Introduction:Flannel filters can preserve rich coffee oils, giving Jamaican Blue Mountain coffee a more mellow and smooth mouthfeel—but do you know which grind size to use? Taking Jamaican Blue Mountain as an example, this article tests the extraction performance of three common grind sizes—BG-5R, BG-6M, and BG-6A—from flow rate changes, flavor profiles to parameter suggestions for dark roasts, helping you master the golden ratio for flannel pour over.
Because flannel filters have larger pores than paper filters, they retain more coffee oils, making the coffee taste fuller and smoother—especially suitable for showcasing dark roasts like Jamaican Blue Mountain with complex flavor layers. But how should you choose the grind size when using a flannel filter? Is it the same as the grind size recommended for a typical KONO dripper? This experiment used 20 grams of coffee, 85°C water temperature, a 1:13 coffee-to-water ratio, and a single-pour technique, testing three grind sizes—BG-5R (with 56% passing through a Chinese No. 20 sieve), BG-6M (45%), and BG-6A (64%)—to find the best combination for Blue Mountain.
The characteristic of flannel filters is that there are no barriers around, allowing water to seep in multiple directions, and the flow rate differs between the front and back. This experiment chose the relatively stable backside for extraction. The Blue Mountain coffee used comes from Clifton Estate in Jamaica’s Blue Mountains region, grown at an altitude of 1310 meters, using the washed method on Typica variety beans. It is medium-dark roasted, naturally possessing a bold base note that complements the smooth characteristics of the flannel filter.
The test results showed that when using the BG-5R grind size, the extraction time after blooming was 2 minutes and 20 seconds. The flavors included cocoa, cream, blueberry, and cedar, but the mouthfeel was overly strong and slightly astringent, due to the overly fine grind causing over-extraction, with more noticeable acidity and bitterness. When switching to BG-6M (45% passing rate), the extraction time shortened to 2 minutes and 17 seconds, and the flavors shifted to raspberry, blackberry, and almond. While the mouthfeel was smooth, it felt thin, with weaker floral and fermented fruit notes in the aftertaste.
To balance the mouthfeel, the grind size was adjusted to BG-6A (64% passing rate)—a slightly coarser medium grind compared to BG-6M. This set had an extraction time of 2 minutes and 15 seconds, with flavors presenting a soft profile of milk chocolate, cane sugar, vanilla, and citrus. Overall, it delivered the most mellow and balanced performance, avoiding both the sharpness of over-extraction and the drawback of an overly thin mouthfeel.
Why are the differences between the three grind sizes so pronounced? The key lies in the flow rate changes of the flannel filter. Right after water is poured in, the flow rate is relatively fast. As the water level drops, the accumulated coffee grounds around the edges create resistance, and ultra-fine particles may clog the flannel’s pores, causing the latter part of the extraction to slow down noticeably. Therefore, when using dark roasts like Blue Mountain, it is recommended to choose a medium grind (close to BG-6A), which can maintain a reasonable flow rate while avoiding over-extraction caused by an overly fine grind, thus preserving flavor balance.
In practice, if you find the flow rate too fast during extraction, you can slightly adjust to a finer grind—but be mindful to observe whether the final mouthfeel becomes too astringent. Conversely, if the flow rate is too slow or the flavor is too light, it is advisable to adjust to a coarser grind. Additionally, for the single-pour technique, it is recommended to start with 40 grams of water for a 25-second bloom, then pour up to 260 grams. Remove the flannel just as the water level is about to expose the coffee bed to ensure even extraction.
The above content is compiled by CoffeeHunters, a coffee news website.
