What Causes Coffee Over-Extraction and Under-Extraction: Understanding Coffee Extraction

Defining what makes a cup of coffee taste good may vary according to everyone's taste preferences. Some people enjoy floral notes, others prefer fruity nuances, while some favor dark chocolate and nutty tones... However, when it comes to undesirable flavors, most people can reach a consensus, such as astringent sourness, burnt bitterness, or mixed woody tastes.

In the realm of pour-over specialty coffee, a cup typically exists in one of three states: under-extracted, over-extracted, or ideally extracted.

If you taste a coffee with moderate concentration, full flavor development, and a comfortable mouthfeel that leaves you wanting more, you can recognize that its extraction rate falls within an appropriate range (though not necessarily meeting Golden Cup standards), even without professional equipment for measurement. Conversely, when a coffee clearly exhibits negative flavors with an off-putting mouthfeel, it indicates that the extraction was likely not properly controlled. In such cases, people typically describe it as "over-extracted" or "under-extracted."

When brewing a pot of coffee, numerous factors influence the extraction of flavor compounds, including grind size, water temperature, time, coffee-to-water ratio, and stirring techniques. All methods that increase extraction rate can potentially lead to over-extraction. Similarly, approaches that reduce extraction rate often result in under-extraction. Therefore, the prerequisite for adjusting your brewing method to achieve optimal results is the ability to correctly identify the coffee's extraction state.

The release sequence of flavor compounds during coffee extraction follows: acidity-sweetness-bitterness. When most coffee particles are insufficiently extracted, meaning only small amounts of soluble compounds have been dissolved by water, the initial acidic flavors emerge first. Without the supporting sweetness and rich bitterness, the coffee tastes primarily weak with prominent acidity, resulting in not only a thin mouthfeel but also minimal aftertaste.

Conversely, when certain particles experience excessive extraction, water has drawn out too many soluble compounds from the coffee. These later-released larger molecular compounds are predominantly bitter and undesirable. Coffee in an over-extracted state is described as having an initially difficult-to-swallow burnt bitterness and woody flavors, sometimes accompanied by an astringent sensation on the palate.

There are many causes of under-extraction, typically including overly coarse grind, excessively low water temperature, too high coffee-to-water ratio, gentle pouring technique, and insufficient water-coffee contact time. Conversely, when analyzing the causes of over-extraction across various brewing parameters, they include: water temperature that's too high, grind that's too fine, excessive water volume, extraction time that's too long, and overly vigorous stirring during pouring.

In reality, during daily brewing, we rarely encounter pure under-extraction or over-extraction. With stable parameters, the most common occurrence is uneven extraction—where some coffee particles are over-extracted while others remain under-extracted. This results in flavor characteristics that simultaneously display defects from both conditions, being simultaneously sour and bitter-astringent, making it difficult to distinguish between them.

Judging Extraction States

When determining extraction state, FrontStreet Coffee believes that one cannot rely solely on taste indicators but should also analyze the coffee bean's flavor profile and the entire extraction framework.

For example, when brewing light-roasted, fruit-acidic coffee with an extraction time under 2 minutes, if the coffee tastes weak and acidic, it's clearly in an under-extracted state. We can address this by adjusting to a finer grind and slightly slowing the pouring pace to see if improvement occurs.

When the timing is well-controlled but the flavor feels hollow and insipid—with the coffee being neither bitter nor sour, and the coffee bed mostly compressed at the bottom—it's likely that some larger molecular compounds responsible for richness have also been extensively released, masking the delicate floral and fruity notes. In this case, adjusting the pouring technique to elevate the coffee bed after blooming can allow more space for acidic and sweet compounds to develop.

When brewing dark-roasted beans, if the coffee liquid hasn't finished dripping after 2 minutes and 20 seconds, with an unusual bitterness in the initial taste accompanied by a hint of straw-like woody flavor that lingers at the back of the tongue, the coffee is likely over-extracted. To troubleshoot, consider adjusting to a coarser grind, increasing water flow force, or switching to a faster-draining brewer.

If, regardless of whether brewing light or dark roast beans, the coffee consistently exhibits strong acidic or bitter sensations with a dry aftertaste, and the coffee bed surface appears sticky and muddy with extended dripping time, it's likely over-extraction due to excessive fine particles. We recommend sieving out some of the fine particles to prevent clogging that would cause the coffee to steep for too long.

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FrontStreet Coffee

No. 10, Bao'an Qianjie, Yandun Road, Dongshankou, Yuexiu District, Guangzhou, Guangdong Province

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