What causes one bean variety to require a significantly different grind than another variety when roasted and brewed (espresso) under the same conditions?

Example: Right now I have two batches I roasted up a week ago. One is a Brazil/Yemen 50/50. The other is all sumatra. The sumatra requires a much finer grind than the brazil/yemen blend, a couple full notches on my mazzer. I've noticed the same thing when dealing with blends containing monsooned malabar, a much finer grind is required.

What is it about these beans that causes this phenomenon? The only thing I can really note is that the Sumatra beans are noticeably larger after roasting compared to Brazil or Yemen.

Right now I've been experimenting with a 50% Sumatra, 25% Brazil, 25% yemen blend and I'm seeing that the high amount of Sumatra seems to lead to unreliable pours. Sometimes the pour will go from mostly one side. I know it's not my technique or equipment, If I pull a shot of only the Brazil & Yemen blend it turns out perfect, but add in the Sumatra and things go haywire.

Has anyone else experienced this? It would be great if someone could explain why this is happening. For now I'll try lowering the Sumatra content until the pours stabilize.

another_jim wrote:Soft beans like Sumatra or aged varieties create fewer fines when you grind them

Thanks, that makes sense. I wonder why though? Is it just a softer bean and that's what causes this?

HB wrote:Tim at Counter Culture pointed out another explanation: Blending coffees with different hardness can lead to grind inconsistencies as the grinder's burr pressure shifts. He demonstrated this effect by grinding coffees separately and then mixing the grounds versus grinding the beans together; the post blend pours were consistently channel free, the pre blend pours were not.

Uh oh... as if daily grind adjustments weren't bad enough already!

This may be worth an experiment, and I'd be curious to know if it worked: Freeze the blend. Grind it without thawing it, right out of the freezer. The soft beans should be hard enough to produce a reasonable amount of fines.