I started digging around and found JimS's extraction yield study, which some may not have seen for while:

http://www.coffeecuppers.com/Espresso.htm

Interesting read - in the Monolith thread folks have been writing about grinder alignment and extraction yield. Jim's study seems to indicate that EY of 25% is the norm for Italian espresso, for example. Or am I missing something?

PC

If I'm not mistaken, that preceded the era of using refractometers to measure EY.

I have no experience measuring EY. But maybe the oven drying method is slightly less precise than refractometry. In that post you linked to Jim cites some challenges with the oven method (e.g. recovering stray grinds from shower screens after a shot etc)

Again, just posing the question. Someone please correct me if I'm wrong.

Breaking the puck up and air drying it at ambient temperature should provide accurate info. When dried in an elevated temperature environment the results will be skewed by the initial moisture in the fresh ground coffee not accounted for as the elevated temperature drying should remove most if not all moisture. Other than that the accuracy should be quite acceptable.

Two things. First the method measures total solids which is of course going to be higher than dissolved solids (as a refractometer reports). The other OldNuc mentioned, the moisture in the fresh grounds.

A simple example:

Suppose you dry a puck and calculate a total solids extraction of 25%. If 5% of the solids in the cup were undissolved that would shift the extraction up by 1%, so the dissolved solids extraction would be 24%. Now suppose that the fresh grounds had a 2% moisture content. That would shift the extraction an additional 2%, to 22%, which is essentially what a refractometer would give you.

OldNuc wrote:Breaking the puck up and air drying it at ambient temperature should provide accurate info.

I don't think it's true that the moisture content of grounds will return to the same level they started.

I'd believe claims about difficult to extract coffees being used to test the efficiency of a grinder more if they used something like Heart Stereo Blend, not a deeply roasted soft Brazilian coffee.

I'm still impressed with the simplicity and value of the Monolith, I just don't buy the claim that anything from Vivace being hard to get higher EY than others. To me, that's no different than Orphan Espresso using Redbird to show how easy the Lido grinder is to crank, when a great deal of the market are using harder, lighter roasts. I don't blame anyone for putting their product in the best lighting, but I wouldn't call it objective evidence either.

jpender wrote:I don't think it's true that the moisture content of grounds will return to the same level they started.

They will be much closer to the fresh ground unextracted coffee than what comes out of an elevated temperature environment. That is probably going to be close enough to induce a negligible error in the extraction calculation.

Have you tried it?

Coffees don't all start with the same moisture content. How would they know to go back to that level? Wouldn't it depend on the ambient humidity?

I haven't allowed a wet puck to sit around for the hours or days it would take to reach some sort of equilibrium. But I've charted weight vs time for dehydrated pucks removed from the oven and exposed to the air. After 90 minutes one of them had gained nearly 5% in weight (the fresh grounds were 1.5% moisture).

This shows two pucks each dried in the oven and then exposed to the air for 90 minutes, then re-dried in the oven and kept tightly covered for 90 minutes. I dried fresh grounds to measure the moisture content. The red coffee had 1.5% moisture and the blue was 3.6%.

There is much more data available on the kiln drying of wood to a much lower moisture content after sawing. After kiln drying process to the nominal moisture content range it is removed and air dried to acclimate to the environment and this is not a long time process. A coffee puck is much smaller and complete disassembly on a full sheet pan will allow relative rapid drying and long term stabilization should not be required. Partial drying can be accomplished in that 120F oven and then placed in the kitchen environment to stabilize.

Yes, I have dried out numerous pucks and decided I could find a better way to spend my time. The process does produce interesting info but on the big scheme of things it does not have that much value as far as I could determine.

You could assume that the moisture content of roasted coffee was zero and go from there and live with the error.

JimS did some measurements on the oven drying I think - worth a look if you're interested.

I air dried mine over a period of days or weeks - the correct process is to keep a sample of unextracted grounds under the same conditions as the extracted pucks, and weigh the extracted pucks until they stop losing weight and stablize.

Anyway, the idea for me was just to get a rough % extraction, which turned out very consistently at just under 20%. I certainly did not have a very precisely aligned grinder, so I'm a bit skeptical on the effects of grinder alignment on % extraction, but others seem to have found evidence for this.

JimS' point was that percent extraction varied with a variety of factors including machine, basket, grinder, dose, bean, etc. I also went back to the original HB posts on refraction - interesting reading.