Extraction Fundamentals: Searching for an Ideal Brew

By: Drew Billups

Feb 3, 2016

Brewing is the final stage in coffee’s lifecycle and the last step in the stewardship of quality. A farmer can work tirelessly to achieve quality, a roaster can expend great effort to source and profile, but it still requires skilled preparation to brew a great cup of coffee. Coffee, the beverage, is a solution that we make by coaxing pleasing soluble materials from the bean into the cup while leaving undesirable solubles behind.

What we will discuss here are extraction basics that can be applied and adapted based on the requirements of the method and the moment. When studying extraction it’s important to understand two key metrics: extraction percentage and brew strength.

Extraction percentage is the percentage of material by weight that we are withdrawing from the coffee beans. The majority of a coffee bean cannot be dissolved into water. This is why we have spent coffee grounds after brewing is finished. On average around 28% of the coffee material is soluble, but as coffee is an agricultural product, this is subject to variation in different individual lots. In any case, we do not want all the soluble compounds in our beverage as some of them simply do not taste good. Fortunately, for the most part, the soluble compounds that taste good, like sugars and acids, dissolve quickly and the solubles that are not as pleasant, like tannins, take longer to dissolve. Because of this, we can withdraw most of the favored compounds and leave the unpleasant ones simply by arresting the brewing process before it gets to the point where the less desirable compounds start to leach out.

The range that has been established as industry standard for extraction is 18%-22%. This means that while a possible 28% of the dry coffee material is soluble by nature, only 18-22% ends up migrating into the final beverage if we have achieved ideal extraction. It is of course possible to under-extract coffee – withdrawing less than 18% of the solubles – resulting in a beverage that tastes sharp, nutty, grassy, and lacking sweetness. Conversely, it’s possible to over extract coffee – moving into the 22-28% range – resulting in wood-like, bitter, and astringent flavors.

Brew strength is a metric to describe how strongly we have flavored our water with soluble coffee material. Obviously, the coffee beverage is mostly water – usually somewhere between 98 and 99%. Brew strength is the percentage of the finished beverage that is composed of coffee material as opposed to water. Brew strength is measured by total dissolved solids (TDS), which can be ascertained by a conductivity meter, or more accurately, by a refractometer like the VST. A TDS reading in parts per million can be easily converted into a percentage by moving the decimal point 4 spaces to the left. There is some diversity of opinion about the ideal strength range, however the Specialty Coffee Association of America recommends 11500 – 13500 TDS (the equivalent of 1.15-1.35%).