Coffee powder expands when it comes into hot water. Do you know why?

The first question is, what is this bubble?

The air inside the bubble is carbon dioxide originally trapped in the roasted beans, and the outer membrane of the bubble is formed by proteins and polysaccharides (the same substances that form the surface of espresso coffee). Some people call these bubbles "impurities", but personally think this is too strict, because whether you remove these bubbles or let them mix in the coffee solution, it will not have a great impact on the taste of the coffee.

Hot water injected into the coffee powder produces bubbles that are some extremely fine, some very large, and some that do not seem to produce bubbles because of the way carbon dioxide appears acquired.

Freshly roasted coffee beans contain a lot of carbon dioxide. If ground immediately, they often emit large bubbles when injected with hot water. Personally, this is not a bad thing, but carbon dioxide will hinder the contact between hot water and coffee powder, affecting extraction. So even if you always brew coffee the same way, sometimes coffee will become more than usual, you have to be careful, usually listen to coffee beans roast well, let it sit overnight, wait for the taste to stabilize before using, I think this is the reason.

Second, when hot water is added, does the coffee powder not expand, that means the coffee beans are not fresh?

There are no bubbles or low expansion of the coffee powder, which occurs when the carbon dioxide content is low. There is also a saying that "coffee does not swell means that beans are not fresh." Roasted coffee beans after a period of time, the content of carbon dioxide is indeed relatively small, so wait. The carbon dioxide content is indeed reduced, so this statement is not necessarily wrong, but sometimes it is not because of this reason. The same situation will occur if the package is packed with a drying agent that absorbs carbon dioxide, or if hot water is slowly injected into the coarse ground coffee powder, or if the hot water temperature is too low.

So there are many reasons why coffee powder swells when hot water is injected, and this requires more practice and more experience.