Coffee Knowledge | Why does coffee powder expand when it comes into contact with water?

Every time you brew coffee by hand, you always encounter a magical thing. After pouring the coffee powder into the filter cup and adding water, the coffee powder will swell up and form the shape of a hamburger. So why does the coffee powder swell up when it comes into contact with water?

During the roasting process, coffee undergoes high-temperature roasting over 160 degrees. The gaps between the internal fibers of the coffee will become looser and more numerous. These fiber gaps often contain a large amount of carbon dioxide and other gases produced by the chemical changes of the coffee beans themselves during roasting. Moreover, these gases are mostly inert gases. Therefore, even if exposed to the air, the gas in the coffee beans will not be released immediately and diluted by the air. Even if they are ground into powder, although the beans will be ground apart during grinding, there will still be gas in some of the fiber gaps.

Therefore, when the water is just started to brew, the gas in the coffee powder expands due to the heat, the coffee powder will absorb water, and the water will gradually fill these gaps. When the fiber gaps in the coffee are truly filled, the gas will expand and begin to expel the excess gas. At this time, because the gas is constantly being discharged, the coffee powder and hot water will form an expanding small hamburger in appearance.

Since the density of water is greater than that of gas, the gas in the powder particle fibers will always be expelled to the outside of the powder particles. However, since water has a certain tension and coffee also releases soluble substances, the surface tension of water is further strengthened. In addition, the powder particles float on the surface, further reducing the distance between water molecules, which in turn increases the surface tension. The gas cannot pass through the surface of the powder layer at once, but after the water is absorbed by the coffee powder, more and more gas is discharged, and the hamburger becomes more and more bloated.

When brewing coffee, it is necessary to steam the coffee. The so-called steaming means pouring hot water lightly, softly, quickly and evenly on the coffee powder when brewing hand-brewed coffee. This will "activate" the coffee powder and release a large amount of carbon dioxide. The coffee powder will form an upward bulge. In other words, steaming is actually a process of allowing coffee powder to emit carbon dioxide.

When water is injected, if the expansion degree is different, the possible factors are the following three:

1. Freshness of coffee beans

2. Roasting degree of coffee beans

3. Grinding of coffee beans

4. The amount of water used to steam coffee beans

The same kind of coffee beans will expand to different degrees depending on the water injection method of point steaming or spiral steaming. If the coffee collapses during the brewing process, there are three main reasons:

1. Over-steaming

2. Too much water injection after steaming

3. The beans are roasted lightly

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