Understanding retention

Grinders are one of the keys to exceptional espresso. Discuss them here.
cunim

Postby cunim » Jan 10, 2019, 9:21 am

I have seen two methods for calculating retention. The easiest is to weigh the beans going in and the grounds coming out. However, that seems flawed in that you don't know what is coming out. Some of your fresh beans remain in the grinder and some of what comes out was there before. In fact, my Mignon Specialita sometimes get more out (by a half gram or so than I put in. I suspect it is not manufacturing coffee so that is unnerving. Mixing is also noticeable if I make a shot of decaf in the evening and feel .... wired. The day's standard beans are still getting into the shot, even though the output is almost exactly the same as the input. In/out is a meaningless specification as far as I can see and my little Mignon does retain more than I would like.

A much more tedious method is to clean the grinder, put through a number of doses and then sweep out everything left and weigh that. This seems to be a more reasonable measure of retention, but it is rarely done. Does anyone have a retention spec that is easier than the clean/residue method but more meaningful than in/out?

Or I guess we avoid the issue and purge - which I find offends my ingrained aversion to wasting food - or go with something designed ground up for single dosing. Expensive.

Markant

Postby Markant » Jan 10, 2019, 10:19 am

In his review of the Niche Zero Grinder, DaveC gives his thoughts on "retention" . A very good read!

There are 3 key areas that we should care
about in a grinder, in addition to grind quality.

Total Retention

Dose Consistency

Exchange


https://www.nichecoffee.co.uk/wp-content/uploads/2017/12/Niche-zero-review.pdf

walt_in_hawaii

Postby walt_in_hawaii » Jan 10, 2019, 1:03 pm

Cunim, some very good points you raise. I don't waste coffee via 'priming' either. There are practical things you can do to minimize or nearly eliminate some of what you are talking about. Putting felt washer arms under your sweepers helps,(flat burrs) and I have made my own sweeping arms (delrin) under my burr to sweep the chamber clean. Nearly zero retention. I haven't solved the little bit of coffee grounds that occasionally lies inside the burr teeth themselves (k10, large conical burrs). The addition of a small cake frosting air bellows O ringed into the hopper space helps to pulse and blow out grounds and the retention is minimized to the point where it doesn't bother me.

cunim

Postby cunim » Jan 12, 2019, 10:37 am

Thanks for the link and the pointers. To my mind, grounds that stay in nooks and crannies should not affect output. Sadly, a machine that has a lot of internal retention will probably not sequester that retention very well and I suspect that total internal retention correlates fairly well with mixing. Therefore, total internal retention is a valuable measurement tool.

When single dosing weighed beans, consistency is not a problem because timing is not relevant. Just grind til it's all done. I usually put 21.5 g in, and get between 21 and 22 g out. Very rare to see an outlier.

Mixing is the key issue, and that is easily demonstrated by grinding beige cleaning beans, and then purging small doses of coffee (similar to this thread WLL Grinder Retention Test). You will see beige grinds mixed in with the first purges, until the cleaning beans have been flushed out. This might provide an estimate of mixing. Perhaps reviewers could give us a "flushes to clean output" observation.

As a practical matter with the Mignon, one 5 g purge pushes out a lot (certainly not all) of the cleaning beans. After seeing that, I feel reasonably safe with a single 5 g purge when going from standard to decaf beans. I might not purge at all when just changing beans, accepting a gram or two of mixing.

'Course, the real answer is to get a low retention grinder designed for single dosing. I will, at some point.

Nunas

Postby Nunas » Jan 12, 2019, 2:57 pm

The easiest is to weigh the beans going in and the grounds coming out.

I agree with your analysis, as evidenced with my Sette. My 7-270Wi is dead nuts on by weight nearly every time I use it from first grind to last on any given day. Even when its off the odd time, it is only by 0.1 of a gram. However, that first shot of the day runs a little more quickly than subsequent shots. This is definitely due to retention. The retained grounds come out on the first shot, but the last few grinds are again retained. Since they are fresher than the previous retention (a few minutes vs a day), the shot runs normally. For this method of measuring retention to work, one would have to first clean the grinder, then put in the measured beans to run the test.
A much more tedious method is to clean the grinder, put through a number of doses and then sweep out everything left and weigh that. This seems to be a more reasonable measure of retention, but it is rarely done.

I've done exactly this several times with my 270-Wi, as it is often written that this is a zero-retention grinder, which does not match my experience. The result is I get 1.7 to 1.8 grams of retained grounds out. The bulk of this is retained in the six little arc-shaped holes in the plastic part in the bottom of the grinding mechanism. I should note that I grind quite fine and roast to FC+. Perhaps a coarser grind or a lighter roast would yield zero retention; I've never tested this as I use a different grinder for drinks other than espresso. So, for my use, at least, I consider the Sette series 'low retention', not 'no retention' grinders.
Or I guess we avoid the issue and purge - which I find offends my ingrained aversion to wasting food - or go with something designed ground up for single dosing. Expensive.

I used to purge, by setting one of the pre-sets on the 7-270Wi to minimum (5 grams). I gave it up mainly because we could not taste the difference between first shot coffee and subsequent shot coffee. Admittedly, I don't consider myself to have 'golden taste buds'. To minimize any effect, I changed the way I do my pulls. My spouse and I each drink a doppio first thing in the morning. I pull two doubles, split. Thus, the amount of coffee from retained grounds is less than one gram in each cup.