Why Doesn’t the Coffee Bed Swell During Blooming in Pour-Over Brewing?

 Whenever I brew a pour-over for a customer, if the coffee bed balloons noticeably during the bloom phase, there’s a good chance someone will ask:

“Why doesn’t my coffee bed swell up like that when I brew at home?”

This is actually one of my favorite topics to discuss. Strictly speaking, the size of the bloom doesn’t directly reflect how the coffee will taste. But it does reveal a lot of other information — such as the condition of the beans and the brewing parameters used. These factors are precisely what determine how much the coffee bed puffs up.

So today, let’s break down how these factors affect the size of the bloom — and why, in the end, a bigger bloom doesn’t necessarily mean better coffee.

---

1. Bean Freshness and Roast Level

First, we need to understand why the bloom happens. The main reason is carbon dioxide (CO₂).

During roasting, coffee beans accumulate a large amount of CO₂, which is slowly released after roasting. When the ground coffee comes into contact with hot water, this gas escapes much more rapidly. The purpose of blooming is to let CO₂ escape before full extraction begins. By wetting the grounds with a small amount of hot water, we allow the gas to release quickly, helping the later pours extract flavor compounds more evenly.

As CO₂ is released, surface tension in the water and capillary action between coffee particles create a kind of airtight “barrier.” Because this “barrier” temporarily traps CO₂ inside the coffee bed, the gas builds up, expanding like a balloon. This expansion is what causes the coffee bed to swell — that’s the science behind the bloom.

From this, we can see that the bloom is primarily affected by the amount of CO₂ in the beans. With all other brewing parameters equal, beans that contain more CO₂ will produce a larger bloom; beans with less CO₂ will bloom less.

This is easy to understand: the more CO₂ there is, the more gas will be released during blooming, and the coffee bed will naturally puff up more.

Two main factors determine how much CO₂ beans retain: roast level and freshness.

• The fresher or darker-roasted the beans, the more CO₂ they contain.

• Conversely, older or lighter-roasted beans hold less CO₂.

So if you use freshly roasted or dark-roasted beans, you’re likely to see a more dramatic bloom.

But as I emphasized earlier, that’s true only if the brewing parameters remain constant — and that’s because extraction efficiency also plays a big role.

---

2. Extraction Efficiency During Brewing

More precisely, we should say extraction efficiency.

When the extraction efficiency is higher, coffee compounds — including CO₂ — dissolve and escape more quickly. This means the faster the CO₂ is released, the more pronounced the bloom will appear in the same amount of time.

In many cases, the reason someone’s coffee bed doesn’t puff up much is that their extraction efficiency during blooming is too low. The gas simply doesn’t escape quickly enough. So even if the beans are fresh or darkly roasted, the bloom won’t look impressive.

There are many factors that affect extraction efficiency — such as contact time, grind size, agitation, and water temperature. However, during the bloom stage, the two biggest influences are water temperature and grind size:

• The higher the water temperature and the finer the grind, the more vigorous the bloom.

• Conversely, cooler water and coarser grinds will produce a flatter bloom.

---

3. A Bigger Bloom Doesn’t Mean Better Flavor

To sum up: the main factors affecting the size of the bloom are bean freshness, roast level, and extraction efficiency.

But again — the bloom size doesn’t directly correlate with flavor quality.

For example, at my shop, I once brewed two pour-overs of the same coffee — El Divisio Estate, Gesha variety — one roasted 2 days prior, and one roasted 25 days prior, using identical parameters.

The 2-day-old coffee produced a much larger bloom, but the flavor was actually less complex.
The 25-day-old coffee, with a smaller bloom, had a far richer and more layered flavor profile.

In addition to notes of grape, passion fruit, apricot, and winey sweetness, I also detected delicate floral aromas and a smoother, more rounded fruit-wine fragrance.

The difference, as I often explain, comes from excess CO₂ hindering proper extraction — but that’s a discussion for another article.

So, it’s best not to judge coffee quality by the size of the bloom. The bloom tells us more about the bean condition and brewing parameters than about taste.

Think of it as a diagnostic “file” — a visual cue that helps us read what’s happening in the brew, rather than a direct measure of how good the coffee will taste.