Why do some countries use screen size for grading?

Screen size grading is a traditional method used by many coffee-producing nations to categorize beans based on their physical dimensions. The process involves passing green coffee through a series of perforated screens with standardized hole sizes, measured in 1/64th of an inch. For example, a 'Screen 18' bean is 18/64ths of an inch in diameter. This system provides a quick, objective way to sort large volumes of coffee.

The primary reason for this is consistency in the roaster. If you have a batch of coffee with beans of wildly different sizes, the smaller beans will heat up and roast much faster than the larger ones. This leads to an uneven roast, where you end up with a mix of light, medium, and dark beans in the same batch. By sorting beans by screen size, producers ensure that the roaster can apply heat evenly, resulting in a more uniform and predictable flavor profile.

However, it is important to note that screen size is not a direct measure of flavor quality. While larger beans are often associated with higher quality, this is not always the case. A smaller bean from a high-altitude farm might be significantly more flavorful than a large bean from a low-altitude, commercial-grade farm. Screen size is a tool for physical uniformity, not a guarantee of taste.

For the consumer, screen size is most relevant when buying single-origin coffees. You might see labels like 'Supremo' or 'Excelso' in Colombia, which are partially based on screen size. These labels help you understand the physical characteristics of the beans you are buying. When you know that your coffee has been sorted for size, you can be more confident that your roaster has been able to achieve a consistent roast, which is the first step toward a great cup of coffee.