The friction created in grinding generates huge amounts of heat, which means that in a working grinder, the coffee temperature can reach 100° C (M Petracco, 2005).1 As any barista has experienced, these temperature changes can have dramatic effects on the way the coffee behaves, with the shots gradually running faster as the grinder gets hotter. The intuitive explanation often given for this is that the metal parts of the grinder expand at higher temperatures, increasing the gap between the burrs. However, this appears not to be the case, as the expected expansion of metal for such a small change in temperature would be negligible.
According to Illy’s Espresso Coffee: The Science of Quality (M Petracco, 2005),1 increased temperature has a second effect that may also affect the way the shot runs. Coffee oils are very viscous at room temperature, but they begin to become more fluid above 40° C. When this happens, the oils can easily flow out from particles through microscopic cracks and coat the outer surfaces with a sticky layer that becomes semisolid again as the grounds cool down after grinding. This causes the particles to stick together into clumps, which can make the extraction of these particles less even and alter resistance to flow.
The high temperatures experienced during grinding may also speed up degassing and oxidation, even in the short time it takes for coffee to pass through the grinder. Coffee grounds inside the grinding chamber can reach temperatures as high as 80–100° C (176–212° F) (M Petracco, 2005).1 Coffee that is ground in an overheated grinder may thus experience some loss of aroma, although there isn’t currently any published research to show whether this is significant.