Essential Guide for Coffee Beginners: Understanding Bitterness and Over-Extraction in Pour-Over Brewing

Many friends who are just starting out often face the frustration of over-extraction! What we call "over-extraction" refers to excessive extraction. In a coffee bean, substances that can be dissolved by water account for about 30% of the total. Although this is all that's available, we cannot extract everything.

This is because, besides the pleasant substances that bring us joy, there are also undesirable substances that can cause coffee to exhibit negative characteristics such as bitterness, astringency, and off-flavors. When these substances are extracted in large quantities, the entire cup of coffee becomes unpalatable. When this situation occurs, we call it over-extraction.

If we want to avoid drinking an unbearably bitter, off-flavor-filled over-extracted coffee, it's best to find the right extraction parameters and control the coffee's extraction rate properly. (Under-extraction can also cause astringency, bitterness, and off-flavors, but they're less prominent.) Among all parameters, grind size, water temperature, and time are three key factors that affect extraction. So today, FrontStreet Coffee will share how to adjust and correct these parameters when we encounter over-extraction in our brewed coffee.

Coffee Grind Size

First, we need to question the coffee grind size, because many novice friends often cause coffee over-extraction due to grind issues. After all, grind size doesn't have a very intuitive numerical value like other parameters, which leads to most of the time we can only make a rough judgment with the naked eye, easily causing coffee over-extraction due to misjudgment. (Guess which of the two coffee powders in the picture below has a coarser grind)

(Answer revealed: the right one is coarser! The left has an 85% pass rate through a #20 sieve, while the right has 75%) Taking the above image as an example, when grinding light roast coffee beans, we need a fineness similar to fine sugar particles, with a 75% pass rate through a #20 sieve, but the current setting produces coffee powder with an 85% pass rate. Judging by the naked eye alone, we can hardly discern that this coffee powder is finer than the setting, we just think it's about right, and then start brewing. Since the coffee particles are finer and have more surface area in contact with water, the dissolution rate of substances increases. With other parameters unchanged, this will cause the coffee to extract more substances than we intended, increasing the probability of over-extraction.

So if we're brewing according to a specific recipe but find the final coffee is very bitter and full of off-flavors, we can first adjust the grind to be coarser. By coarsening the coffee particles, we can reduce extraction efficiency and prevent over-extraction. After all, other parameters are visual; when the values of other parameters differ from those in the recipe, we can directly detect and correct them. If everyone wants to know the approximate value of coffee powder ground at the current setting, FrontStreet Coffee recommends purchasing a #20 sieve online, which can give us a general range. For example, the 75% pass rate through a #20 sieve often mentioned by FrontStreet Coffee means that 75% of the ground coffee powder can pass through the sieve.

However, we need to know that the pass rate can only provide a general reference. For example, if 75% passes through the sieve, how fine is this 75% of coffee? Is it uniform? How much is extremely fine powder? These are things it cannot determine. (Related article → How to use a #20 sieve) Besides coffee powder being ground too fine, there's another grinding issue that can lead to easy over-extraction: poor grinding quality that produces too many extremely fine particles.

When the quality of the grinder isn't high enough, not only is the grind size very uneven, but there's also an excessive amount of fine particles. FrontStreet Coffee often mentions that an appropriate amount of fine particles helps enhance complexity, but once there's too much, coffee easily becomes over-extracted! Because substances in fine particles are more easily dissolved, including bitter compounds. Moreover, because fine particles are extremely small, they can easily clog the filter paper during brewing, extending extraction time and causing other normally-sized particles to also be over-extracted.

In such cases, FrontStreet Coffee recommends using a flour sifter to remove some of the fine particles before starting the actual brewing. For specific details, you can refer to FrontStreet Coffee's previous articles, so we won't elaborate further here.

Extraction Time

Next is extraction time, which refers to the time water is in contact with coffee powder to dissolve substances. When we use a longer extraction time during brewing, relatively more substances are dissolved. In other words, when the extraction time is longer, coffee is more likely to become over-extracted. But what constitutes a long time isn't standardized, because whether the extraction time is 2 minutes, 3 minutes, or 4 minutes, we can brew a delicious pot of coffee. It specifically depends on whether the combination of time and other parameters is appropriate, just like in "What Water Temperature Makes the Best Tasting Coffee?" recently shared by FrontStreet Coffee.

Using the same example, when FrontStreet Coffee brews a 15g pot of light roast coffee, the parameters used are: "water temperature 92°C, coffee-to-water ratio 1:15, grind size with 75% pass rate through #20 sieve, combined with three-stage pouring." Under these parameters, 2 to 2.5 minutes is the optimal extraction time. If the time reaches 3 or 4 minutes, then for these parameters, the extraction time is too long, and it's highly likely to extract all the negative characteristics of bitterness, off-flavors, and astringency from the coffee. If the time really reaches 3-4 minutes, then we need to appropriately reduce the extraction time to decrease the dissolved negative substances. There are many factors that affect time, including the fine particle clogging mentioned earlier by FrontStreet Coffee. Another is the use of the dripper! The size of the dripper's drainage holes, the number of flow ribs, and their protrusion degree all directly affect drainage speed. If we want to shorten extraction time and increase drainage speed, we might consider using a fast-flow dripper similar to V60, which has prominent ribs and large drainage holes.

Additionally, time is also controlled by our pouring speed. Without forming clogs, when our pouring speed is higher, the drainage speed is faster, extraction time is shorter, and vice versa. So if we want to shorten extraction time, appropriately increasing pouring speed is also a good method. (Simply put, increase the water flow to pour more hot water in the same amount of time)

Water Temperature

Finally, there's water temperature. The level of water temperature determines the rate at which substances dissolve, but because FrontStreet Coffee has previously shared this in detail in "What Water Temperature Makes the Best Tasting Coffee?", we won't elaborate too much here. But actually, water temperature is the easiest to determine. Generally, when other parameters aren't too extreme, water temperature will be in the 86-93°C range. Using the same parameters as above, with other parameters unchanged, FrontStreet Coffee controls water temperature between 86-88°C when brewing dark roast coffee; while brewing medium-light to light roast coffee, it's controlled between 90-93°C. When we feel the brewed coffee is too bitter, we can reduce extraction efficiency by appropriately lowering the water temperature.

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