How are coffee beans graded by size?
Coffee beans are graded by size using a series of screens with specific hole diameters, measured in 1/64th of an inch increments. This process, known as screen sizing, helps ensure uniform roasting, as beans of similar size absorb heat at the same rate. Larger beans are often perceived as higher quality, though size alone does not guarantee superior flavor.
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Screen sizing is a fundamental step in the post-harvest processing of coffee. After the beans have been dried and hulled, they are passed through a series of vibrating screens with holes of varying sizes. These screens are measured in 1/64th of an inch increments, with a 'Screen 18' bean being 18/64ths of an inch in diameter. This mechanical sorting is essential for consistency, as it separates the beans into uniform lots that will behave predictably in the roaster.
Uniformity is the primary goal of screen sizing. If a batch of coffee contains a mix of very large and very small beans, the smaller ones will roast much faster than the larger ones, leading to an uneven roast where some beans are under-developed while others are burnt. By sorting the beans by size, roasters can dial in their roast profiles with precision, ensuring that every bean reaches its full potential. This is particularly important for , where the goal is to highlight the delicate nuances of the origin.
While larger beans are often marketed as 'premium' or 'fancy'—such as the large, prized beans from the variety—size is not a perfect indicator of quality. A small, dense bean grown at a high can often taste significantly better than a large bean grown at a lower . However, size grading remains a standard industry practice because it provides a reliable way to categorize coffee and ensure that the final product is consistent and easy to work with.
For the reader, understanding helps demystify the labels you see on coffee bags. Terms like 'Screen 17/18' or 'AA' (a common grading term in Kenya) refer to these size classifications. While you shouldn't judge a coffee solely by the size of its beans, knowing that they have been sorted for uniformity gives you confidence that the roaster has taken the necessary steps to provide a high-quality, consistent experience in your cup.
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