In typical ground coffee, nearly 70 percent is insoluble, so you could brew a batch of grounds endlessly and there would still be some spent grounds to throw away afterwards. What can be dissolved in water are the compounds that make up coffee’s flavor in the cup. Theoretically, your maximum extraction was thought to be around 30 percent of the coffee you brew with.

The coffee industry used to talk about a certain range of extraction being ideal. 18–22 percent was considered a good ratio for a good-tasting cup. It can be an awful thing to think about, so let’s put some numbers behind it.

Let’s say you brew a pour-over with coffee, using 500g of water, and you extract 20 percent of the coffee. You then take your spent grounds and dry them slowly in the oven until all the moisture is gone, and then they’d now only weigh 20 percent of their initial weight: 24g. The coffee contains the missing 6g of co dissolved in the liquid, giving it color and flavor.

In the past, this is pretty much how measurement was done: Oven-dry ground coffee is the technique used to develop the target range of coffee. However, in the last decade or so, modern technology has replaced the use of a refractometer (an instrument that can measure the concentration in a liquid) to measure the liquid coffee. This means that we can now convert the refractive index of the coffee to an expression of strength.

In the example on the previous page, the refractometer might show a strength of 1.36 percent. If you weigh your liquid coffee (you can’t use the soot you started with because some of that water has been absorbed by the ground coffee) and you have 440 g, then it is easy to calculate how much coffee you extracted (440 x 1.36 = 6 g). Thus, your extraction percentage is 6 g/30 g (your starting dose of grounds), which is 20 percent.

You could always dehydrate your cup of coffee and measure what is left. This is, in a much more simplified way, how instant coffee is made. Your teaspoon of instant coffee is pure, soluble coffee material that has been brewed and then freeze-dried, formed into attractive little clumps that look a bit like fresh coffee grounds, then packaged and sold.

Measuring extraction is primarily the concern of the coffee industry; it is used for research and development, diagnostics, or helping standardize drinks in a café. However, this understanding is useful because of two terms that are extremely important and commonly used: underextraction and overextraction.

Underextraction and overextraction

Underextraction was previously defined as a brew of coffee where the total extraction was below the target window, and overextraction was when the total extraction exceeded the window. A deeper understanding of what is happening in coffee brewing has led to a reassessment of how we use those terms and what they really mean.

The flaw in the simple definitions of under- and overextraction is that they don’t really help us understand why a brew might taste bad. In the past, the solution to underextraction was to grind finer, and conversely, to grind more coarsely for an overextracted cup. This approach seems reasonable at first—the coarser a coffee is ground, the less total surface area there is, so the less access water has to the flavors in the ground coffee. Frustratingly, in the real world, it didn’t solve our problematically bad cups of coffee in the way that we would have liked.