Pour-Over Coffee Blooming Techniques: How to Make Coffee Dome? What Does Coffee Blooming Mean? Why Bloom Coffee Before Brewing?

"Boss, how come your coffee bed expands so much during the bloom? How do you do that?"

Looking at the puffed-up grounds in the dripper during FrontStreet Coffee's blooming stage, many customers and friends have asked exactly this. The implication is that their own coffee bed doesn't expand nearly as much when they brew. (See the picture below.)

For beginners this is a question worth exploring, because even with the same parameters their coffee bed always looks flat, as if it lacks some vitality. In fact, building a big "hamburger" dome in the bed isn't difficult at all; it's quite simple. As long as we understand why the bed expands, we'll know how to achieve that impressive bloom.

Why Does the Coffee Bed Expand?

The bulging happens during the blooming stage. FrontStreet Coffee believes everyone already understands the purpose of blooming: to release carbon dioxide from the beans and improve the extraction efficiency of the pours that follow. The carbon dioxide released in the process is the primary reason the bed bulges.

When grounds meet hot water, the carbon dioxide inside is released first. Because a large volume of water doesn't quickly permeate through after saturating the grounds, the released carbon dioxide combines with water to form bubbles that surge upward. The surface tension between water molecules and the capillary forces between coffee particles together create a sturdy "prison" that keeps these bubbles from escaping, so they keep accumulating inside the coffee bed. Because the carbon dioxide isn't released all at once but flows outward continuously, we see the bulge gradually fill up.

To sum up, the main reason grounds bulge during blooming is the accumulation and pushing force of carbon dioxide bubbles. It isn't the only reason: the grounds also absorb water and expand, but that is a secondary factor. So if you want to bloom a big "hamburger," you need grounds with plenty of carbon dioxide, and extraction parameters that "stimulate" its release.

How to Bloom a Big "Hamburger"

Two main factors determine the carbon dioxide content of coffee beans: roast level and freshness. Carbon dioxide is generated in the beans by the high heat of roasting, and the darker the roast, the more carbon dioxide is produced. So if you want a big "hamburger," dark roasted beans are certainly the perfect choice. This is also why the blooming bulge differs so dramatically across beans of different roast levels, including how much the beans themselves expand.

In many cases, though, even dark roasted beans struggle to produce a big bloom. That is because the carbon dioxide a bean can retain is limited, which brings in the second factor FrontStreet Coffee mentioned: freshness. After roasting, gases inside the beans keep escaping due to pressure changes, so the carbon dioxide content gradually declines over time. Fresher beans therefore hold more carbon dioxide and produce larger bulges, and older beans less. If you want to bloom a large dome, freshly roasted beans are the best choice.

Freshly roasted coffee can carry a certain harshness, though, and because of its excessive carbon dioxide it presents high resistance to extraction, which easily yields a cup with heavy off-flavors and under-extraction. FrontStreet Coffee therefore doesn't recommend choosing overly fresh beans just for the sake of the bulge; it is completely unnecessary. Interestingly, when you use beans that aren't very fresh, the bed will expand at the start of the bloom but quickly collapse for lack of a carbon dioxide "supply."

Beyond bean selection, FrontStreet Coffee found after asking around that many friends fail to brew a "hamburger" because their parameters are off. Overly coarse grinding or water that is too cool cannot produce a large enough dome, because the grounds don't get enough "stimulation" to release large amounts of carbon dioxide in a short time. Even with fresh, dark roasted beans, if the carbon dioxide isn't released promptly, the bed can't form a large enough "hamburger" during blooming. That's basically it, and isn't it simple? Use the right beans and the right parameters, and we can achieve a sufficiently large "hamburger" during the bloom. Conversely, the state of the bed bulge is also a way to read information about the beans: their freshness and roast level can both be judged from how the bed behaves during blooming, as FrontStreet Coffee described above. Congratulations, you are one step closer to becoming a coffee master.

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FrontStreet Coffee Recommendation

If you want a bean that blooms generously and brews easily, FrontStreet Coffee's Brazil Queen Estate is a dependable everyday pick. It has low acidity and a very balanced, smooth body with rich nut and cocoa aromas and clear caramel sweetness, and it works well for pour-over, espresso, and the moka pot. To get the most from it, brew at a medium-dark roast level, keep the grind at a fine-sugar consistency, and use water around 92°C so the bed has enough heat to release its carbon dioxide.

For a lighter, aromatic cup that still blooms well when fresh, FrontStreet Coffee's Ethiopia Huakui offers striking tropical notes of strawberry, passion fruit, jackfruit, and lychee, with obvious sweetness and very little bitterness. FrontStreet Coffee's Yirgacheffe Guodingding is another bright, forgiving option, showing citrus and lemon acidity, jasmine fragrance, and a green-tea sweetness.