What is Coffee Extraction? How to Identify Under-extracted and Over-extracted Flavors in Pour-over Coffee

Under-extraction and Over-extraction: Common Misconceptions

"Under-extraction" and "over-extraction" are the two most commonly misunderstood brothers. Many people think that coffee tasting watery means "under-extraction," while coffee tasting very bitter means "over-extraction." In reality, these two brothers are not as simple as people think...

Q1: What is "coffee extraction"?

Coffee extraction, simply put, is the process where water, as a solvent, combines with soluble substances as it passes through coffee particles, ultimately resulting in flavorful coffee liquid.

Within a coffee bean, only 30% consists of soluble substances, while the remaining 70% is woody fiber. Among the 30% soluble substances, only about 20% are desirable flavor compounds, with the remaining approximately 10% being bitter/negative flavor compounds. Since each substance dissolves at different times and speeds, generally positive flavors are dissolved first and most easily, while negative flavor compounds require longer time to be dissolved.

Therefore, while 30% is the maximum extraction rate for coffee, the extraction rate suitable for public taste is actually between 18%-22%. This is also considered the optimal coffee extraction rate according to the Golden Cup extraction theory from SCA.

Q2: How to calculate coffee extraction rate?

When calculating extraction rate, you need to use a coffee refractometer (TDS detector) for measurement. A coffee refractometer can quickly detect the proportion of extracted coffee substances in the total coffee liquid, where the main subject of concentration is the coffee liquid. (If you want to calculate the concentration yourself, the formula is: weight of extracted coffee substances ÷ weight of coffee liquid. Too complicated! Don't bother...)

The extraction rate is the proportion of extracted coffee substances to the total coffee bean weight, where the subject of extraction rate is the coffee beans/ground coffee. Therefore, the calculation formula is: coffee liquid concentration × coffee liquid weight ÷ coffee bean (ground coffee) weight.

With a refractometer, we can very conveniently calculate the coffee extraction rate. If you're calculating the extraction rate for pour-over coffee, you need to note that the blooming stage absorbs twice the weight of the coffee grounds in water. Therefore, the coffee liquid weight calculation formula is: water amount - coffee bean (ground coffee) weight × 2.

Didn't understand? Let's use an example: If you use 15 grams of coffee grounds with a 1:16 coffee-to-water ratio for drip extraction, and the extracted coffee liquid measures 1.37% concentration through the refractometer, then the coffee extraction rate is calculated as follows: 1.37% × [(15×16) - (15×2)] ÷ 15 = 19.18%

Q3: What is "under-extraction"? What are its characteristics?

If we use the Golden Cup extraction theory to define it, extraction rate below 18% = under-extraction. FrontStreet Coffee also knows that doing math problems just to drink a cup of coffee is very painful, so FrontStreet Coffee will now explain what flavor characteristics coffee has when under-extracted~

Sharp Acidity & Lackluster Flavor

The most obvious characteristic of under-extracted coffee is prominent acidity. The order of flavor substance release during coffee extraction is: acid - sweet - bitter. In under-extracted coffee, the early acidic substances are basically extracted, but sweetness and bitterness are insufficient to reduce the expression of acidity and enhance the coffee's body, making the coffee taste very sharp and bland with a short aftertaste.

Salty Taste

Under normal circumstances, salty taste rarely appears in coffee flavor because it's masked by sweet and sour tastes. Saltiness comes from minerals and inorganic compounds in coffee beans. Sweetness and saltiness enhance each other, so in cases of under-extraction where sweetness is insufficiently extracted, we might be able to detect the saltiness in coffee.

Q4: What is "over-extraction"? What are its characteristics?

If we use the Golden Cup extraction theory to define it, extraction rate above 22% = over-extraction. Again, calculations are painful! Let's now look at what flavor characteristics coffee has when over-extracted~

Bitterness

The bitterness mentioned here is very unpleasant/unusual bitterness, and bitterness alone doesn't represent coffee over-extraction. First, everyone's perception of bitterness varies, while the bitterness from over-extraction is unacceptable to most people, such as burnt bitterness.

Astringency and Dryness

When over-extracted coffee enters the mouth, not only will you experience unusual bitterness, but your tongue will also feel astringent, meaning over-extracted coffee exhibits very strong bitter and astringent sensations in the mouth. After swallowing, our throat will immediately feel dry, or as if there's a lump of smoke stuck in the throat, persisting for a relatively long time.

Q5: How should we view "under-extraction" and "over-extraction"?

"Under-extraction" and "over-extraction" are actually conclusions derived from the SCA Golden Cup standards, which are proposed based on American taste trends and scientific research. When a coffee's extraction rate is below 18% or exceeds 22%, it doesn't necessarily mean the coffee is unpleasant to drink.

FrontStreet Coffee has found in actual brewing that, depending on the coffee bean's own variety/processing method/roast level, etc., some coffees with extraction rates below 18%, while somewhat bland, exhibit very clear flavors; some coffees with extraction rates above 22%, while strong in taste, don't necessarily show astringent sensations. In these cases, FrontStreet Coffee prefers to describe them as "low extraction" or "high extraction."

Important Notice :

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