Detailed explanation of the grinding rules of espresso powder

Coffee extraction is closely related to coffee flavor, so what is the best grinding method for coffee powder? Let's take a look at the rules of coffee powder grinding coarseness!

Concentrates that are light in color and thin in oil can basically be judged as insufficiently extracted.

The color is very dark and the color of the surface oil is inconsistent, which can be judged as over-extraction.

Spots of grease on the surface can be judged as perforation leading to partial over-extraction

So how finely should the coffee powder be ground? It depends mainly on three factors:

A. Coffee bean grinding method and selected utensils

According to the size, it can be divided into three types: coarse grinding, medium grinding and fine grinding. Different coffee utensils are used for different degrees of grinding coffee powder. The specific usage is as follows:

Fine grinding: espresso machine, moka pot, Turkish pot;

Medium grinding: hand pouring kettle, siphon kettle;

Coarse grind: French press, electric drip kettle.

You can also do a sort:

French press (thickest) > electric drip pot > hand pour pot, siphon pot, smart cup > moka pot > espresso machine > Turkish pot (thinnest)

In addition, the following three points should be noted when grinding coffee beans:

a. Frictional heat should be suppressed to the minimum, because heat will cause the loss of fragrance.

b. Whether the size of the coffee beans is uniform. If the coffee beans are not uniform, the concentration of the brewed coffee will be uneven.

c. Grind just before brewing.

B. Roasting degree of coffee beans

Dark roasted coffee beans have a higher solubility rate, so when we encounter dark roasted coffee beans, we need to use a slightly coarser grind. On the contrary, the lighter the coffee beans are roasted, the finer they should be ground.

C. The flavor of coffee

The flavor of coffee is closely related to the coarseness of the coffee powder. Generally speaking, the coarser the coffee powder, the stronger the acidity; the finer the coffee powder, the more bitter it is. For the same type of coffee beans, if you want a mellower cup of coffee, you might as well grind the powder finer; if you want to drink a fruity flavor, you can grind it coarser.

The above are some common methods to judge whether the espresso is extracted ideally. Insufficient extraction can be easily judged, while ideal extraction is hard to come by. Over-extraction is a problem that occurs in most espressos - due to holes, different thicknesses of powder cakes, and uneven filling pressure, part of the water will be concentrated in a certain position of the powder cake to extract more substances - water is an inert substance and will find a channel from the place with the least resistance. Therefore, excluding the problem of too long extraction time, over-extraction of espresso is caused by the "channel effect".

Channel Effect

When you are extracting espresso, if the flow rate suddenly increases and unevenly colored crema appears, it means that the powder cake is perforated, resulting in a "channel effect". The "channel effect" makes it easier for water to flow through the cracks and loose places in the powder cake, resulting in uneven coffee extraction and a bad cup of espresso. Common reasons are uneven distribution of powder, uneven force when pressing the powder, or too much powder (overfilling).

Now I will tell you a simpler and more direct way to determine whether the espresso you made is over-extracted, and you can also accurately determine from which point of the powder cake the channel is formed to cause over-extraction! No need to smell or drink, just take a look!

Did you see it? The picture shows the coffee bowl after the coffee cake is knocked off immediately after it is made. There is coffee oil remaining on the powder net, which is proof of over-extraction. If your coffee is over-extracted by perforation, the "point" of residual oil is where the water flow forms a channel!

Why is this the case? Let’s look at the previous diagram that explored the principle of flavor release in hand-poured coffee, which is similar to espresso extraction.

In the early stages of extraction, the sour taste is released first, followed by the sweet taste, and finally the bitter taste.

Sweet and sour taste molecules are small molecular weight compounds and are more easily extracted by hot water, while bitter, off-flavor and other unpleasant tastes have relatively large molecular weights and will only be extracted in the middle and late stages. Moreover, due to the large molecular weight of burnt, bitter and off-flavor molecules, the number of them increases rapidly with the amount of extraction. At the end of the extraction, the water flows faster due to the lack of resistance from the coffee contents. Therefore, the flowing coffee liquid suddenly "turns white", and some of the over-extracted coffee liquid will remain in the powder bowl. Therefore, the more coffee liquid is retained and the whiter the color, the more serious the over-extraction of your espresso.

To correct the problem of uneven concentrated extraction, you need to adjust the grinding degree, powder distribution, filling pressure and other aspects. Also, don't ignore the coffee machine. If there is a problem with the brewing head or the water distribution network, the water flow cannot fall evenly, which will also cause over-extraction in the channel. At this time, you should find a professional to perform maintenance.

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