Why Bubbles Appear in Pour-Over Coffee: Is the Blooming Step Important?

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Introduction

Watching hot water slowly pour into the coffee powder layer, which gradually forms a "hamburger-shaped" bulge, the blooming process is always so healing. What role does the "hamburger-shaped" bloom play in brewing? Does the amount of water during blooming affect coffee extraction? This article, FrontStreet Coffee will tell you through experiments.

Is the purpose of blooming merely to release carbon dioxide from the coffee powder?

Freshly roasted coffee beans continuously release carbon dioxide for several weeks after roasting. When fresh coffee beans are ground into powder, carbon dioxide emission accelerates. Therefore, when we pour water into the coffee powder layer, gas creates numerous bubbles in the water, significantly increasing the surface area of the coffee powder.

Without blooming, the foam produced would block the spaces between coffee particles, altering water flow channels and easily causing uneven extraction. Meanwhile, carbon dioxide would prevent contact between water and coffee flavor compounds. If blooming is not performed to eliminate carbon dioxide, the subsequent extraction stage would see carbon dioxide release affecting the release of coffee aromatic compounds and oils.

Does the amount of water during blooming affect coffee extraction?

Since blooming releases carbon dioxide while also participating in the extraction of coffee flavors, does the amount of water during blooming really affect the entire coffee extraction process? This time, FrontStreet Coffee will conduct experiments using a Clever Dripper, using immersion methods to reduce experimental errors caused by brewing techniques. We'll test using equal amounts of water to coffee powder, 2 times the water amount, and 4 times the water amount for 30 seconds of blooming (with piston open), then close the piston, add water to 225g, steep for 2 minutes and 30 seconds, then open the piston again, allowing the extracted coffee liquid to blend evenly with the bloom extraction.

This experiment uses Costa Rica Musician Series Mozart coffee beans. Brewing parameters: 15g coffee powder, water-to-coffee ratio 1:15, grind size 0.85mm with 75% pass-through rate on sieve, water temperature 90°C. (PS: Remember to preheat all equipment before brewing~)

Group A: 15g water bloom for 30 seconds

Coffee flavors: fermented aroma, grape acidity, overall bland, insufficient aroma, poor clarity, woody taste after cooling.

Group B: 30g water bloom for 30 seconds

Coffee flavors: raisin fermentation, berry-like sweet and sour, orange blossom aroma, pineapple-like sweet and sour aftertaste, prominent aroma, smooth mouthfeel, overall balanced.

Group C: 60g water bloom for 30 seconds

Coffee flavors: slight fermented aroma and grape acidity, insufficient aroma, average clarity, bland flavors.

After comparing the three groups, FrontStreet Coffee believes that Group A's bloom water amount was too small, causing water to not make uniform contact with the coffee powder. Some coffee particles were in degassing and extraction state, while others remained dry and unable to degas, resulting in poor overall clarity, insufficient flavor, and woody tastes. In Group C, the bloom water amount was excessive, causing the coffee powder layer to collapse. Most water merely passed through the coffee powder layer without achieving extraction effects, thus diluting the concentration of the entire cup of coffee, making it appear bland overall.

Summary

From the experiment, it can be seen that using twice the amount of coffee powder for blooming is most appropriate. At this point, some friends might ask: "If I exceed 30g by a little bit when blooming 15g of coffee powder, will it have an impact?"

Generally, as long as the bloom water amount can evenly moisten the coffee powder without large amounts of coffee liquid dripping, it's fine. Due to factors like the degree of roasting, freshness, and coffee bean variety, the water absorption of coffee powder will vary. Of course, this "little bit" shouldn't be excessive. When blooming 15g of coffee powder, using 30-40g of water is still acceptable.

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