Coffee extraction refers to the process of extracting desirable compounds from coffee beans during the brewing of coffee. Extraction occurs when hot water is poured over coffee grounds, causing compounds such as caffeine, carbohydrates, lipids, melanoidins and acids to be transferred from the grounds to the water.
Extraction is everything water takes from the coffee. When you mix coffee and water, a lot of things happen. The most relevant and easy to understand of all these things is that water dissolves a lot of coffee’s flavors. These dissolved flavors make up almost everything you taste when you drink a cup of coffee.
Water is pretty good at dissolving those soluble chemicals, but it needs help. If you throw a handful of coffee beans in hot water, you don’t extract much more than the outside layer. This is because the coffee bean’s structure is incredibly dense and complex; water can’t just pass through and collect all the flavor on its way. To help, we have to increase the surface area of the coffee beans; we need to ‘open them up’ so the water can easily get at all of the flavor. This is achieved rather handily by the use of a coffee grinder. It crushes the beans into a powder, exponentially increasing their surface area and allowing the water to do its work.
Under-extraction occurs when you haven’t taken enough flavor out of the coffee grinds. There’s still a lot left behind that could balance out the following undesirables.
A well extracted coffee has a finish that lingers for minutes (or hours if you’re lucky). This finish can feel as though someone has left dark brown sugar on your tongue, or as though you’ve just finished a toffee. An under extracted coffee doesn’t have this finish.
Over-extraction occurs when you take too much of the soluble flavors out of the coffee. This level of extraction results in unfavorable flavors.
It’s important to realize that over extraction isn’t a switch that you flip on and off. It’s a slow, gradual change. It’s possible for coffee to be only slightly over extracted, while another mug can be very much so.
While experimenting to find your personal preferred extraction yield, we suggest starting with coffee strength and brew method. Immersion brewing methods, such as a French Press, produce oily cups with a heavier mouthfeel than say a drip method. Coffee strength also interacts with extraction. If you enjoy a strong coffee, you likely need to use a higher grounds-to-water ratio.
Bottom line, assuming your beans are fresh and your grind correct, if your coffee tastes sour, it is likely brewing too quickly; if your coffee tastes bitter, it is likely brewing too slowly.