What does the bloom actually do in pour-over coffee?

The bloom releases gas trapped inside the coffee grounds before the main pour, and it sets up even extraction. Skip it, especially with relatively fresh beans, and the cup tends toward under-extraction. The stage also doubles as a freshness check: the way the bed swells and settles tells you how recently the coffee was roasted.

Freshly roasted is not the same as freshly ground, and beans are not better the sooner they are brewed. Coffee usually needs four or five days of resting after roasting before extraction shows full flavor. Roast level, storage time, water temperature, water volume, bloom duration and grind size all shift the result, and comparative tests show how the number of pouring stages changes both extraction time and flavor.

The blooming stage in pour-over coffee may look like simply wetting the coffee grounds with hot water, but it actually determines whether the whole cup can be extracted evenly. Drawing on the hands-on experience of Front Street Coffee, this article systematically sorts out the core purposes of blooming—releasing gases inside the coffee beans, ensuring even extraction, and judging bean freshness—and analyzes one by one how factors such as coffee bean storage time and roast level, water temperature, amount of water poured, blooming duration, and grind size affect the blooming result. Through comparative experiments, the article also verifies the actual impact of the number of pouring stages on extraction time and flavor performance, helping enthusiasts understand the logic and applicable boundaries of three-stage pouring.

Many friends who are just getting into pour-over often come to Front Street with the same question: why is blooming such a necessary step in pour-over coffee?

In the pour-over process, blooming is almost an unavoidable step. On one hand, it allows the gases in the coffee grounds to be released; on the other hand, it lays the foundation for subsequent even extraction, and it can also be used to judge how fresh the beans at hand are. Front Street has always focused on freshly roasted coffee beans, but some people mistake "freshly roasted" for "freshly ground," which is actually a concept some merchants swap in during marketing. That said, coffee beans are not necessarily better the fresher they are. If they are brewed right after roasting, they usually still need four or five days of resting before the flavor can be fully displayed during extraction. The beans supplied by Front Street are generally already two or three days past the roast date, unless a customer specifically requests same-day roasting, in which case separate arrangements are made.

The purpose of blooming

Blooming is usually written as Bloom in English, and it refers to the preparatory action of evenly wetting the surface of the coffee grounds with a small amount of hot water before the formal pouring begins.

Front Street has verified through repeated experiments that coffee lacking blooming is more prone to under-extraction. The reason is that during extraction—especially with relatively fresh beans—there is quite a lot of gas hidden inside the coffee grounds, and the main task of blooming is to expel this gas. If blooming is not done first, then as soon as the coffee grounds touch water, the internal gas will immediately rush out in large amounts; if brewing begins directly, the gas wraps around the coffee bed, making it hard for water to truly contact the coffee grounds. The water can only extract the outer portion and continuously washes the already soluble areas, ultimately causing uneven extraction.

Factors affecting blooming

Coffee bean storage time, roast level, water temperature, water amount, blooming duration, and grind size all affect the blooming result.

[Coffee bean storage time and roast level]

There are two situations that make the expansion during blooming不明显, or even produce no reaction at all: freshness and roast level.

First, freshness. Fresh coffee grounds contain a certain amount of gas, and this gas gradually dissipates over time. In addition, coffee grounds absorb moisture from the air, so the original gas naturally decreases. Therefore, stale roasted beans often do not expand much during blooming, or even show no reaction at all.

Next, roast level. Light-roasted coffee beans usually do not expand very dramatically either, because light-roasted beans have a denser texture, and their overall gaps are not as numerous as those of medium or dark roasts, so relatively less gas is hidden inside.

[The level of blooming water temperature]

Water temperature directly affects the dissolution efficiency of coffee. The higher the temperature, the faster the dissolution rate, and the more substances can be dissolved within the same extraction time. Therefore, generally speaking, if the water temperature exceeds 95°C, burnt bitterness and miscellaneous astringency are likely to emerge quickly; the lower the temperature, the slower the dissolution rate, and the fewer substances dissolve within the same time, making the coffee likely to taste thin and making it hard for good flavors to come out, while blooming is also less likely to be completed smoothly.

[The amount of water poured in pour-over]

If the amount of water used for blooming is too small, some coffee grounds will remain not fully soaked, and the brewed coffee will have a noticeably rough and astringent mouthfeel, with a short finish as well; if too much water is poured, then the hot water used in the early blooming stage does not extract enough coffee components, and the resulting cup will have a relatively one-dimensional flavor, with the acidity extracted in the early stage standing out especially strongly and overpowering other flavors.

[The length of blooming time]

How long the blooming should last must be judged in combination with the freshness and roast level of the beans. Generally speaking, the blooming duration is about 30 seconds to 60 seconds. When the expansion on the surface of the coffee grounds settles down and begins to show signs of shrinking, that means blooming is complete and the first pour can begin.

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

[Pour-over coffee grind size]

Grind coarseness also affects the size of the "hamburger" shape formed during blooming. The coarser the particles, the smaller the contact area between grounds and water, and the less carbon dioxide the coffee releases. If the particles are too fine, although the contact area becomes larger, the grounds are so fine that water stays on the surface and cannot go down, so the blooming result is likewise unsatisfactory.

Therefore, pour-over needs a grind size suitable for pour-over so that the "hamburger" shape during blooming will be relatively obvious. FrontStreet Coffee uses a standard No. 20 sieve, determining the grind size for each bean by a sieving pass rate of 80% (light-roasted beans) and 70-75% (medium-dark roasted beans).

The phenomenon of pour-over coffee blooming: expanding into a small hill

When pouring water for pour-over, the gas inside the coffee grounds expands when heated (if you magnify the coffee grounds, their structure resembles a honeycomb, filled with gaps), and after expanding, they begin to release excess gas. Because the gas continues to be released, the coffee grounds and hot water form a continuously expanding small hill in appearance. At the same time, in addition to gas being released from inside the grounds, hot water is also slowly penetrating into the coffee grounds, causing soluble substances to begin dissolving and forming a more concentrated coffee liquid.

At the final stage of blooming, once the gas has mostly been released and the overall temperature has also dropped, some air that has not yet escaped begins to shrink back. Visually, the small hill gradually dries from being wet, or shows slight cracking and collapse, which means blooming is finished and the second pour can begin.

Pour-over coffee steps

Having covered blooming, next let's talk about the pour-over coffee steps after blooming. Front Street uses three-stage pouring, controlling the total extraction time at about 2 minutes, divided into blooming, the second pour, and the third pour. The staged extraction method of three-stage pouring can clearly present the front, middle, and back flavors of the coffee, ensuring the overall flavor is displayed.

[First pour: bloom (to help degassing)]

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

For the blooming time, FrontStreet Coffee sets it at 30 seconds. When the expansion on the surface of the coffee grounds ends, you will see the surface begin to shrink, which means blooming is complete and pouring can continue. Basically, rested beans have a shelf life of within 1 month, and a 30-second bloom at FrontStreet Coffee is just enough to release all the gas.

[Second pour]

For the second pour, FrontStreet Coffee lasts 30 seconds, with the timer showing 1 minute at this point (including the 30-second bloom). Starting from the center, pour with a small stream into the bottom of the coffee bed. To keep the stream's penetrating force concentrated, the circling movement should be small, then spiral outward.

[Third pour]

Starting from the third pour, FrontStreet Coffee recommends paying attention to the drop in water level and beginning the third pour just before the coffee bed is about to be exposed. Likewise, start from the center and pour in circles, with the water amount not exceeding the height of the coffee bed. Stop pouring at 1 minute 50 seconds, and finish dripping by 2 minutes.

Pour-over coffee steps: the impact of the number of pouring stages

Front Street has also encountered some friends asking: why must three-stage pouring be used, and does one more or one fewer stage make a difference? So Front Street conducted an experiment to see the results.

Front Street set the number of pouring stages as the variable and carried out three groups of brewing respectively. The control group was the two-stage pouring extraction mode after blooming that Front Street normally uses; Group A had only one pour after blooming; Group B had three pours after blooming. After brewing, evaluations were carried out by slurping with a cupping spoon.

The variable parameters are as follows:

This experiment used Front Street Ethiopia Yirgacheffe Konga Cooperative. The coffee bean information is as follows:

[Extraction parameters]

[Experimental results]

[Analysis of results]

From the table, the most intuitive result is: extending extraction time can extract more substances. Increasing the number of pouring stages is equivalent to adding extra intervals between stages, and during these intervals the coffee is still continuously extracting. So after the number of stages increases, the total extraction time is also lengthened, thereby extracting more substances.

We know that time is an important factor affecting coffee extraction and taste. Too long an extraction time easily leads to over-extraction, making the coffee bitter and harsh; too short an extraction time means insufficient flavor substances are released, making the coffee likely to be bland and tasteless, unable to display its proper flavor.

Seeing the results of this experiment, many friends will probably ask: "Is three-stage pouring the best pouring mode?" The answer is not necessarily. This experiment only concluded that the number of stages affects extraction time; it did not prove that three-stage extraction is the optimal solution. One-stage or multi-stage pouring can likewise change extraction by adjusting factors such as grind size and coffee-to-water ratio. For example, Mr. Tetsu Kasuya's earlier 46 brewing method used coarse-ground coffee for five-stage pouring; later he also introduced a fine-grind, no-bloom one-stage pouring extraction plan.

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The above content is compiled by CoffeeHunters, a coffee news website.