What does 'screen 18' mean?

The term 'screen size' is a fundamental metric in the coffee industry, used to categorize beans by their physical dimensions. The process involves using a series of metal or wooden sieves with holes measured in 64ths of an inch. A 'Screen 18' bean is one that is large enough to be retained by a sieve with 18/64-inch holes but small enough to pass through a larger screen, such as a Screen 19 or 20.

Why does this matter? Size is not a direct indicator of flavor, but it is a critical indicator of uniformity. When a batch of coffee consists of beans that are all roughly the same size, they will absorb heat at a similar rate during the roasting process. If a batch contains a mix of very small and very large beans, the smaller ones will roast faster and potentially burn, while the larger ones may remain underdeveloped. This leads to an inconsistent, muddy flavor in the cup.

In many coffee-producing regions, specific screen sizes are tied to traditional grading systems. For example, in Kenya, the 'AA' grade typically refers to larger beans that fall into the Screen 17/18 range. While these larger beans are often marketed as premium, it is important to remember that size is only one piece of the puzzle. A smaller bean from a high-altitude farm can easily outperform a larger bean from a lower-altitude, less-carefully-managed farm.

For the reader, understanding screen size helps demystify the labels on coffee bags. When you see a reference to a specific screen size, it tells you that the producer has invested time in sorting the coffee to ensure consistency. While you shouldn't assume that 'bigger is always better,' you can be confident that a uniform screen size is a sign of a well-processed, professional coffee lot.