Coffee Knowledge: Why is Bloom Necessary for Pour-Over Coffee? How Long Should Bloom Last and How Does It Affect Extraction

Professional coffee knowledge exchange, more coffee bean information, please follow Coffee Workshop (WeChat public account: cafe_style)

Brewing a cup of pour-over coffee is a rigorous process, as every factor is an important element in creating a good cup of coffee. A small mistake can lead to complete failure. This is FrontStreet Coffee's experience, a very painful lesson. So how important is the beginning of making a pour-over coffee? An operation called "bloom" - how exactly does it affect the quality of a cup of coffee? Here you'll find the answers you need.

What is Coffee Bloom?

"Bloom" is a term everyone learns when encountering pour-over coffee; it refers to the first water pour over the coffee grounds in the filter, emphasizing that the water should fully saturate the coffee grounds. Theoretically, the coffee bed will expand at this time, typically lasting 30 seconds.

However, in actual practice, you'll encounter many questions: How much water counts as "fully saturated"? Why do coffee grounds sometimes not expand and instead collapse? Why 30 seconds? What effect does skipping the bloom have on coffee flavor?

The Science Behind Coffee Bloom

Here, we return to the perspective of physical chemistry to explore this matter. First, roasted coffee beans have a cellular structure that is honeycomb-like, filled with many holes. These holes store coffee's aromatic compounds and continuously release carbon dioxide, waiting to be extracted.

The moment ground coffee comes into contact with hot water, carbon dioxide inside the holes is rapidly released. We can observe that many bubbles are produced during pouring - that's the phenomenon of carbon dioxide combining with coffee grounds. Since gas rises, these bubbles will block the channels in the coffee bed, preventing water from continuing to flow to deeper layers, which actually hinders subsequent extraction.

Therefore, first using a small amount of water to wet the coffee grounds allows appropriate release of carbon dioxide. When there are no gas obstructions between the coffee bed's channels, proceeding with the next stage of pouring can smoothly achieve our ideal extraction state.

Based on the above, we tend to call this water pouring for the purpose of wetting coffee grounds "pre-infusion" rather than "bloom," which better aligns with physical explanation. The amount and time of pre-infusion are not difficult to imagine. To fully saturate the coffee, at least the amount of water that the coffee grounds can absorb should be poured (typically 1g of grounds absorbs 2g of water). The pre-infusion time can be determined by observing the gas release status of the coffee bed, using as a benchmark the state where most gas is released but the coffee bed doesn't collapse.

Different Roast Levels and Bloom Characteristics

Darker roasted coffee has lower moisture content, absorbs more water when in contact with it, releases more gas, and therefore shows greater expansion.

Lighter roasted coffee has higher moisture content, doesn't absorb water as much when in contact with it, releases less gas, and therefore shows less expansion.

What to Pay Attention to During Blooming?

Just the Right Amount of Water

If too much water is used for blooming, and a large amount of coffee liquid drips down, the situation is that the falling water doesn't stay but directly carries substances from the exterior of the coffee grounds. When the outer layer's components of the coffee grounds are dissolved while the inner layer is still dry, when hot water penetrates to the inner layer, the bitter and astringent tastes from the outer layer also begin to dissolve. Coffee brewed this way is both weak and has over-extracted unpleasant flavors.

If there's insufficient blooming water and no water drips down at all, this means the coffee grounds haven't absorbed enough water, and some coffee hasn't released enough gas, which will cause under-extraction.

Don't Pour Too Gently

When you pour particularly gently, or believe in the so-called "spreading water" approach, if there isn't enough force to let water penetrate the surface layer of grounds during pouring, just moistening the surface, allowing water to seep into deeper layers only through capillary action and gravity, the time for blooming water to seep from the surface to the bottom layer will be extended. Additionally, some water will spread from the surface and flow away through the edge ribs, reducing the actual amount of water received by the coffee grounds during blooming, naturally diminishing the blooming effect.

1. Increase pouring force, which can be approached from two aspects: water stream size and pouring height. After all, some narrow-spout kettles are truly only suitable for fine water flow. When the water flow is larger, the water column easily tilts. At this point, increase the pouring height slightly and let gravity acceleration provide sufficient penetration force for you. Of course, be careful not to create splashing.

2. Appropriately increase the amount and time of blooming. Since the penetration force is insufficient, which reduces the actual blooming water amount and slows down the speed, just increase a bit more water and extend the blooming time.

Is Blooming Necessary?

But is the blooming step absolutely necessary when making pour-over coffee? We'll use [Yirgacheffe Worka] roasted 8 days ago as an example to compare how much impact blooming versus not blooming has on a cup of coffee!

Water temperature 90°, Grind size: BG-6L, Powder-to-water ratio: 1:15, Time: 2:05

Brewing method: 27g water bloom for 30 seconds, slow small-flow pour to 120g in sections, when water level drops and is about to expose the coffee bed, pour to 225g and stop brewing

Water temperature 90°, Grind size: BG-6L, Powder-to-water ratio: 1:15, Time: 1:46

Brewing method: Direct pour to 120g in sections, when water level drops and is about to expose the coffee bed, pour to 225g and stop brewing

Comparison: In terms of flavor, the cup with the blooming step tastes richer, the floral aroma is much more obvious than the non-bloomed version, and it has richer layers. The cup of [Worka] without the blooming step shows some sourness and astringency, with grassy notes, clearly exhibiting under-extraction.

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473