Recently I was asked about the bloom in a French press and whether or not it is important to accommodate for it.
This is a really good question and to answer it I’ll start by explaining what the bloom is and why we’re concerned with it.
The bloom is a rapid release of gases, mainly CO2, when hot water comes in contact with freshly ground, freshly roasted beans. This double-freshness is important because the gases begin to slowly escape the beans as soon as the roasting process ends and rapidly after the beans are ground. Therefore the fresher the roast and the fresher the grind, the bigger the bloom.
Knowing that the bloom is the rapid release of gases, we can then apply that to the brewing process, which is the addition of hot water to the ground beans. The hot water penetrates the porous bean fragments and extracts and then transports volatiles. With a finer grind we get more surface area of beans and more exposure to hot water and ultimately more extraction of volatiles. Back to that bloom; when the hot water contacts the beans, gases flow out and that flow stops water from coming in. Think about when jumping into water, you blow air out of your nose to prevent water from shooting up into it. It’s the same principle.
With pour-over brewing, gravity is constantly pulling the water trough the ground coffee and eventually the filter. Fresh water that hits a degassing bean fragment will be pushed past by the escaping gas and that means less extracted coffee. But in a French press, the coffee is steeped in the hot water. It still blooms and the gas movement still interferes with extraction, but the water remains available. As soon as the degassing is done, extraction continues with the original volume of water.
Therefore in a French press, to accommodate for reduced extraction during the bloom, we can simply add the bloom time (typically less than a minute) to the total brew time (optimally around four minutes). If our bloom was 45 seconds in duration, we want to decant the brewed coffee from the press in about 4:45. Brew longer for a stronger coffee—strength is a measure of dissolved solids in the brew and brew shorter for a weaker coffee (The Craft and Science of Coffee, 2017, p. 360).
My last tip about French presses is this; decant when you’ve reached your brewing goal. Leaving the coffee in the press allows it to continue to steep and also cool rapidly. The glass of a press is not a good insulator. Instead, decant to a pre-heated, insulated vessel (I pour whatever hot water I have left over after brewing, right into my thermos to pre-heat it while I wait for my coffee to finish brewing).
Understanding and accommodating for the bloom will help ensure that you get the optimal amount of goodness extracted from your coffee beans when using a French press.
The Craft and Science of Coffee. (2017). Academic Press.