Introduction:The flavor expression of pour over coffee largely depends on the balanced release of soluble substances during extraction. When over-extracted, coffee tends to develop unpleasant negative tastes such as bitterness and dryness. This article delves into the typical characteristics of over-extraction, explores the impact of two key factors—water temperature and grind size—from the perspective of the chemical composition of coffee beans, and provides practical parameter suggestions for different roast levels to help you precisely adjust your brewing approach and avoid common extraction pitfalls.

During the pour over brewing process, when hot water comes into contact with coffee grounds, the soluble flavor compounds within gradually dissolve. The types and proportions of these compounds directly determine the final flavor profile of the coffee. Once extraction exceeds the ideal range, the coffee often develops disagreeable bitter or dry tastes, negatively affecting the overall drinking experience.

So-called over-extraction refers to the hot water extracting an excessive amount of soluble components from the coffee grounds, particularly those that contribute negative flavors. In the composition of coffee beans, about 70% consists of woody fiber, while the remaining 30% is soluble substances. Of these, only around 20% contribute positive flavors, whereas approximately 10% are associated with undesirable flavors such as bitterness. Therefore, the ideal extraction yield for a well-balanced cup of coffee typically falls between 18% and 22%.

It’s worth noting that different flavor compounds dissolve into water at varying rates. Generally speaking, pleasant acidic and sweet compounds have smaller molecular structures and are more easily extracted quickly by hot water. In contrast, the molecules responsible for bitterness and off-flavors are larger in size and tend to be released only during the mid-to-late stages of extraction. If only the small molecules from the early stage are extracted, the coffee may exhibit prominent acidity and sweetness but will taste thin, lacking body and balance—this situation is commonly referred to as under-extraction.

Conversely, if the small molecules have already been fully released and the large molecules are excessively dissolved, the originally bright acidity and sweetness will be overshadowed by bitterness, and the mouthfeel may become dry and difficult to swallow. This is the hallmark of over-extraction. To avoid these issues, it’s crucial to control two main factors: water temperature and grind size.

The roast level of the coffee beans also affects how easily flavor compounds are released. Light to medium roasted beans undergo shorter roasting times and have a denser internal structure, making their flavor compounds less soluble. For brewing such beans, it’s recommended to use a medium-fine grind, with particle size similar to that of fine sugar (approximately 75–80% passing through a Chinese Standard No. 20 sieve), and to set the water temperature between 90 and 91 degrees Celsius. This helps achieve a better balance in flavor extraction.

For medium-dark to dark roasted beans, due to longer roasting times and the beans’ more porous internal structure after the second crack, flavor compounds are relatively easier to extract. In this case, a medium-coarse grind is recommended, with particle size close to that of coarse sugar (approximately 65–70% passing through a Chinese Standard No. 20 sieve), and the water temperature should be set at around 88 to 89 degrees Celsius to prevent over-extraction.

In actual brewing, it’s also important to note that factors such as coffee variety and processing methods can influence extraction performance. Some beans may yield a slightly flat flavor even with an extraction yield below 18%, yet still maintain a clear core flavor profile. On the other hand, some beans may have an extraction yield exceeding 22% and taste rich without necessarily exhibiting obvious dryness. In such cases, it’s more appropriate to describe them as “low extraction” or “high extraction” rather than simply labeling them as under-extracted or over-extracted, allowing for more flexible adjustments to the brewing strategy.

The above content is compiled by CoffeeHunters, a coffee news website.