Why do some countries grade coffee by screen size?

Coffee grading by screen size is a fundamental practice in many producing nations, particularly in East Africa and Colombia. The process involves passing green coffee beans through a series of metal sieves with holes of specific diameters, measured in 1/64th of an inch. For example, a 'Screen 18' bean is 18/64 of an inch wide. This mechanical sorting is not just about aesthetics; it is a vital step in quality control.

Uniformity is the roaster's best friend. When a batch of coffee contains beans of vastly different sizes, the roasting process becomes unpredictable. Smaller beans have a higher surface-area-to-volume ratio and will heat up and reach the 'first crack' much faster than larger, denser beans. If you roast a mix of sizes, you end up with a 'mottled' roast where some beans are light and others are dark, leading to a muddy, inconsistent flavor profile.

Beyond roasting, screen size is often used as a proxy for quality. In many regions, larger beans are perceived to have had more time to develop nutrients and sugars while on the tree, potentially leading to a more complex cup. While this is not a universal rule—some smaller varieties are exceptionally flavorful—it remains a reliable way for exporters to categorize their inventory and set price points for different market segments.

In practice, this means that when you see a grade like 'AA' in Kenya or 'Supremo' in Colombia, you are looking at a product that has been physically sorted to ensure that the beans in your bag are roughly the same size. This allows you to achieve a more even extraction during brewing, as the water interacts with the grounds in a predictable, balanced way. It is a testament to the effort producers take to ensure that the final cup in your kitchen is as clean and delicious as possible.