Low Extraction = Uneven Extraction = Worn or Bad Burrs?

Grinders are one of the keys to exceptional espresso. Discuss them here.
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#1: Post by another_jim »

peppersass wrote:A few years ago, mitch, who is one of our members, was dissatisfied with the shots coming from his Robur and Strada EP. He felt they were under-extracted. Mitch bought a refractometer and found that, indeed, the extraction yield was quite low. After much troubleshooting, Mitch determined that his Robur had a bad set of burrs.
This post by Dick has rung a geometric bell in my head. My theory is simple. People who have chronic low extraction problems, i.e. which cannot be solved simply by grinding finer (and, for espresso, dosing lower) may actually have uneven extraction brought about by misaligned or worn burrs.

Here's the geometrical part. People have assumed all uneven extraction would show up as a bad tasting shot that has a proper extraction level; after all half the coffee overextracts, half underextracts, so it evens out. This is true if there is improper percolation; but it misses the geometry of grinding: The large particles underextract, the small particles overextract, so most of the coffee mass, in the large particles, is underextracting. Therefore, when coffee is not extracting evenly due to grind problems, it will measure and taste underextracted.

If you can't get your brewed coffee tasting buttery and mild, no matter how fine you grind; or your espresso tasting mild, no matter how low you dose and fine you grind; it may be that your grinder needs to have its burrs replaced and its alignment checked.
Jim Schulman