Posted Tue Feb 10, 2009, 2:48pm Subject: espresso versus drip grinding
I came across a posting on another site yesterday that claimed that the burrs designed for drip coffee are different from those designed for espresso.
The argument was that espresso burrs are designed to produce two humps in the coffee particle distribution - the normal size of the grind and smaller fines - that are required for a good espresso, but that burrs designed for drip coffee are made with only one hump - a uniform grind without fines.
The claim was that espresso burrs end with a flat rim at the outer edge of the burr that produces the fines, but that drip burrs do not.
Now, common sense tells me that if the burrs come to a flat rim at the outer edge, which mine certainly do, that would be the size of the ground coffee particle since no larger particle would make it through the burrs. If indeed there are fines, they would have to be smaller than that outer rim.
The posting was claiming that grounds that have no fines don't produce good espresso, which may or may not be true. What I'm more curious about is the basic claim that the burr design for espresso and the burr design for drip are fundamentally different and that a grinder that's good for espresso would of necessity be bad for drip, and vice versa.
Does anyone know anything about the theory of burr design? Roy
I'm afraid this statement doesn't hold water. Coffee is an agricultural product that is roasted, then literally smashed, broken, and crushed during grinding. This cannot possibly produce particles of exactly the same size. Particle sizing invariably shows a fairly broad range of grind particle diameters. For espresso grinders, you typically see two peaks: a larger diameter particle peak centered around 400-500um, and a much smaller diameter peak (the fines) centered around 40um.
This bimodal particle size distribution is thought to be desirable for espresso because the fines help regulate flow through the puck. For non-espresso brewing, fines overextract, muddy presspot coffee, and clog drip coffee filters.
Does anyone know anything about the theory of burr design?
Unfortunately, no. But there's a nice discussion of different burr types here. Note Jim's identification of different grinding zones on the burrs. Some burrs (including Mazzers) have channels that extend to the rim, and may allow certain sizes of particle to escape.
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