Why does coffee grind size change the flavor in the cup?

Grind size sets the contact area between water and coffee particles and the time extraction takes, so it directly shapes how a cup tastes. Finer grounds extract faster and can turn bitter or harsh; coarser grounds extract slower and can taste weak or sour. Grind is the variable many home brewers leave unadjusted, even when everything else is dialed in.

Fresh roasting and resting come first: beans release carbon dioxide for 3 to 7 days, and brewing too soon makes extraction unstable. Rest medium-light roasts 3 to 4 days and medium-dark roasts longer. From there, calibrate pass rate with a 0.85mm cupping sieve, using separate reference ranges for light, medium, and dark roasts, or judge coarseness by brewing device if no sieve is on hand.

How coarse or fine you grind coffee directly affects the contact area between water and coffee particles and the extraction time, and it is also a key variable that determines whether a cup of coffee tastes good. But grind size is exactly the part many enthusiasts are least sure about. Starting from Front Street Coffee's practical experience, this article first explains the prerequisite effects of fresh roasting and bean resting on grinding results, then analyzes the different flavor tendencies of fine and coarse grounds, afterward gives a specific method for calibrating the pass rate with a 0.85mm cupping sieve, and lists reference ranges for light, medium, and dark roasted beans, with a practical summary at the end for judging coarseness by brewing device if you do not buy a sieve.

Whether coffee particles are ground coarse or fine changes the contact area between water and coffee grounds, and also changes the time spent on extraction, all of which is ultimately reflected in the taste in the cup. Yet grind size is precisely the thing that leaves many enthusiasts unsure where to start. Next, Front Street will explain how grind coarseness should actually be set for different coffee beans.

The first step is to confirm that the single-origin beans or espresso blend beans you have are freshly roasted.

When buying single-origin coffee beans or beans for espresso, pay attention to whether the packaging has a one-way exhaust valve and whether the roast date is marked. The so-called one-way exhaust valve is that small button-like hole at the top of the coffee bag. Its function is to let the carbon dioxide released by the beans escape while keeping outside air out of the packaging. Every bag of coffee beans produced by Front Street Coffee (FrontStreet Coffee) is freshly roasted and comes in packaging with a one-way exhaust valve.

After receiving the beans, you can first check the roast date. During roasting, coffee beans accumulate a large amount of carbon dioxide inside, so they need 3 to 7 days to release gas on their own, which is commonly called bean resting. If freshly roasted beans are taken straight to brewing, the continuously released carbon dioxide will interfere with the entire extraction process and make the results very unstable. As for how long to rest them, Front Street's suggestion is: rest medium-light roasted beans for 3 to 4 days, and medium-dark roasted beans for 4 to 5 days.

As mentioned earlier, the most intuitive sign of whether beans are fresh is the release of carbon dioxide. In addition, freshness also determines the flavor and aroma of the coffee. The aromatic substances that make up aroma are small molecules and are more volatile than the large molecules that bring bitterness. Therefore, the longer the beans are stored, the less carbon dioxide and aromatic substances remain, and naturally less can be extracted. If the beans themselves are already stale, no amount of care with grinding can save them.

So how does grind size influence flavor?

1. With the same number of grams, the finer the grind, the more cuts are needed, the larger the surface area becomes, and the larger the area of coffee grounds that water can contact.

2. The finer the grind, the smaller the particles are cut, the shorter the distance water must travel from the surface of the particle to its center, and the shorter the time water needs to enter the particle and extract flavor substances, or in other words, the easier it is to extract flavor.

To put it plainly, under otherwise identical conditions, the finer the particles, the higher the extraction yield. When the grind is fine, the area exposed to water is large, so the dissolved concentration is high; conversely, when the grind is coarse, the surface area exposed to water is small, so the extracted concentration is low.

What signs does over-fine grinding produce?

In appearance, the particles are smaller and the gaps between them are narrower, so water does not pass through easily, and the drip time during brewing is prolonged. Fine grounds mean a large particle surface area, more places where water can extract substances, and both concentration and extraction yield rise. It tastes intense, with prominent bitterness and astringency, while other flavors are suppressed.

What happens when the grind is too coarse?

In appearance, the particles are larger and the gaps between them are wider, so water passes through more easily, and the filtration time during brewing is correspondingly shortened. Coarse grounds mean a small particle surface area, fewer places where water can extract substances, and both concentration and extraction yield decline. It tastes weak and bland, with a thin mouthfeel and almost no perceptible flavor.

So how exactly should grind size be determined?

Front Street recommends using cupping calibration sieve No. 20 (0.85mm mesh) to judge. Take 10g of coffee beans, grind them roughly, pour them into the sieve and sift. After sifting, see how much ground coffee has fallen into the tray and use the pass rate to calibrate the grind size. Front Street Coffee's (FrontStreet Coffee) standard grind-size sieve pass rates are: cold drip > pour-over, siphon 80% > moka pot, American drip 75% to 80% > cupping 70% to 75% > French press 65% to 70%.

Of course, beans with different roast levels also correspond to different grind sizes. For example, when Front Street brews light roasted beans, it uses a grind with a 0.85mm sieve pass rate of 80% to 85%; for medium roasted beans, it uses a grind with a pass rate of 70% to 75%; and for dark roasted beans, it uses a grind with a pass rate of 65% to 70%. However, everyone has different preferences for strength, so these numbers are only for reference and can absolutely be adjusted to your own taste.

What if you do not want to buy a calibration sieve?

Front Street also knows this whole process is quite troublesome, so it made a brief summary of the grind sizes corresponding to common brewing devices. Front Street divides coffee ground coarseness into five types: extra-fine, fine, medium-fine, medium, and coarse.

(From left to right: coarse, fine, extra-fine)

Extra-fine: used for espresso-style coffee (moka pot, espresso machine).

When making espresso, you must tamp the coffee grounds firmly so that they create resistance to the high-pressure hot water and the essence can truly be extracted. The more uniform and delicate the grind, the denser the puck after tamping, and the more even the resistance to water flow, so that the espresso "pressed" out this way has a rich crema.

Fine: used for drip bag coffee.

This coarseness is Front Street's "soul mate" for making drip bag coffee. The main reason is that drip bags are entirely filter paper and water flows through very quickly, unlike pour-over, which has a restricted opening - for example, the commonly used three-hole dripper actually serves to slow the water flow. Therefore, drip bags need fine grounds even more to expand the extraction area and extract more substances.

Medium-fine: used for brewing medium-light roasted coffee (various pour-over drippers).

When Front Street brews medium-light roasted beans, it uses medium-fine grounds, roughly the size of fine granulated sugar, with a sieve pass rate between 75% and 80%. Medium-light roasted beans usually have rich acidity, and a medium-fine grind can enhance the overall sweet-and-sour performance.

Medium: used for brewing medium-dark roasted coffee (various pour-over drippers).

When Front Street brews medium-dark roasted beans, it uses medium grounds, roughly the size of coarse granulated sugar, with a sieve pass rate between 70% and 75%. Medium grounds are less likely to produce extra-fine powder, and are also less likely to cause a large amount of water to sit in the dripper and soak the coffee grounds because the flow is unstable at the beginning of pouring, which would cause over-extraction.

Coarse: used for making coffee with a French press.

A French press relies on a 4-minute steep to extract coffee's most original taste. Compared with pour-over, its coffee liquid is relatively cloudy because the metal filter mesh has larger holes. Therefore, a coarse grind should be used: first, so the flavor is released slowly, and second, so that when the filter is pressed down, coffee grounds do not "slip" through the gaps in the mesh into the coffee and affect the mouthfeel.

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