Grinding is a simple process involving cutting and crushing the rhizomes into small particles, then sifting through a series of several screens. Depending on the type of mill, and the speed of crushing, the spice may heat up and volatiles be lost. In the case of turmeric, heat and oxygen during the process may contribute to curcumin degradation. Cryogenic milling under liquid nitrogen prevents oxidation and volatile loss, but it is expensive and not widespread in the industry. Ground spices are size sorted through screens, and the larger particles can be further ground. Most quality control laboratories use the U.S. Standards (U.S.S.) screen size system. However, there are other systems that use a different numbering, and comparisons between specifications may be difficult. For instance, the U.S.S. screen numbering goes from 4 to 80 mesh screens (i.e. 4 to 80 openings per inch), while the Mill screen system goes from 4 to 55 mesh with different increments than the U.S.S. system.
