[vallode]

The paper[1] seems to imply agitation is exactly what this method is promoting: "Furthermore, acoustic streaming induced greater mixing and enhanced mass transfer during brewing.". I assume the 100W of ultrasonic energy would be pretty hard to reproduce by just shaking your cold brew container though!

[1]: https://www.sciencedirect.com/science/article/pii/S135041772...

[dahart]

Very interesting, and I missed the 100W bit, thanks! Yeah that would be really hard to do by hand for 2 minutes. Maybe Guinness records needs to see who can shake their cold brew hardest/longest. So this begs further questions for me, like can I shake with 10W for 30 minutes, or 10 minutes, or…? Does the frequency matter? Can we use one of those chem lab agitator machines to cold brew?

[kurthr]

I find it interesting that 100W for ~120 seconds is ~0.3kcal which for a 100ml cup is ~3C. They are right at the limit of power to flow rate. Much faster flow and presumably the cavitation wouldn't "brew" enough, while much slower and it would warm up the coffee noticeably. I'm doubtful the frequency matters much if the cavitation is what is causing the mixing since those are just bubbles emerging and popping, but the efficiency of coupling from ultrasonic wand to liquid could change a lot.

Since you could presumably put 2 of these in parallel and have 2x100ml cups in 2min with 200W without changing the recipe (or 1 cup in half the time), this seems pretty scalable with increased cost and area.

Unagitated cold brew is in the 10hour region, but with agitation/pump through it seems like you can do 8 cups in 20min which is almost as fast as the cavitation method. I suspect the grind size starts having really big effects here.

https://instantpot.com/products/instant-cold-brewer

[petre]

Yes, it's the frequency and also the amplitude that makes it faster. One could use a lab agitator but it would still be too slow. I think if you pour the water into an ultrasonic cleaner along with the coffee and filter the mix you might get the same result.

[fch42]

That's precisely the thought which has taken hold of me now as well.

The ultrasonic cleaner is always stashed away somewhere safe for its once-a-year use ... I think can't resist to experiment.

[aaarrm]

I think the size of the vibrations is important here. The paper mentions acoustic cavitation, which I believe would only really occur at small frequencies like the ones stated in the paper, not large shakes that you or I would do.