What Do We Mean When We Say Coffee Blooms? with Jamie Smith

Around here, we love a good analogy to help make the science behind coffee feel a little more accessible. Not everyone is a chemical engineer or a food scientist (myself included), so using familiar comparisons can turn a complex subject into something we can easily wrap our minds around. Because let’s be honest, your morning cup is anything but simple (yes, even if you’re an automated drip kind of coffee person). 

WE SAY COFFEE BLOOMS
And because we’re excited for spring, let’s use flowers as an example. A flower blooms only after a period of growth and cellular change, when environmental cues such as light, temperature, and moisture tell its petals to unfurl and release their strongest fragrance to attract pollinators. As coffee roasts, it goes through a similar kind of transformation: the beans are heated, and inside them begins a flavorful change called the Maillard reaction (the same browning magic that gives toasted bread its aroma or seared food its rich flavor). During roasting, as sugars, proteins, and lipids transform and build flavor, the beans also produce carbon dioxide (CO₂). Some of that gas escapes naturally, but much of it becomes trapped within the bean’s tiny cellular structure. Just as a flower holds its fragrance in a closed bud until the moment it blooms, roasted coffee holds onto that CO₂ until hot water wakes it up and invites it to open and express its aroma and flavor.

ALLOW US TO GEEK OUT A BIT MORE
Just a little more science, coming in hot. So those tiny pockets of CO₂ we talked about, they help carry aromas into the cup, but they can also make brewing a little tricky. If, after roasting, the coffee hasn’t had enough time to release its CO₂—or “off gas” as we call it in the biz—it can create a cup that tastes sour or harsh. This happens because when very fresh beans meet hot water, the trapped CO₂ reacts to form carbonic acid, which naturally has a sour taste. 

Fun fact: this is the same chemical reaction that produces the “bite” in sparkling water.

LET’S KEEP GOING
Even with proper resting, some CO₂ remains trapped, and when you grind your coffee and add water during brewing, it begins to escape rapidly. This visible release of gas is the bloom you see in pour-overs and similar manual brewing methods. And just like flowers in bloom, timing matters: release the gas too quickly, and the coffee can taste flat or dull because many of the aromatics escape too fast. Release the gas too slowly, and you risk extra sourness because carbonic acid remains present longer than you want.

Another fun fact: this “off-gassing” is also why most coffee bags have a valve—it’s not there just so you can smell those delicious aromas, though that’s one seriously good perk. Without that valve, the bag would puff up like a balloon and may even burst as the CO₂ releases.

BACK TO OUR FLOWERS
Just as a blossom unfolds at the right pace to share its fragrance to attract pollinators, coffee needs a balanced release of CO₂ to unlock its best flavors and attract its coffee sippers (that’s you… bzz). The bloom is your coffee’s moment to open up, preparing the grounds to extract all the complex and beautiful notes you want in your cup. And if you brew with an automatic coffee maker like a Moccamaster, you may have been blooming your coffee all along without realizing it—many automatic brewers are designed to encourage even extraction and a balanced cup. But if you are brewing manually with a Chemex or V60, blooming becomes a step you must guide with intention. We can help with that.

HOW TO MANUALLY GET THAT BLOOM
You, the brewer, will create the conditions for your coffee to open up. Just as a blossom needs the right balance of warmth and moisture to reveal its fragrance, so do your grounds to become a cup that smells wonderful and tastes just as good. Here’s how to make the bloom happen:

·      Start with freshly ground coffee and add it to your brewer of choice

·      Pour hot water in slow, even circles clockwise or counterclockwise, whichever feels natural

·      Add just enough water to wet the grounds evenly, aiming for roughly twice the weight of the dry coffee, or 2:1

·      As you pour, the coffee bed should begin to rise or dome. This is the bloom: trapped CO2 pushing outward (take a moment to enjoy the aromatics during this step, it’s the best part!)

·      Allow it to rest for about 30–45 seconds before continuing your brew

Note: Pour water too fast, and the CO₂ trapped in the grounds will push the water away and it won’t extract properly; you’ll end up with a sad, sour brew. But wait too long, and you’ll start pulling out bitter flavors before you even get to the real brewing. Get more brewing details here.

ENJOY THE MOMENT, ENJOY THE BREW
We like to think of that less-than-a-minute rest as an important part of the morning ritual—an opportunity to pause before the day speeds up again, take in the aroma, reflect on how you’ve given your coffee the same thoughtful energy that nature gives her flowers. We hope you enjoyed learning more about your brew. We loved sharing it. Happy brewing—or rather, happy blooming.

- Author Jamie Smith is Peet’s R&D Manager and all-around coffee superstar

Check out Jamie's other article, “How Coffee Brewing Methods Affect Flavor”