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. This is the double hump: where a larger minority (by volume) of grinds in the form of fines are over-extracting, the goal posts shift to the left of the brew control chart.
Home baristas often struggle to produce espresso shot times much over 20 seconds, even on the finest grind setting. The cost of installing a good home setup can create limits on extraction targets for home espresso. The extreme example of this is the spice grinder.
These are the main issues:
Spice grinders produce a massive amount of fines and a disproportionately large amount of boulders; probably the widest spread of any grinder type.
The excess of fines leads to over-extracted flavours which make the user naturally want to reduce extraction.
The excess of the boulders adds two more problems into the mix. The low surface area to mass ratio of the boulders promotes under-extraction while their larger diameter results in higher flow rates.