What does "screen 17/18" mean in coffee?

The practice of grading coffee by screen size is a legacy of the commodity coffee trade, where uniformity was the primary goal. By sorting beans through a series of metal sieves with specific hole sizes, exporters can separate a harvest into different lots based on physical dimensions. A 'Screen 17/18' lot consists of beans that are large and relatively uniform, which historically commanded a premium because they were easier to roast evenly in large-scale drum roasters.

In the modern specialty coffee industry, the importance of screen size has diminished. We now understand that a bean's size is not a direct indicator of its flavor quality. A smaller bean from a high-altitude farm can be significantly more delicious than a large bean from a low-altitude farm. However, screen size remains a useful tool for logistics and roasting consistency. When beans are of a similar size, they tend to absorb heat at a similar rate, which helps the roaster avoid defects like 'tipping' or uneven development.

While you might see 'Screen 17/18' on a bag of coffee, it is best viewed as a technical specification rather than a quality guarantee. It tells you that the roaster has taken care to ensure the beans are uniform, which is a good sign of quality control. However, it should not be the primary factor in your purchasing decision. Instead, look for information about the origin, the variety, and the processing method, as these will give you a much better idea of what the coffee will taste like.

In practice, if you are a home roaster, having beans of a consistent screen size can make your life much easier. It allows you to dial in your roast profile with more precision. For the average consumer, however, the screen size is just one piece of the puzzle. It is a testament to the meticulous work that goes into preparing a high-quality lot of coffee, ensuring that every bean in your bag is ready to be roasted to perfection.