The grind size of coffee directly determines the contact area between water and coffee grounds as well as the extraction time, which in turn affects the final taste. But when faced with different roast levels and different brewing devices, how do you determine the right grind size? This article starts with the freshness and resting of coffee beans, systematically explains the actual impact of grind size on flavor, analyzes the manifestations of grinding too fine and too coarse, introduces the cup test correction sieve method recommended by Front Street Coffee, and finally provides specific coarseness references for common devices such as espresso, drip bags, pour-over, and French press, helping coffee enthusiasts find a grinding plan that suits them.

Whether coffee is ground coarse or fine directly changes the contact area between water and coffee as well as the time required for extraction, and is ultimately reflected in the taste in the cup. At the same time, choosing the grind size also troubles many enthusiasts. Today, Front Street will focus on this topic and talk about how the grind size should actually be determined for different coffee beans.

First, one thing must be confirmed: the single-origin beans or espresso blend beans you have must be freshly roasted.

When purchasing, pay attention to whether the packaging has a one-way exhaust valve and whether the roast date is marked. The one-way exhaust valve is usually located at the top of the coffee bag, in the form of a small button-shaped hole. Its function is to allow the carbon dioxide released by the beans to escape while preventing outside air from entering the bag. Every bag of beans produced by FrontStreet Coffee insists on fresh roasting and uses packaging with a one-way exhaust valve.

After receiving the beans, first check the roast date. During roasting, the beans accumulate a large amount of carbon dioxide, so 3 to 7 days need to be set aside for the gas to be released naturally. This process is called resting. If you skip resting and brew directly, the continuously escaping carbon dioxide will interfere with extraction and make the whole process very unstable. So how long is appropriate for resting? Front Street's recommendation is: light-medium roasted beans should rest for 3 to 4 days, and medium-dark roasted beans should rest for 4 to 5 days.

As mentioned earlier, the most intuitive manifestation of whether beans are fresh is the release of carbon dioxide. In addition, freshness is also related to the aromatic substances on which flavor and aroma depend. These small-molecule aromatic substances volatilize much faster than the large-molecule substances that present bitterness. Therefore, the less fresh the beans are, the less carbon dioxide and aromatic substances they have, and the fewer extractable contents they contain. Once the beans have lost freshness, no amount of grind adjustment can help.

How exactly does grind size affect coffee flavor?

1. At the same weight, the finer the grind, the more times the coffee is cut, the larger the total surface area, and the larger the area water can contact.

2. The finer the particles, the shorter the distance for water to travel from the particle surface to the particle center, the faster the internal substances are extracted, and the easier it is for flavor components to dissolve out.

To put it more plainly: when other parameters remain unchanged, the finer the particles, the higher the extraction yield; the finer the grind, the larger the contact area with water, and the higher the dissolved concentration. Conversely, the coarser the grind, the smaller the contact surface area, and the lower the extracted concentration naturally is.

What happens when the grind is too fine?

Visually, the particles are smaller, and the gaps between particles become narrower, making it harder for water to pass through, so the drip filtration time during brewing is prolonged. Fine grounds mean increased surface area, more locations where water can extract substances, and both concentration and extraction yield rise. It tastes strong, with prominent bitterness and astringency, while other flavors are suppressed.

What happens when the grind is too coarse?

Visually, the particles are larger, and the gaps between particles are wider, making it easier for water to pass through the coffee bed, so filtration time is shortened. Coarse grounds mean reduced surface area, fewer locations available for water to extract, and both concentration and extraction yield decrease. It tastes weak, with a light mouthfeel, and the flavor is almost imperceptible.

So how should the grind size be determined?

Front Street recommends using a No. 20 cup test correction sieve (0.85 mm mesh) as an aid. Take 10 g of coffee beans, grind them roughly, pour them into the sieve and sift. After sifting, observe how much grounds fell into the tray, and correct the grind size based on the pass-through rate. FrontStreet Coffee's reference for the coarse-fine standard sieve pass-through rate is as follows: cold brew > pour-over, siphon 80% > moka pot, American drip 75-80% > cupping 70-75% > French press 65-70%.

Of course, different roast levels also correspond to different grind sizes. For example, when Front Street brews light-roasted beans, it uses a grind with an 80-85% pass-through rate on a 0.85 mm sieve; medium-roasted beans use a grind with a 70%-75% pass-through rate; dark-roasted beans use a grind with a 65%-70% pass-through rate. However, everyone has different preferences for strength, so these data are for reference only, and in practice you can still adjust according to your own taste.

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

Front Street also understands that this operation is rather cumbersome, so it has compiled a grind size quick reference to make it easier for everyone to compare with their own brewing tools. Front Street divides coffee grind coarseness into five levels: extra fine, fine, medium-fine, medium, and coarse.

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

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

When making espresso, you need to tamp the coffee grounds with greater force 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 more compact the puck after tamping, and the more balanced the resistance to water flow, so that the extracted espresso has a rich crema.

Fine: used for drip bag coffee.

This coarseness is Front Street's capable partner when making drip bag coffee. The drip bag as a whole is made of filter paper, and water flows out very quickly. Unlike pour-over, it does not have a narrowed opening to limit flow speed (for example, the commonly used three-hole dripper serves to slow the water flow), so drip bags need fine grounds even more to expand the extraction area and dissolve out more substances.

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

When Front Street brews light-medium roasted beans, it uses medium-fine grounds, roughly equivalent to the coarseness of fine granulated sugar, with a sieve pass-through rate between 75%-80%. Light-medium roasted beans usually have rich acidity, and medium-fine grinding can better showcase 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 equivalent to the coarseness of coarse granulated sugar, with a sieve pass-through rate between 70%-75%. Medium grounds are less likely to produce very fine powder, and are also less likely to cause a large amount of water to accumulate in the dripper and soak the coffee grounds because of unstable water flow at the beginning of pouring, thereby avoiding over-extraction.

Coarse: used for making coffee with a French press.

The French press relies on steeping for about 4 minutes to extract the most original taste of the coffee. Compared with pour-over, coffee made with a French press is more turbid because the metal filter mesh has a larger pore size. Therefore, the grind size should be coarse: first, to allow the flavor to be released slowly; second, so that when the filter is pressed down, coffee grounds will not "slip" through the gaps into the coffee and affect the mouthfeel when drinking.

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