Beginner's Guide to Pour-Over Extraction Fundamentals: What is the Purpose of Blooming in Pour-Over Coffee?

FrontStreet Coffee often encounters many pour-over beginners asking one question: why do we need to bloom pour-over coffee?

The most indispensable step in pour-over coffee is the blooming process. The purpose of blooming is to allow the coffee to release gases and ensure even extraction, as well as to clearly understand whether the beans being brewed are fresh. FrontStreet Coffee specializes in freshly roasted coffee beans, and some people mistakenly think that fresh coffee beans mean freshly ground coffee, when in fact this concept has been swapped by certain merchants' marketing tactics. However, coffee beans are not necessarily better the fresher they are. If you get freshly roasted coffee beans, you need to let them rest for four or five days so that the brewing can fully extract the coffee flavors. FrontStreet Coffee generally provides coffee beans that are two to three days from the roasting date, unless customers request same-day roasted beans, which will be provided additionally.

Purpose of Blooming

Blooming, often referred to as "Bloom" in English, refers to the preparatory action of evenly moistening the surface of coffee grounds with a small amount of hot water before the official pour-over brewing begins.

FrontStreet Coffee has demonstrated through experiments that without blooming, brewed coffee tends to show under-extraction. This is because when extracting coffee, especially with relatively fresh beans, there are more gases inside the beans, and blooming is mainly for releasing these gases. Without blooming, the gases inside the coffee grounds start releasing in large quantities as soon as they contact water. If brewing directly, because the gases surround the coffee grounds, water has difficulty reaching the coffee grounds and can only extract the surface layer of coffee grounds, continuously washing away the extractable parts, which easily leads to uneven extraction.

Factors Affecting Blooming

The storage time of coffee beans, the degree of coffee bean roasting, water temperature, the amount of water used, the duration of blooming, and the grind size.

Coffee Bean Storage Time and Roasting Degree

Two situations can cause the expansion speed during blooming to be unobvious or even show no reaction: freshness and roasting degree.

First, fresh coffee grounds contain a certain amount of gas. The gas inside the coffee grounds gradually releases over time, and the coffee grounds absorb some moisture from the air, causing the original gas to decrease. Therefore, stale roasted beans are less likely to expand or even show no reaction.

Second, lightly roasted coffee beans are also less likely to expand dramatically because light-roasted beans have a denser texture, so the overall gaps are not as numerous as medium or dark roasts, thus containing less gas.

Blooming Water Temperature

The level of water temperature affects the overall dissolution efficiency of coffee. The higher the temperature, the faster the dissolution rate, and more soluble substances dissolve within the same extraction time. Therefore, generally speaking, if the water temperature is too high, exceeding 95 degrees, it tends to quickly dissolve bitter, astringent, and miscellaneous flavors. The lower the temperature, the slower the dissolution rate, and fewer soluble substances dissolve within the same extraction time. Therefore, generally speaking, if the water temperature is too low, the coffee tends to be weak, and good flavors are not easily extracted, making blooming difficult to complete.

Amount of Pour-over Water

Too little blooming water can lead to incomplete wetting of coffee grounds, resulting in obvious rough astringency in the mouth and a short aftertaste. Too much blooming water causes the hot water in the front blooming section to not extract enough coffee components, resulting in brewed coffee with relatively monotonous flavor in terms of taste, and the acidity from the front extraction will be prominently突出, thus masking other flavors.

Duration of Blooming

The duration of blooming depends on the freshness and roasting degree of the beans. Generally speaking, blooming time is about 30 to 60 seconds. When the expansion state of the coffee ground surface returns to calm, and the surface is about to shrink, it represents that blooming is complete, and you can begin the first water injection.

When FrontStreet Coffee brews beans that have rested for four or five days, the blooming time is generally around 30 seconds.

Pour-over Coffee Grind Size

The coarseness of coffee grinding also affects the size of the "hamburger" shape formed during blooming. When the grind particles are coarser, the contact surface between powder and water is smaller, and the carbon dioxide released by the coffee is less. When the grind particles are too fine, although the contact surface with water increases, because the powder is too fine, water will stay on the surface layer and cannot penetrate, resulting in poor blooming.

Therefore, pour-over requires a grind size suitable for pour-over coarseness to make the blooming "hamburger" shape more obvious. FrontStreet Coffee uses a standard #20 sieve, with a powder passing rate of 80% (for light-roasted beans) and 70-75% (for medium-dark roasted beans) to determine the grind size of each bean.

Pour-over Coffee Blooming Phenomenon: Expanding into Small Hills

When water is poured over pour-over coffee, the gases inside the coffee grounds expand due to heat (if you look at coffee grounds magnified, they look like honeycomb with many gaps). After the gas expands, it begins to release excess gas. At this time, because gas continues to be released, it causes the coffee grounds and hot water to visually form a continuously expanding small hill. At the same time, besides gas releasing from inside the coffee grounds, hot water also slowly penetrates into the interior of the coffee grounds, causing the soluble substances in the coffee grounds to begin dissolving and forming high-concentration coffee liquid.

In the final stage of blooming, because most of the gas has been released and the entire temperature has also decreased, some unreleased air also begins to contract. Therefore, visually, the small hills will go from moist to gradually showing a drying or slightly cracked and collapsed phenomenon, which also represents that blooming is complete and you can begin the second pour-over water injection.

Pour-over Coffee Steps

After discussing blooming, let's talk about the pour-over coffee steps after blooming. FrontStreet Coffee uses three-stage water injection to control the total extraction time to about 2 minutes, divided into blooming, second water injection, and third water injection. The three-stage water injection segmented extraction method can clearly distinguish the front, middle, and back flavor segments of coffee, ensuring the presentation of coffee flavor.

First Water Injection: Blooming (Helps release gas)

FrontStreet Coffee focuses on fresh roasting. Freshly roasted coffee beans accumulate a large amount of gas (mostly carbon dioxide) inside and need to rest for four or five days before brewing.

For blooming time, FrontStreet Coffee determines it to be 30 seconds. When the expansion of the coffee ground surface ends, you will see the coffee surface showing shrinkage, which represents that blooming is complete and you can begin water injection. Basically, for rested beans with a shelf life within one month, FrontStreet Coffee's 30-second blooming is just right to complete gas release.

Second Water Injection

For the second water injection time, FrontStreet Coffee injects water for 30 seconds, at which point the timer shows 1 minute (counting the 30 seconds of blooming). Starting from the center, inject a small water stream to the bottom of the powder layer. To concentrate the penetrating power of the water stream, the range of circular movement should be small, and then move outward in circles.

Third Water Injection

Starting from the third water injection, FrontStreet Coffee recommends observing the degree of water level drop and beginning the third stage water injection just before the powder bed is about to be exposed. Also start from the center and move in circles, with water amount not exceeding the height of the powder layer. Stop water injection at 1 minute 50 seconds, and complete filtration at 2 minutes.

The Impact of Pour-over Coffee Water Injection Segmentation

FrontStreet Coffee also encounters some friends asking why we determine three-stage water injection, and whether there would be a difference in coffee with one more or one less stage. FrontStreet Coffee will conduct an experiment to see the results.

First, FrontStreet Coffee takes the number of water injection stages as a variable and conducts three groups of brewing respectively. The control group uses FrontStreet Coffee's usual extraction mode of two-stage water injection after blooming. Group A uses the extraction method of only one-stage water injection after blooming, while Group B uses the extraction method of three-stage water injection after blooming. After brewing, cupping spoons are used for slurping evaluation respectively.

The variable parameters are as follows:

Group | First Stage (Blooming) | Second Stage | Third Stage | Fourth Stage | Total Water Amount

Experimental Group A | 30ml | 195ml | - | - | 225ml

Control Group | 30ml | 95ml | 100ml | - | 225ml

Experimental Group B | 30ml | 65ml | 65ml | 65ml | 225ml

This experiment uses FrontStreet Coffee's Ethiopia Yirgacheffe Konga Cooperative. Coffee bean information is as follows:

Name | Yirgacheffe Konga

Altitude | 1850 to 2050 meters

Variety | Heirloom

Processing | Natural processing

Extraction Parameters

Powder amount | 15g

Powder to water ratio | 1:15

Water temperature | 91℃

Grind degree | Medium-fine grind (78% pass rate through #20 standard sieve)

Filter cup | Hario V60 #01

Water injection speed | 4g/s

Experimental Results

Group | Total extraction time | Flavor description

Experimental Group A | 1 minute 43 seconds | Overall taste is relatively light, citrus acidity is obvious, fresh floral aroma, high cleanliness, slightly short aftertaste

Control Group | 2 minutes | Citrus, honey flavors, sweet and sour balance, mild, juice-like mouthfeel

Experimental Group B | 2 minutes 10 seconds | Sweetness significantly improved, citrus acidity in the front segment, mandarin orange, brown sugar flavors in the middle segment, tea-like feeling in the tail segment, with astringency, overall high complexity in mouthfeel

Analysis Results

From observing the table, the most intuitive result is that extending extraction time extracts more substances. Increasing the number of water injection stages adds additional interval time between stages, and during this increased interval time, coffee continues to extract, so after increasing the number of stages, the total extraction time also increases, thereby extracting more substances.

We know that time is a relatively important factor affecting coffee extraction and coffee taste. Too long extraction time can easily lead to over-extraction of coffee, making the coffee taste bitter and astringent; too short extraction time means insufficient extraction of coffee flavor substances, making the coffee taste bland and unable to express the coffee flavor characteristics.

From the results obtained in this experiment, I believe many friends will ask, "Is three-stage the best water injection mode?" The answer is not necessarily. This experiment only concluded that the number of stages affects extraction time, but did not prove that three-stage extraction is the optimal water injection mode. Single-stage or multi-stage water injection can also adjust extraction by modifying other factors such as grind size or powder-to-water ratio. For example, Mr. Tetsu Kasuya's previous 46 brewing method uses coarse-ground coffee powder for five-stage water injection; then he also introduced a fine-grind degree, non-blooming single-stage water injection extraction scheme.

For professional coffee knowledge exchange and more coffee bean information, please follow Coffee Workshop (WeChat public account: cafe_style).

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