Pour-over coffee is a deceptively simple way of brewing that involves pouring hot water over freshly ground coffee. Whether you’re brewing with a Chemex or a single-cup brewer, perfecting your pour-over technique takes some work. There are lots of tips for making great pour-over coffee, but we’re going to focus on “The Bloom.”

Carbon dioxide is a major byproduct of roasting coffee. The gas is trapped in the beans, and is released when the grounds are hit with hot water. If you’ve ever made a pour-over, that’s why the coffee bubbles up when you first wet it.

The escaping carbon dioxide prevents water from reaching the grounds and can mess with your extraction. It’s a good idea to wet the grounds entirely with hot water, then let them rest for about 30 seconds before continuing to brew. Wet with twice as much water as dry grounds (ex: 30g coffee = 60g water). During the 30-second pause, the gas escapes and you might see the grounds rise and expand. That’s the bloom.

Check back every Tuesday for articles on coffee processing, brewing tips, and more. Have questions about how coffee is produced or brewed that you want to see answered in a post? Shoot an email to


  1. I never understand why allowing a bloom is of any benefit. The co2 is going to get out anyway. I question the need for this.

    1. When the coffee is letting off CO2, its not extracting coffee oil into the water. If you continue to pour water over it you will under extract and add CO2 to your coffee.

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