Reasons for Pour-Over Coffee Blooming Failure: Guide to Bloom Water Amount, Time Parameters, and Methods

Understanding Coffee Blooming: The Science Behind Freshness and Expansion

A few days ago, when FrontStreet Coffee was discussing coffee degassing, we briefly mentioned that "carbon dioxide within coffee beans 'inhibits' the release of volatile flavor compounds." This is why there's a "blooming" step during brewing—by adding a small amount of water to wet the coffee grounds, allowing them to quickly release gases. During this process, the coffee grounds expand, forming a dome-like shape.

If the gases in the coffee beans have completely escaped, then during blooming, the coffee grounds have no gas to release, so they won't expand. Therefore, it's generally believed that if coffee doesn't bloom and expand, it indicates the beans are no longer fresh and have lost their flavor. Regarding this issue, FrontStreet Coffee believes that coffee expansion can serve as a reference for bean freshness, but one shouldn't rely too heavily on this method, as there are other reasons why coffee grounds might not expand.

Roast Degree

Have you noticed that those impressive "hamburger-shaped" domes typically come from darker roasted coffee beans? This is because the darker the roast, the larger the coffee bean volume and the more porous its internal structure. On one hand, there's more carbon dioxide, and on the other, the porous structure expands more easily when exposed to water. This is why we can see beautiful, perfect "hamburger" domes when brewing dark roasted coffee.

Conversely, the lighter the roast, the harder the coffee bean texture, and there's less carbon dioxide compared to dark roasted beans. Therefore, the "hamburger" dome during blooming will be smaller, sometimes showing no obvious expansion characteristics.

For example, high-altitude Geisha coffee beans, when brewed two weeks after roasting, show no significant expansion in the coffee grounds layer. This is because light-roasted Geisha beans are quite hard and release gas slowly, and because they're lightly roasted, there isn't excessive gas. During the lengthy 30-second blooming period, sometimes you might only see a slightly swollen small dome, sometimes no expansion at all, and sometimes a large bubble emerges from the flat grounds surface. This doesn't indicate staleness and doesn't affect the flavor of Geisha coffee.

Another example: once, a friend brought a bag of extremely light-roasted coffee beans to share with FrontStreet Coffee. When grinding, the sound from the grinder suggested these beans were very hard. Similarly, the grounds didn't expand during blooming, yet the brewed coffee had very bright and pleasant acidity, without any staleness-related flavors.

Grind Size

The size of the coffee dome is also related to grind size. If the grind is quite fine, gases can't easily find gaps to "escape" through, so the gas pushes up the coffee grounds to find larger spaces to escape. If there's continuous, rapid gas release, large bubbles may appear.

Conversely, if the grind is coarse enough, there's sufficient space between particles for gas to escape, so the grounds layer won't expand.

Therefore, if you want to see beautiful blooming "hamburgers," choose dark roasted coffee beans and grind them fresh before brewing.

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473