plant solids. Acids dissolve pretty easily, sugars take longer to dissolve, and those bitter, dry plant solids are pretty dense and are made up of large molecules that take the longest to dissolve. Because these all dissolve in the same order, when we calculate an extraction yield as 15%, 21%, or 25% of the total soluble material, we can usually assume a certain concentration of those four main categories of coffee solids. From 0-18%, we’re seeing the fruit acids dissolving in high concentrations. From 18-22%, the fruit sugars and caramelized sugars start dissolving in higher concentrations, and from 22% to 28%, we see the dry, bitter plant solids dissolving in higher concentrations. Therefore, ideally, we’d like coffee extraction yield readings to land around 19-21% for the sweetest brewed expression of what that coffee has to offer.
The extraction yield is just an average though — anytime you’re brewing coffee, the smaller particles will dissolve quicker, and the larger particles will dissolve slower. So in a 19% extraction yield, we’re assuming the larger particles might only be extracted to 17%, while the smaller particles maybe would have extraction to 21%. The more consistently even your grind setting, the more consistent your extraction yield will be. This is the main crux for the importance of not only just using a burr grinder, but a quality, precision burr grinder.
The reason for this disparate extraction yield has to do with how the coffee actually extracts. There are three main phases: Wetting, in which the coffee particles absorb water and push the remaining gasses out; Extraction, in which the solubles inside the particles actually begin dissolve into the water and start exiting the coffee particle into the overall solution; and Hydrolysis, in which larger insoluble carbohydrates are broken down into soluble ones. With smaller coffee particles, the water can penetrate and dissolve the solubles very quickly. Larger coffee particles require the water to penetrate deeper, and have more solubles to extract, so it takes longer inside the particle to dissolve the desired solids before the brew leaves the particle to join the overall brewed coffee solution.
It’s because of this relationship between brew time and particle size that we are able to control our coffee brewing process more specifically. When paired with an appropriate ratio of coffee to water, a brew temperature around 200ºF, and controlled agitation, extracting the positive flavor solids and leaving the negative ones behind becomes an easy solution. (Please forgive the pun).