There are a lot of options to make when you brew coffee by hand, including the shape of the brewer. Pour-over brewers can have either a semi-conical shape, which looks like a funnel. Another option is a flat-bottom brewer, which has a wider base and is featured on many commercial brewers. Coffee professionals might argue about their favorite, and why one shape brews better coffee than the other.
In fact, the geometry of each of these brewing devices has been shown to have a significant impact on the overall flavor profile we achieve with each cup.
How does this happen?
For the last few years of my coffee career, this has been a big question and ongoing experiment for me. Not only is it important for people who brew coffee to understand, but knowing how coffee will be brewed in cafes gives roasters a direction for creating roast profiles. In this post, we’ll review these different devices, and discuss what’s occurring within our brew based on the shape of the brewer.
Semi-Conical Vs. Flat-Bottom Devices
Semi-conical brew devices are built with steeper walls than a flat-bottom brewer. They also have a much larger opening at the bottom, allowing water to flow through much quicker. With a semi-conical brewer, coffee tends to sit in a high column, increasing the chances of dry pockets.
Flat-bottom brewers, by contrast, have smaller holes on the bottom of the device which slows the drain time. Its flat bed also creates a larger surface area of coffee at the top of the brewer, which gets exposed to water and agitation. This can help ensure even saturation.
In a semi-conical brewer, coffee tends to brew faster. Flat-bottom brewers slow down brew times and create a longer overall extraction. In a recent YouTube video, Lauren explained that a fast brew will create higher perceived acidity, with accentuated citrus and tart flavors. As the brew time slows down, the extraction yields more sweetness, floral, and fruit-forward notes. With a slower brew, the body of the coffee can feel heavier and fuller.
In an article published by 25 Magazine for the SCA, researchers Dr. Scott Frost, Professor Jean-Xavier Guinard, and Professor William D Ristenpart explore the importance of basket shape in brewing.
In the experiment, which you can read about at the link above, he brews were standardized using the Breville Precision Brewer on the Gold Cup setting. Dose and water amount remained consistent. Only the baskets were switched out to assess the impact of brew bed geometry.
The study found that the geometry of the basket directly affected the TDS (total dissolved solids).
TDS is a measurement that correlates to how strong a cup of coffee is. The stronger the cup, the higher the TDS will be. TDS is denoted as a percentage, with the “ideal” range for brewed coffee measuring at between 1.15%-1.35% TDS.
When you take a sip of coffee, you can feel the weight of it on your palate. It may feel heavy like whole milk or lightweight like nonfat milk or, on the lightest end of the spectrum, tea. The heavier-bodied coffee will likely have a higher TDS.
The amount of solids in your coffee will impact the flavor of the cup. TDS can also be increased by brewing with hotter water temperature, changing the water minerality, or in this case choosing a specific filter shape.
Results of the experiment by the SCA researchers show profound effects of brew bed geometry.
As a former barista, this information is super useful in determining how I want to brew a specific coffee. When I’m dialing in, I can use information from the roaster about how the coffee may have been processed and roasted to determine where it will shine best. Sometimes I’ll brew on both to compare, and I might find highlighted sweetness in one cup with acidity emphasized in the other.
For the roaster, being able to understand how your customers prefer to drink their coffee, either in semi-conical brewers or flat-bottom brewers, will help you develop the right profile. The SCA article references the relationship between roast profile and filter shape: “we see that for a given roast (e.g., the light roast at left), the basket shape also yielded a meaningful difference. The flat-bottom basket yielded flavor attributes with more dried fruit, sweet, and floral flavor intensities, while the conical basket yielded more citrus, berry, and sour. ”
Our Colombia Guava Banana green offering from Forest Coffee has been a great example of this. At home on my Kalita brewer (flat-bottom), I get a cup full of tropical fruit sweetness, some bubblegum, and red punch. Brewing this same coffee on my ceramic Hario V60, as expected, we get higher acidity with notes of grapefruit, blood orange, and hibiscus. Try it for yourself.