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.
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