Dear Blue Bottle,
I know that one never wants a too-long brew time for fear of overextraction, but I also know that an eight-cup Chemex exists because there has to be a way to make pour-over for a large group of people (right?). Can you give me some tips for how to brew at this volume?
–Longing for Chemex
Chemex is a terrific method for making coffee for a crowd, and there are a few tricks for ensuring that you don't overextract.
Adjust the Grind Size
The first thing to assess is the size of the grind. Every brewing method has an ideal grind size, but you will want to make your grind slightly coarser as you increase your amount of coffee. This is because the larger amount of ground coffee will provide increased resistance, slowing down the water as it flows through the grounds. A coarser grind will ensure that the water flows through more quickly, preventing overextraction.
The next step is to adjust the water-to-coffee ratio based on the coffee you are using and your preferred brew strength.
Here are some basic parameters: To brew an eight-cup Chemex with a blended coffee, aim for a 1:12 ratio (one unit of coffee per twelve units of water). To brew a single origin, which you will want to make slightly "lighter" so that the nuanced flavor profile can come through, add more water, and aim for a ratio of 1:15.
It generally takes about three to four minutes to brew a cup of coffee, but an eight-cup Chemex will take slightly longer, because there is more water working through more coffee grounds. Try to stick with a range between four and five minutes and you should be safe from overextraction.
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For smaller batches, you can find our Chemex brewing guide for three to six cups here.