What are the signs of under-extracted coffee? What causes coffee to taste sharp and sour? Why is my pour-over coffee too acidic?

Introduction

In the previous article, FrontStreet Coffee elaborated on "how over-extracted coffee performs in flavor," and this installment of the Barista Training Series continues to discuss how under-extracted coffee performs!

What is Under-Extraction?

In the SCA Golden Cup extraction theory, it's believed that 18-22% of the water-soluble substances in coffee contribute to good flavor. Therefore, when brewing coffee, ensuring the extraction rate falls within the 18-22% range provides relatively guaranteed flavor quality. Under-extraction, then, occurs when the extraction rate doesn't reach the lower limit of 18%.

What are the Characteristics of Under-Extraction?

In sensory terms, the characteristics of under-extracted coffee are divided into the following situations:

① Sharp Acidity

The most obvious characteristic of under-extracted coffee is prominent sharp acidity. This is because during normal extraction, acidic compounds are released first, followed by sweet compounds, and finally bitter compounds. Therefore, in under-extracted coffee, the early-stage acidic compounds are basically extracted, while the sweet and bitter compounds are not. This results in an overall extraction rate that is slightly low, and without the balancing effect of sweetness and bitterness, the coffee exhibits sharp, stimulating acidity.

② Salty Sensation

The saltiness in coffee comes from minerals and inorganic compounds in the coffee beans. In normal brewing, these salty substances are also extracted into the coffee liquid, but we don't perceive saltiness in the coffee because sweetness masks the saltiness (salt and sweet enhance each other). When sweetness is insufficient (under-extraction) and acidity is too strong, the salty sensation becomes prominent.

③ Empty Aftertaste

Another characteristic of under-extracted coffee is a short aftertaste, which makes the entire coffee sensory experience incomplete.

How to Avoid Under-Extraction?

① High Concentration, Insufficient Extraction Rate (Rich, Acidic, and Salty)

This situation is likely caused by too small a coffee-to-water ratio, meaning using relatively little water to brew a large amount of coffee grounds. This leads to insufficient overall extraction efficiency by the water, resulting in only extracting the first half of the coffee's flavor compounds, while the latter half remains in the coffee grounds. Additionally, because the coffee-to-water ratio is too small, it creates a flavor that is both strong and acidic.

To address this situation, appropriately increasing the coffee-to-water ratio during brewing can effectively solve the problem. FrontStreet Coffee generally recommends using a coffee-to-water ratio of 1:15 or 1:16.

② Low Concentration, Insufficient Extraction Rate (Weak and Sharp Acidity)

This situation involves more factors. First, you should check whether your water temperature, coffee grind size, and coffee-to-water ratio are reasonable. For example, too low water temperature, too coarse grind, etc., can make a coffee weak and flavorless, or produce irritating acidity (low water temperature reduces extraction efficiency, while too coarse coffee grounds lead to short contact time between water and coffee grounds).

Next, observe the condition of the coffee bed in the filter to see if channeling has formed - that is, if water breaks through the coffee bed wall or flows over the coffee bed during pouring, causing water to flow from the side of the filter and resulting in under-extraction.

Final Thoughts

Finally, it's important to note that coffee development is a dynamic process, during which new perspectives will replace old ones. When defining whether coffee is under-extracted, we cannot completely rely on the 18% figure as the only indicator. We use Golden Cup knowledge as the basis for brewing coffee, but the most important point is to brew a good cup of coffee. Today, some brewing schools only extract the front and middle flavor compounds from coffee, then dilute to an appropriate concentration by adding water. Although this method results in an extraction rate below 18% when converted, it doesn't produce the flavor characteristics typically associated with under-extraction.

Therefore, FrontStreet Coffee believes that the term "under-extraction" places more emphasis on describing phenomena such as sharp acidity, saltiness, and empty aftertaste.

Important Notice :

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FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473