Hand Brew Theory: What Factors Cause Over-Extraction and Under-Extraction in Coffee?

When it comes to brewing coffee, everyone should share the same goal: how to make it taste good. Although people's definitions of "good taste" may vary, as long as we ensure it's neither over-extracted nor significantly under-extracted, we naturally know it's a decent cup of coffee. Therefore, for those who want to improve their brewing skills, learning to identify coffee extraction situations through practical examples is particularly crucial.

Today, instead of discussing brewing techniques, let's focus on each stage and talk about the characteristics of both over-extracted and under-extracted coffee.

The Golden Cup Standard

Friends who have specifically studied coffee theory should know that mainstream research has proposed that coffee with an extraction rate between 18-22% is considered within the reasonable extraction range. Below 18% is under-extraction, while above 22% is defined as over-extraction—this is what many articles refer to as the Golden Cup Standard.

However, brewing coffee is ultimately an everyday activity. Not to mention ordinary enthusiasts, even experienced baristas with years of brewing experience won't frequently pull out laboratory instruments to measure concentration and then seriously calculate the extraction rate of each cup of coffee.

FrontStreet Coffee, through a series of comparative experiments, discovered that coffee with extraction rates precisely within the Golden Cup range can still produce unpleasant flavors, while some coffee with extraction rates exceeding 22% can remain delicious. This mainly relates to the flavor compounds contained in the beans and their dissolution differences, involving many details.

Factors Affecting Coffee Extraction

When brewing a pot of coffee, many factors affect the extraction of compounds, such as the hardness of the beans themselves, roast level, bean resting time, grind size, water temperature, coffee-to-water ratio, filter cone structure, stirring technique, etc. All these details can more or less alter the final taste of the coffee. In FrontStreet Coffee's view, brewing coffee should be a matter of adapting to circumstances rather than something to get bogged down in details. Therefore, rather than relying on instrument measurements each time and calculating a bunch of data, it's better to learn to judge whether your brewed coffee is over-extracted based on taste indicators combined with actual situations.

Under normal circumstances, the dissolution order of flavor compounds follows a certain pattern: light substances related to acidity dissolve first, followed by sweet molecules, and then the bitter components with the largest molecular weight, which dissolve more slowly.

Characteristics of Under-Extraction

When most particles are under-extracted, it means only a small amount of flavor compounds have been extracted by water. At this point, the acidity from the early extraction stage dominates, while the components responsible for sweetness and body haven't had time to release, and extraction has already terminated. Therefore, the coffee first tastes lacking in aroma, with no distinct flavors detectable on the palate. Light-roasted fruit-acid coffees typically exhibit harsh raw astringency and sharp acidity, with relatively thin mouthfeel and no aftertaste. Dark-roasted coffees, lacking the most exquisite parts (body, caramel notes, pleasant aftertaste), appear watery and lack depth.

If you also taste saltiness in your coffee, it might be another manifestation of under-extraction. Under normal circumstances, we rarely have the chance to taste saltiness in coffee because it's masked by acidity, sweetness, and bitterness. However, under-extracted coffee lacks the masking effect of sweet substances, thus exposing the saltiness. However, compared to other flavors, the presence of saltiness in coffee is quite low, so it's often ignored by the taste buds.

There are many causes of under-extraction, typically due to overly coarse grind size, water temperature that's too low, brewing time that's too short, or insufficient water volume.

For example, FrontStreet Coffee once encountered a customer who mentioned that she purchased our Colombia·Sidra and followed our extraction recommendations, but the brewed coffee was extremely acidic with no aftertaste可言. Upon investigation, we learned that she had the habit of stopping the flow early—removing the filter cone before the coffee liquid had finished flowing. This not only disrupted the intended coffee-to-water ratio but also resulted in sharp acidity due to the missing tail section.

Characteristics of Over-Extraction

Another extraction state is what we more commonly encounter when brewing at home—over-extraction, where hot water extracts too much from the coffee. This mainly consists of large molecular substances released in the later stages, often manifesting as bitter, dry, astringent, woody, straw-like, and other harsh characteristics, making them more easily perceived. The appearance of these flavors is likely due to excessive fine particles causing the coffee to steep too long or water temperature that's too high. For specific solutions, you can refer to FrontStreet Coffee's previous article "Coffee tastes too bad? Helping you solve various difficult problems."

Dealing with Over-Concentration

In addition, some friends have reported that their brewed coffee, while not having any obvious negative flavors, just doesn't taste very good, with indistinct flavors leaning toward dullness, and a wave of bitterness appearing in the throat when swallowing.

If you've experienced this problem, it's likely because your coffee is brewed too concentrated, possibly due to an unexpectedly high extraction rate that hasn't reached the level of "over-extraction." In this case, we can add 10-20ml of bypass water to the pot to try to save the overly high concentration through dilution.

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