What does screen size mean in coffee grading?

In the world of specialty coffee, consistency is king. When you see a grade like 'Screen 18' on a bag of green coffee, you are looking at a measurement of the bean's physical size. This process happens at the dry mill, where beans are passed over a series of metal screens with precisely sized holes. These holes are measured in 1/64th of an inch; for example, a Screen 18 means the bean is at least 18/64 of an inch (approximately 7.1mm) in diameter.

The primary reason for this grading standard is heat transfer. During roasting, the size and density of the bean dictate how quickly it absorbs heat. If a batch contains a mix of very small and very large beans, the smaller ones will roast much faster than the larger ones. This leads to an uneven roast profile where some beans are underdeveloped and grassy, while others are dark and potentially burnt. By sorting by screen size, roasters can ensure a more uniform roast, which is a hallmark of high-quality coffee.

It is important to note that while larger beans are often associated with higher quality or higher altitudes, size alone does not guarantee flavor. A smaller bean, such as a peaberry or a specific high-altitude variety, can be just as delicious as a large one. However, uniformity within a lot remains a critical factor for professional roasters.

In practice, when you buy coffee, you might see a range like 'SC 14/16.' This indicates that the lot consists of beans that passed through a size 16 screen but were retained by a size 14 screen. This level of sorting helps the roaster dial in their equipment to achieve the best possible sensory experience, ensuring that the final cup is balanced and free from the defects caused by uneven roasting.