19% and Below: When to go Low
James Hoffmann, 2007 World Barista Champion, coined the term the double hump [of coffee extraction] in a blog post from 2010 with some help from Scott Rao and Andy Schecter. In short, they observed there can be a slight improvement in the flavour of under-extracted coffee around 15-16% extraction yield. This extraction yield was optimised for surface extraction rather than surface and internal grinds extraction. With a less effective grinder, a barista will tend to find the symptoms of over-extraction in an under-extracted brew.