Note that this habit can easily make your coffee less delicious! Why is it easy to block up the water when making coffee by hand? What is the problem with uneven steaming and extraction in making coffee?

A surprise question from Front Street: What would you choose to do when you poured coffee powder into a filter cup and poured the scene shown in the picture below?

A: Flat it B: Flat it C: Choose one or two

I believe that most of my friends, like Qianjie, will choose to flatten this unevenly distributed powder bed by tapping the filter cup. This is not only to make it look more beautiful, but also because people think that flattening the powder bed will make the extraction more uniform. Then it is indeed the case! Compared with an uneven powder bed, a flat powder bed makes it easier for hot water to achieve uniform extraction, making it easier for us to make a cup of delicious coffee.

But! Yes! The way to flatten the powder bed is also very particular. If we don't pay too much attention, the coffee we brewed after we flatten the powder bed may not taste so good! For example, when we tap very frequently, it may have an impact on the extraction due to the emergence of the "Brazil Fruit Effect."

The so-called "Brazil Fruit Effect" means that when we apply external force to a container containing particles of different sizes by shaking or beating under normal conditions, a phenomenon will appear in the container: the larger particles will gradually rise to the surface layer, while the smaller particles in the container will settle to the bottom. Qianjie often shares this effect because this phenomenon is very common in coffee! It is not only suitable for coffee beans, but also suitable for coffee powder!

The size of the coffee powder ground by the bean grinder is not the same, and there are both large and small grains. When such differences exist, it will lead to the Brazil Fruit Effect! When we beat the filter cup, the smaller particles of coffee powder will accumulate at the bottom of the filter cup, and the larger particles of coffee powder will concentrate on the upper layer. This will easily lead to two extraction problems:

1. Uneven extraction When the smaller particles of coffee powder are piled up at the bottom, it will be difficult for the hot water during steaming to uniformly wet all the coffee powder, which will affect the subsequent extraction.

Because the gap between small particles will be narrower, especially after frequent knocking, the "connection" between the particles will be closer, and the flow space of hot water in the coffee powder will be greatly reduced. As we all know, water is inert and will concentrate in places where it flows easily. Therefore, when the powder bed at the bottom is very solid because it is filled with small particles of coffee powder, hot water will preferentially flow to spaces with large gaps and easy flow such as the edge of the filter cup. If enough water is injected, then this effect can be completely ignored. For example, in the main extraction stage of hand-brewed coffee, it is because enough water is injected and there is no need to worry about channel effects during extraction like espresso.

But it's different when steaming, because we don't use much water when steaming (usually twice the amount of water used with powder), so when the powder bed is too firm, a small amount of hot water will be inert. It's difficult to wet all the coffee grounds, because the hot water will run to other places that flow easily after coming into contact with the consolidated powder bed. Qianjie has conducted many tests for this, and then found that when we use conical filter cups that can stack a high powder bed thickness and coffee powder with a high density, beating the filter cup multiple times can easily lead to hot water. The situation where all the coffee powder cannot be wetted. When the steaming time is over, dig up the coffee powder, you can always see that part of the coffee powder at the bottom is still dry.

If this happens, these coffee powders will not be able to fully exhaust air during steaming, and will not be able to release enough flavor substances during the extraction process. The coffee we will eventually brew will not taste so good! Because of the decrease in extraction rate, the flavor is not so prominent and the taste is not so full.

Second, excessive extraction. Secondly, excessive tapping can easily lead to excessive coffee extraction! The reason is also very simple. Qianjie mentioned earlier that the Brazil nut effect will cause small particles of coffee powder to settle downward, and these small particles of coffee powder of course also include the ultra-fine powder of coffee. When we beat these ultra-fine powder. When the powder drops to the bottom, they can easily block the filter holes of the filter paper! The more times/force you tap, the higher the chance of clogging. When the filter holes of the filter paper are blocked, the space for water flow will be further reduced, which will extend the coffee extraction time. Too long extraction time will lead to higher extraction efficiency, and eventually we will easily brew a cup of over-extracted coffee.

The above are the negative situations that are likely to occur after the coffee powder is flattened. I believe there are friends who want to say this at this time. To sum up, when we encounter uneven coffee powder, can we only use other methods to flatten it instead of flattening it?

Of course not! You can also shoot, but it's best not to tap the filter cup too often or too hard, so that too much coffee powder will not migrate, and naturally these negative situations will not occur. Then of course we can choose other methods to smooth it, but the same reason is that we should not go too far ~

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