What is Coffee Blooming According to the Science

Your coffee is talking to you. 

No, not literally. Even though it might feel like the only thing pulling you out of bed every morning is the call of your morning brew, your coffee is actually communicating with you in a much more subtle way. As you pour water over your ground coffee, waiting patiently for that first sip of your pour-over that promises to bring you back to life, take a peek at what’s happening in the brew bed. What you will see is a language expressed in bubbles.

When you pour hot water onto coffee grounds, bubbles will pop to the surface for the first few moments, almost like the coffee is exhaling—and in a sense, it is. The process of hitting coffee with just a splash of hot water to release these bubbles is called blooming, and the bloom can tell you a lot about what’s happening during brewing and how to adjust your method to get the best possible cup.

What’s a Coffee Bloom?

To bloom your coffee is to pour a small, initial stream of hot water on it, just enough to saturate the brew bed, and then wait briefly before adding more water.

Coffee brewing recipes vary widely, but almost any good one—save for immersion brewing methods, which we’ll talk about later—includes a bloom phase. Usually this step requires folks to add about double the weight of the coffee in water (so if you’re brewing 30 grams of coffee, you’d add 60 grams of water, but it doesn’t have to be precise), which is usually enough to saturate the coffee bed. Most recipes will ask you to then wait as the coffee blooms for 30-45 seconds. 

It’s during the bloom phase that you’ll see our chatty friends, the bubbles, rise to the surface. These little spheres are filled with carbon dioxide, or CO2, which has been trapped in the coffee beans ever since they were roasted. If you’ve ever looked at green (unroasted) coffee, it’s much smaller than its roasted counterpart, and CO2 produced during roasting is the reason for this change in size. Just as oxygen fuels a bonfire and produces carbon byproducts like CO2, roasting coffee beans converts oxygen and hydrocarbons in the bean into CO2, which expands the beans’ cell walls as it accumulates, with more produced the longer you roast coffee. Over time, CO2 slowly leaves a coffee bean in a process called degassing, but there’s no specific time limit or range for how long that takes.

In some ways, CO2 is both a good and a bad thing. It creates a protective layer around well-sealed coffee, keeping it fresh for longer by protecting coffee from being exposed to oxygen, which, like many foods, oxidizes in the presence of O2—that’s why you’ll see many roasters use one-way valve bags, which let out CO2 as it naturally leaves a coffee bean over time, but won’t allow oxygen in until the seal of the bag is opened. But CO2 also gets in the way of brewing. “While CO2 does not bear a direct impact on the taste of filter coffee,” writes Jonathan Gagné in The Physics of Filter Coffee, “it can interact negatively with the process of extraction.” 

Basically, Gagné is saying that CO2 won’t impart any particular flavor to a coffee, but it will prevent water from extracting the flavors out of coffee that we want. That’s because, as soon as the ground coffee is exposed to hot water, a more rapid degassing is triggered. With all that CO2 rapidly bubbling out of the coffee particles, the water is less able to soak in and extract flavor. Gagné’s book has a really helpful visual of this process* where he illustrates a close-up of one particle of ground coffee, and how water molecules essentially cause the CO2 to “leave” the particle. 

*If you want to get very technical with coffee, I can’t recommend this book enough, and the drawing is on pg. 85.

Why Bloom Coffee?

You might wonder whether a bloom phase can really help—after all, we’re dousing beans with hot water throughout the brewing process, so wouldn’t there be enough time to still get what we need out of the coffee as more and more water is added? Does an initial wetting phase before we pour the rest of the water really change things? 

We’ve gone over the importance of blooming in the past, and Gagné’s book suggests why, but I wanted to find out by doing an experiment: I brewed the same coffee—a naturally processed Ethiopian coffee from my friends at Rabbit Hole Roasters in Montreal, Quebec—using a standard recipe which I’ll be using as my “control” recipe throughout this testing. This is the protocol: 

• A light-medium roast (ideal for pour-over and filter brewing) 
• Roasted nine days from testing (ten days for a specific test, which we’ll talk about below)
• 30 grams of coffee 
• 500 grams of water 
• Ground at the 22 mark on my Baratza Virtuoso
• 205°F filtered water using a temperature-controlled gooseneck kettle 
• Brewed on a Kalita Wave
• Pre-wet the paper filter
• A 45-second bloom of double the coffee weight (60 grams +/- 2 grams) 
• Four pulse pours of 110 grams, poured in concentric circles

Yes, that’s a lot of detail, but I want to make sure I could isolate as many variables as possible. To measure how important the process of blooming is at all, I compared a “control” brew to one I did with a continuous pour, meaning I poured all 500 grams of water onto the coffee bed in one go without stopping. You can see what that looked like below: 

As you can see, the continuous-pour brew looks…like a mess. “As the first drops of water are released on the dry coffee bed, several things begin happening at once,” writes Gagné. “The water initially wets only partial regions of the coffee bed, and the hydraulic resistance goes down drastically in these specific regions…like sand, dry coffee is much more resistant to the flow of water compared with wet coffee.” 

Gagné’s observation notes that by prewetting the coffee, you’re cutting down the resistance the coffee puts up to the water, which helps to make sure that subsequent pours of water move through evenly and don’t create channels—basically, all the coffee will brew at the same rate. However, Gagné also notes you want that to happen uniformly throughout the brew bed. As you wet the brew bed, water will naturally fall towards the bottom because of gravity, but it will also move in all directions (and uniformly wet the entire brew bed) because of capillary action. 

“Capillary action is the phenomenon that causes a towel to progressively get wet farther upward, even if you dip its bottom tip in water,” writes Gangé. Using the towel analogy, the waiting period in a bloom is like waiting for the entire towel to get wet, and you want to make sure that your entire brew bed is saturated before your next pour. I had never considered this until I read Gagné’s book, and then I began noticing it in other parts of my brew, like when I pre-wet the coffee filter. Pre-wetting a coffee filter removes the papery taste it might have, but I realized that I usually just aim for the bottom of the filter, knowing the water will travel up over time. 

Again, this is where bubbles come in. “If you noticed a lot of large bubbles occurring after the preinfusion, it means you could probably improve your brew…the appearance of carbon dioxide bubbles after preinfusion is a problem, not only because they indicate some part of the coffee bed was still dry and will end up lagging behind the rest in terms of its extraction yield, but also because bubbles flowing upward will cause localized channels of much lower hydraulic resistance,” he writes. Simply put: coffee blooming is necessary to ensure your coffee bed is evenly saturated—and saturated for long enough to give the gas time to exit—and if you see bubbles happening after the bloom, it means you might not have bloomed the coffee thoroughly. 

How Long Should You Bloom Coffee?

A few coffee folks have tips for improving your coffee bloom and thoroughly saturating the bed and letting all the CO2 out of your ground coffee. Many recipes recommend agitating your coffee bloom by sticking a spoon or stirring stick right into the grounds and giving it a good stir. James Hoffmann, on his popular coffee YouTube channel, has a V60 technique where he encourages people to swirl their brewers to ensure even saturation. But none of these techniques really answer the question of time: How long should you bloom your coffee? 

In general, you should wait until all the bubbles are done creeping out the top of your brew bed, and while you can bloom your coffee for too short of a time, there’s really no limit to how long you can bloom. 

In the article Measuring Coffee Extraction and Why It Makes People Mad, coffee expert Umeko Motoyoshi proposes that you try extending your bloom, something that became colloquially known as the “long ass bloom” on social media and Reddit. Motoyoshi suggests blooming a coffee for a really long time (as long as ten minutes) to increase extraction. “For some reason, this suggestion really makes coffee people lose their minds—but blooms do not have to be 30-45 seconds. You can bloom a coffee for 3 minutes. You can bloom it for 10 minutes,” they write. “Nothing explodes. Increasing bloom time will almost always increase extraction, and it can taste really delicious too.” 

So I decided to try that out as well. 

I extended the bloom for three minutes, and on tasting, I did notice more sweetness in the cup, particularly in the finish, but I’m not sure it was enough for me to justify this as an everyday technique. What’s important is that it doesn’t seem like there’s an upper time limit on blooming: You should aim for it to be longer rather than shorter, and only once you see the bubbles start to fizzle out is it usually a good time to end your bloom. Forty-five seconds is a good rule of thumb, but if your coffee is very fresh or darker roasted, which equates to having more CO2, you can try extending the bloom longer.

The Coffee Whisperer: What Your Coffee is Saying and How to Listen and Adjust Accordingly

After confirming the value of blooming coffee, I decided to do three more sets of experiments to demonstrate how we can “listen” to the bubbles in our coffee and adjust our brewing method depending on what they’re telling us. I started by isolating one of the variables in my “control” recipe and changing it slightly to determine how each affected the amount of bubbles I saw, how it affected the taste of the coffee, and what the bloom ultimately told me about each variable. Again, here’s that recipe, and in bold are the three variables I planned to change: 

• A light-medium roast (ideal for pour over and filter brewing) 
• Roasted nine days ago — the bag was sealed until the day of testing
• 30 grams of coffee 
• 500 grams of water 
• Ground at the 22 mark on my Baratza Virtuoso
• 205°F filtered water
• Brewed on a Kalita Wave
• Pre-wet the paper filter
• A 45 second bloom of double the coffee weight (60 grams +/- 2 grams on each end) 
• Four pulse pours of 110 grams, poured in concentric circles

There were other variables relevant to the bloom that I wanted to test, but they proved much trickier to control. I tried to examine the grind setting, for example, but grinders are notorious for being finicky the minute you begin using them: a coffee ground on cold burrs will grind differently than one with hot burrs because steel expands when it’s hot (and burrs can start to get hot after a few uses). Water hardness, humidity, bed depth, pre-ground versus freshly ground coffee, type of brewer, etc. are other variables on interest, but the three I focused on were the most manageable to test, and also ones that are most likely to change from day to day for most coffee drinkers at home.

Let’s take a look and listen to each.

Roast Date

The variable I was the most excited to play with was roast date. If you buy coffee that was roasted four days ago and intend to brew over the course of the next two weeks, it’ll taste noticeably different on the first day than it will on the last. I theorized that the amount of bubbling I’d see in an older coffee would be less than a fresher one given that roasted coffees naturally degas over time, and that might indicate that a coffee needs to be treated differently. To find out, I asked my friend David Lalonde of Rabbit Hole Roasters to send me the same coffee roasted on four different days (each was roasted a few days apart: 8/17, 8/22, 8/26, and 8/29), then got to testing.

What the coffee is saying: I opened the sealed bags, each roasted on a different date, at the same time, then brewed them. They all performed relatively the same. I should have expected this: The one-way valves we discussed earlier do a really good job of keeping coffee fresh. “In situations where coffee is left in a good-quality bag without ever breaking the initial seal, it can remain fresh for a surprisingly long period of time,” says Gagné, noting that he was unable to tell the difference between a freshly roasted coffee and a three-month old coffee that both kept sealed until tasting.

Next, I closed the coffee bags up and sat on my hands and waited until a day had passed to brew them again.

While I waited, I decided to try one other thing—this was outside of my initial experiment, but I looked for the oldest beans I could find. Like, the oldest. I found a mason jar sitting in a cabinet with unidentified beans from who knows when. I think they were roasted about four months ago? I brewed this coffee using the same ratios and brewing technique, and as you can guess, there were virtually no bubbles in this brew (and it tasted flat and hollow), confirming that poorly sealed coffee eventually degasses fully as it loses freshness. 

Unfortunately, there’s no clearly measurable rate at which coffee loses freshness (a lot of websites indicate that a coffee “loses its freshness” after three weeks, but there’s no hard and fast rule, and that ignores all the ways we can store coffee to maintain freshness). 

The next day, I returned to my coffee bags. I didn’t notice much of a difference in the 8/22 and 8/26 bags (it’s hard to make a quantitative analysis of bubbles without a lot of fancy lab gear) but when I compared the 8/17 bag to the 8/29 sample, I noticed some stark differences: 

Within a day of being opened, the 8/17 beans lost a fair amount of CO2, as displayed by the lack of bubbles during the blooming process, and when I tasted it, it didn’t have the same vibrancy it did the day before. The 8/29 bag still produced vigorous bubbling and tasted relatively the same as the prior day (which was surprising as well—I expected some degradation in flavor but couldn’t pick up on any). This makes sense, in that a coffee that’s older will have released more gas into its sealed bag than a more recent roast; the one-way valve on the bag will have kept oxygen out of both, preserving flavor, but once opened the older coffee, having already lost more CO2, will be more vulnerable to oxygen than the fresh coffee that’s still more actively degassing. Which is to say, once opened, bags of even slightly older coffee will be on a more rapid decline than their newer counterparts.

How to account for freshness: As we mentioned before, CO2 inhibits extraction and freshly roasted coffee has a lot of CO2, meaning you’re at risk of not getting the full flavors out of a coffee if you don’t account for this. The downside of not accounting for the freshness of the coffee is that your brew can end up with a very hollow flavor—not necessarily flat like old coffee, but like there’s something missing. If your coffee is very fresh, try grinding finer (to increase surface area for CO2 to escape) and giving your coffee extra time to bloom; consider a “long ass bloom” or perhaps even agitate the brew bed to try to get that excess gas out and ensure your coffee brews evenly. 

There are a lot of conflicting narratives about what to do with coffee when it’s old (say, more than two weeks from its roast date), and no one seems to agree. Some folks on the internet suggest upping the dose (relative to the water) and grinding coarser; the rationale here, which isn’t supported by any evidence I know of, is to offset the loss in flavor with more body by brewing a stronger cup (but the higher dose requires a coarser grind to increase the flow rate and keep the brew times consistent). Some of my coffee-industry friends told me they did the exact opposite. I texted one friend to ask what they did and they admitted they don’t make a single adjustment to their brew recipe if a coffee is old. When I googled “how to brew old coffee” I got a lot of hits on how to make cold brew.

Probably the biggest takeaway here is: as your coffee gets older, there’s less and less you need to do to ensure an effective bloom. With each passing day, you’ll have to accept that your coffee is a little bit less good than the day before, and the faster you can use up your beans, the better. That’s not to say there’s no value in blooming older coffee, but you’re at less risk of your bloom phase being insufficient.

The issue of coffee age is perhaps largely a preventative maintenance one rather than a fix-in-the-moment one. Look for coffee bags with one-way valves, and try not to buy more coffee than you need for a week or two. There are emerging ideas on ways to make freezing coffee work, so you can dive down that rabbit hole, but if you’re simply trying to keep a regular, 12-ounce bag as fresh as possible for as long as possible, try a vacuum-based storage container or store your bag away from sun, light, and air—Gangé mentions he stores his coffee bags in a cabinet.

Roast Profile

When I messaged David at Rabbit Hole Roasters about the experiments I wanted to do, he mentioned that darker roasted coffee would have a more aggressive bloom, and he was right. The darker-roasted coffee he provided me with (although I should note that the coffees weren’t wildly dark—we’re not talking a very oily dark roast, but rather a slightly more developed bean) had a more vigorous bloom, but it deflated much quicker than the lighter-roasted coffee. This was tricky to capture in photos, but the darker roasted coffee bubbled up more quickly and had lots of little bubbles scattered around that popped quickly, whereas the lighter roast had bubbles that slowly crept up and stayed longer. 

What the roast level is saying to you: Darker roast coffees have more CO2, so they’ll give off way more bubbles. But darker roasted coffees are also more soluble and brittle, meaning that water can penetrate the ground particles more easily since their cellular structures are wide open. So while the bloom was initially more aggressive, it dissipated quicker. 

How to account for roast level: I wouldn’t necessarily say this is a thing to “fix,” but roast profiles are definitely something to be aware of—and something that isn’t often indicated on many specialty bags. No one roaster has the same roast profile, but Gangé notes in his book that light roasted coffees could benefit from a longer bloom since they’re denser and not as soluble. If you know your coffee is fresh and the bubbles aren’t coming up to the surface quickly, it might just mean your coffee is a light roast and would benefit from a longer bloom. Darker-roasted coffees have more CO2 (which is created during roasting) but because their cellular structure is more brittle (again, from roasting), CO2 can more easily escape. So you might observe a more aggressive bloom early, but it’ll dissipate quickly.

Temperature

Think of temperature in terms of energy: The hotter something is, the more it can do and the faster it can do it. When it comes to coffee brewing, hotter water is often associated with a quicker brew since the water can extract the coffee’s flavor more rapidly. 

What water temperature is saying to you: In general, most coffee brewing recipes call for brewing water between 195-205°F, so I tried brewing one batch with 185°F water and another with water just off the boil. 

Predictably, the 185°F water didn’t bloom as much and it tasted flat and under-extracted (more sour, less balanced and sweet). Strangely, the boiling water did this weird thing where there were no bubbles at all, and instead there was a distinct crack in the middle of the brew bed. One reason for this is that boiling water might simply evaporate, not really giving a chance for the bed to make bubbles. I do think the brew sufficiently degassed through that fissure, but I’m unclear if it did so evenly. I thought the resulting brew was kind of acrid, but I’m unclear if that was from the hot water itself or from an uneven extraction—I’d guess it was the former. 

How to account for temperature: Water temperatures are really important to be consistent with—but hard to do if you don’t have a kettle that gives you real-time temperature readings or you’re not taking the temperature actively throughout the brewing process. Hotter water can increase extraction, so if you have a very fresh coffee, it might be worth using a slightly hotter water to get the CO2 out of the beans and pull the flavors from the coffee you’d like. But a not-so-bubbly brew bed might indicate that you’ve lost too much temperature in your water and you might need to work to increase extraction or even re-brew—it’s difficult to recover from a drop in temperature.

Conclusion

While it may not be the first conversation one might expect to have in the morning, the bubbles in your brew can communicate a lot. But as with human-to-human communication, that conversation isn’t always clear and direct. Instead, those bubbles at the beginning of your brew are whispering, and you have to pay attention and get close to hear what they’re telling you—because coffee bubbles can communicate a lot more than you expect.