Why do coffee grounds expand when they come into contact with hot water?

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The first question is, what exactly is this bubble ?

The air inside the bubbles is the carbon dioxide originally trapped in the roasted beans, and the outer membrane of the bubbles is made of proteins and polysaccharides (the same substances that cover the surface of espresso). Some people say that the bubbles produced during dripping are "impurities", but I personally think this is too strict, because whether these bubbles are removed or mixed in the coffee liquid, it will not have a big impact on the taste of the coffee.

When hot water is poured into coffee powder, some bubbles are produced that are extremely fine, some are large, and some seem to produce no bubbles at all. This is because of the way carbon dioxide appears.

Freshly roasted coffee beans contain a lot of carbon dioxide. If you grind them right away, large bubbles will often appear when you add hot water. I personally don't think this is a bad thing, but carbon dioxide will prevent the contact between hot water and coffee powder, affecting extraction. In this way, even if you always make coffee in the same way, sometimes the coffee will become better than usual, so you must be careful. I often hear that coffee beans should be left to stand overnight after roasting, and then used after the flavor stabilizes. I think this is the reason.

Secondly, if the coffee powder does not expand when hot water is poured in, does that mean the coffee beans are not fresh?

The absence of bubbles or the low expansion of the coffee powder occur when the carbon dioxide content is low. There is also a saying that "the coffee does not expand, which means the beans are not fresh". After the roasted coffee beans are left for a period of time, the carbon dioxide content is indeed relatively low, so this statement is not necessarily completely wrong. However, sometimes it is not because of this reason. If a desiccant that absorbs carbon dioxide is added during packaging, or hot water is slowly injected into the coarsely ground coffee powder, or the hot water temperature is low, the same situation will occur.

So there are many reasons why coffee powder expands when hot water is poured in. This requires more practice to accumulate more experience.

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