What causes under-extraction and over-extraction in pour-over coffee?

Under-extraction leaves coffee sour, astringent, and thin because water does not pull enough soluble material from the grounds. Over-extraction pushes past the ideal range and brings burnt bitterness and off-flavors. The fix starts with the numbers: aim for concentration between 1.2 and 1.4 TDS and an extraction yield of 18% to 22%.

Adjust one variable at a time: grind size, water temperature, pouring technique, brew time, and dripper material all shift the result. Coffee extracted in small concentrated amounts sits at the under-extracted end, so finer grinding, hotter water, or a slower pour can pull more from the grounds and bring the cup back toward balance.

In the practice of pour-over coffee, under-extraction and over-extraction are two common yet frustrating situations. The former makes coffee sour, astringent, and thin, while the latter brings burnt bitterness and off-flavors. To solve these problems, we first need to understand the principles of extraction—the process in which water passes through the coffee grounds and dissolves out flavor compounds. The ideal "Golden Cup" extraction requires concentration to fall between 1.2 and 1.4 TDS, with an extraction yield maintained at 18% to 22%. Exceeding or falling short of this range corresponds to over-extraction and under-extraction, respectively. This article will analyze the causes one by one from aspects such as coffee bean selection, brewing duration, pouring technique, and dripper material, and provide specific adjustment suggestions to help you achieve balanced flavor more consistently in pour-over brewing.

A few days ago, Front Street Coffee was chatting with friends and the topic of extraction came up. Whenever extraction is mentioned, two states that trouble many coffee enthusiasts cannot be avoided—under-extraction and over-extraction. What are their respective characteristics? And how can we correct them during the pour-over process?

Extraction

Let's first clarify the concept of extraction!

So-called coffee extraction refers to the process in which water flows through the coffee grounds and carries out the soluble flavor substances within them. In this step, there are quite a few variables to pay attention to, including water temperature, grind size, pouring amount, stirring strength, and so on, any one of which will directly affect the final extraction result. We usually divide extraction situations into three categories: under-extraction, Golden Cup extraction, and over-extraction.

Golden Cup Extraction

Golden Cup extraction refers to brewed coffee with a concentration between 1.2 and 1.4 TDS, while the extraction yield reaches the range of 18% to 22%; at this point it is considered to fall within the ideal range. Anything that has not yet reached this range is under-extraction; anything that exceeds this range belongs to over-extraction.

Under-extraction & Over-extraction

Under-extraction means that water does not carry out enough substances from the coffee grounds, and a large amount of soluble material still remains in the grounds. Such coffee has a sharp acidity on the palate, noticeably lacks sweetness, and is mixed with a slight saltiness; the overall mouthfeel is thin, and the finish is short.

The reason is that during extraction, the first substances dissolved out are small-molecule substances such as sour and salty notes, followed by sweetness, and finally bitterness. Because the total amount of dissolved substances is insufficient, it cannot balance the acidity released in the earlier stage, so the acidity of this cup of coffee appears especially sharp and lacks balance.

Over-extraction

The soluble substances in coffee beans account for about 30%, but the part we actually need is at most only 20%; anything beyond this 20% is over-extraction. Because too many substances are dissolved out, the coffee shows obvious burnt bitterness, off-flavors, and astringency.

So, how should we make adjustments?

Many friends experience over-extraction when making espresso, and the root cause is that the coffee beans were not chosen correctly. The darker the roast level, the more caramelization products there are, and the easier it is to bring out bitterness when making espresso. Therefore, Front Street Coffee suggests that if you do not like bitterness, buy medium or light roast beans, with the top choices being Front Street Coffee Fruit Ding Ding Yirgacheffe, Front Street Coffee Butterfly Panama, Front Street Coffee Sidra Colombia, and Front Street Coffee Mozart Costa Rica.

In addition to the factors of the coffee beans themselves, bitterness in coffee is most likely also caused by the following problems:

1. Brewing time is too long

As is well known, the release rates of various substances in coffee under the action of hot water are not consistent. Large-molecule substances, mainly bitterness and astringency, tend to dissolve concentratedly in the final stage. Therefore, once coffee has astringency, it is usually accompanied by an unpleasant bitter note. From the logic of extraction, the longer the steeping time, the stronger the taste, and the later the stage, the easier it is for bitter and astringent components to be extracted.

So to achieve reasonable extraction in coffee, brewing duration must be controlled well. Taking a single serving of 15–16 g of coffee grounds as an example, Front Street Coffee suggests the total duration should not exceed 2 minutes 30 seconds. If the actual brewing time exceeds two and a half minutes and you do indeed taste bitterness and astringency, then the parameters need to be adjusted according to the specific situation.

For example, if after brewing a large amount of wet, sticky muddy grounds remains on the surface of the coffee bed, it indicates that too many fines caused water retention, and at this point the grind size should be adjusted coarser;

If most of the coffee grounds sink to the bottom and the surface looks very flat, then it is likely that the pouring force was too small, the coffee bed could not be lifted, and the water was forced to pass slowly through the thick layer of grounds, thereby prolonging the brewing time. The solution is to appropriately increase the flow after blooming ends, so that the water level rises as much as possible to two-thirds of the height of the dripper.

2. The water flow scouring force is too strong

According to Front Street Coffee's observations, there are mainly two situations in which pouring causes astringency: first, unstable water flow causes uneven extraction of particles, making the coffee show both acidity and astringency at the same time; second, friends who are used to using a strong flow to agitate the coffee bed can easily wash out off-flavors and astringency during the later stage of extraction.

Unlike other parameters, pouring is a practical skill that requires repeated practice to master. To achieve even brewing, Front Street Coffee suggests using a vertical stream of water, keeping a distance of 3–4 cm from the coffee bed, and then pouring slowly in circles.

3. The dripper's extraction efficiency is too high

For many beginners, dripper material is one of the most easily overlooked causes of over-extraction. For example, ceramic drippers have a relatively high specific heat capacity. Preheating them in advance can maintain good heat retention throughout the pouring process, but at the same time it also "quietly" improves the coffee's extraction efficiency, thereby increasing the probability that astringency will be released in the final stage.

Therefore, when we taste astringency in coffee, and the dripper being used happens to be made of ceramic, and the first two causes have already been ruled out, then we may as well start with temperature and try lowering the brewing water temperature by 2–3°C to reduce the risk of over-extraction.

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