![]() Credit: Rebeckah Burke, University of Rochester. The cells are about 40 microns in diameter. The cellular structure of roasted coffee seen under an electron microscope. When we grind finer, we apply more cuts to the beans, and force them through a smaller gap, and both effects tend to increase the production of fines. Rather, the amount of fines will change, which means that the height of the fines peak will change if we represent it in a figure like the one above. But in short, the most important point here is that the location of the fines peak does not move left and right as you change your grind size. ![]() I think this is a consequence of fractal-like patterns of fractures that arise when crushing beans that stop propagating to smaller scales when they get to a similar size as the coffee cells, and they have no significant structure smaller the cells to propagate into. I believe it is not a coincidence that, regardless of the grinder, burr, and grind size, this peak is always located near the actual size of coffee cells ( about 40 microns). Those are what I usually call coffee fines, and I’ll therefore refer to this peak as the “fines peak” they are a consequence of crushing the coffee beans rather than cutting them cleanly, and their size depends on the microscopic structure of coffee beans. The other peak at left consists of particles of a much smaller size. The rotation speed of a grinder can also affect the width of the target peak, but in my experience the exact effect it has seems to be very different for different types of burrs. The width of the target peak can change from one grinder to the next, and it also depends significantly on burr alignment and perhaps even more importantly, on the geometry of the burr teeth, and on the burr materials and coating, both things that I do not think get their fair share of appreciation. This is why I refer to this peak at the right as the “target peak”, because it consists in coffee particles of the desired size, and which we have control over. As you change your grind size, this peak will move left or right, towards smaller or larger particle sizes. Basically, you have a peak at larger particle sizes that depends on how much spacing you have between the grinder burrs, which is usually how we adjust our desired grind size.
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