Nicholas, Jon, and all other hopper versus single dose testers -- please listen up.

The Hypothesis

In all the heat about whether single dose grinding is worse than hopper grinding, the really interesting question has been lost: how the heck can you get two different grinds from the same burrs at the same burrs spacing just by changing the ways the beans feed? As a penance for contributing to all the heat, I thought hard about this. Here's what I have:

The diagram shows the two alternatives by looking at the upper and lower burrs edge on, with the beans exiting from the gap between them.




Testing the Hypothesis

There is no way the contrast between single layer/single dose, multiple layers in hopper grinding is 100%. But the hypothesis implies there will be more multi-layer ground particles in the hopper grind. This means that the distribution of coarse grinds will be more dispersed for the hopper grind than in the single dose grind.

In the TGP, we found that the conicals were more forgiving than the flat burr, since the dispersion of coarse grinds was wider. This means that if the hypothesis is correct, hopper grinding will be more forgiving than single dose grinding..

This should be a lot easier to test than taste. Once you have both grinders dialed in, change the setting by a fixed amount on each. The flow and shot timing on the hopper one should be less affected than on the single dose one.

It may also mean that with meticulous grinder prep, there will be little difference between the methods, while with more casual prep, the hopper grinds will produce more successful shots.

another_jim wrote: there will be more multi-layer ground particles in the hopper grind. This means that the distribution of coarse grinds will be more dispersed for the hopper grind than in the single dose grind.

You lost me there.

This is an interesting conundrum, from both a practical and logical standpoint. I expected per-dose grinding to result in more fines and a wider distribution of particle sizes, but the need for significantly finer grind settings suggests the opposite.

Another possible explanation: in per-shot dosing, the bean fragments bounce around more, allowing larger particles to find exit paths from the burrs more easily. With a hopper load, the beans may be forced into "crushing" trajectories.

I'm still trying to get my head wrapped around this.

RapidCoffee wrote:Another possible explanation: in per-shot dosing, the bean fragments bounce around more, allowing larger particles to find exit paths from the burrs more easily. With a hopper load, the beans may be forced into "crushing" trajectories.

That seems plausible, and I think that's similar to what Jim is saying.

Here's another way of looking at it: with hopper grinding, the space between the burrs is packed more fully with coffee. With per-dose grinding, the coffee is less tightly packed.

The tighter the pack, the greater the tendency for coffee to be ground against itself. As well, a tighter pack forces the coffee into more intimate (and destructive) contact with the burrs.

Hence, we get finer grinding with the hopper filled. That's my story and I'm sticking to it.

John, is there any way we can get a particle size analysis done on a couple of grind samples so we can see what the difference is? I bet we could take up a collection to cover the cost...or maybe you could just bring the grinder and an espresso maker and pay with cappuccinos

This would seem to be an elementary piece of information to help us attempt to understand what is really happening as far as the difference between grinding with a full hopper and grinder per shot. It is hard to imagine why we wouldn't want to get this done.

It would be pretty straightforward to get samples: make a few espresso shots each way and when the timing for a measured dose gets consistent then grind a sample and bag it & tag it.

Once this data is measured then we could correlate the information with taste testing and maybe (MAYBE) get some real idea of what is really happening when we grind loaded or single-dosed.

AndyS wrote:You lost me there.

The argument for a wider distribution is a little more flimsy than the one for finer grinds with a hopper. There's two points:

• With single dosing, all the beans are relatively unobstructed, while with hopper grinding, some of the beans are unobstructed, and some are obstructed. Hence with hopper grinding, the bean face more varied circumstances as they go through the burrs. This creates a wider distribution.
• A wider distribution around the same mean will get slower flow due to better packing. So the hopper grind is likely to be both finer and more widely dispersed

The counterargument may be more compelling. The dispersion goes from zero to whatever the gap between the burrs is. The gap is the same for both, so the hopper grinds' average particles are smaller.

Deciding between these two versions means trying to figure how grinding affects skewness, kurtosis and the unnamed higher moments about the mean that describe the particle distribution. At this point, even a lazy cheapskate like me starts preferring data to inference.

My main point is that if this argument is roughly right, then at single dosing, a grinder will be more finicky for grind adjustments than with hopper grinding. This is clearly easier to test and replicate than taste differences.

JonR10 wrote:John, is there any way we can get a particle size analysis done on a couple of grind samples so we can see what the difference is? I bet we could take up a collection to cover the cost...or maybe you could just bring the grinder and an espresso maker and pay with cappucinos

There are soooooo many interesting things to examine with a laser diffractometer: different grinders, grind adjustments, different coffees, different roasts, staling-related changes in grind, bean load etc. Testing a couple of samples sounds easy enough, but since the diffractometer is owned and managed by a diffferent department, usage is not that simple. I need to make advance arrangements for its rental, working around official scheduled use; calibrate it for coffee grinds; actually run the tests; extract the data from a proprietary format; yada yada. So I'd rather run a set of tests, all in one swell foop. Realistically, that will have to wait until the summer.

RapidCoffee wrote:So I'd rather run a set of tests, all in one swell foop. Realistically, that will have to wait until the summer.

Makes sense....
Too bad that diffractometer isn't in your garage!

another_jim wrote:Deciding between these two versions means trying to figure how grinding affects skewness, kurtosis

Kurtosis?

Is that like, bad breath?