Selecting green Bean - screen size

All green coffee is bought with a screen size reference. This is a process where a set amount [we use 300g] of green coffee is passed through a series of interlocking trays placed one on top of the other. The largest screen size [19] is on the top, smallest size [13] on the bottom. The screens are shaken vigorously by hand and beans are either retained by the screens or fall through the various screens. Retained beans in each tray counted and a percentage can be worked out of each different screen.

Whys is screen size important ? 

Consistency in roasting.