Measuring the Concentration of Espresso Shots

If you are wondering why it is widely recommended to use syringe filters when you measure the concentration of espresso shots with a refractometer, or how bad exactly a measurement would be if you don’t use such a filter, I highly recommend this recent blog post by my friend Mitch Hale. And while you’re there, you should also check out his new blog, which already has a method on how to align your grinder, and much more geeky stuff to come.

Mitch ran a very dedicated experiment to compare the precision and accuracy of filtration by VST syringe filters as well as centrifuging espresso samples. The results are very clear: while the precision inherent to syringe filters is as good as the internal precision of the VST Coffee lab III refractometer (0.01% total dissolved solids), not using the syringe filters will give you concentration measurements around 0.38% too high, and way less precise on top of that.

This graph shows a kernel density estimation of the frequency of measurements that are discrepant by a given number. Mitch obtained two samples and two measurements with three different methods, for a total of 20 espresso shots. These curves tell you how often the two measurements disagreed by a given quantity. You can see that the unfiltered samples can disagree by a larger number on average, and even worse, the disagreement is highly unpredictable. This means unfiltered espresso samples are not reliable for measurements of concentration.

Even in a best-case scenario where all coffee roasts and origins have the same amount of oils and suspended solids (this is most likely false), deciding not to filter your espresso sample, and instead subtract 0.38% from it, would result in a very degraded precision of about 0.1%, instead of 0.01%. Hence, I highly recommend that you always filter your espresso shots before you measure their concentration.

Another thing that Mitch concluded from his experiment is that centrifuging allows to obtain measurements as accurate as the VST syringe filters, and that contrary to some popular worries, the syringe filters do not bias measurements by filtering out some of the coffee’s dissolved solids.

This graph is very similar to the last one, but this time it shows the difference between measurements taken with unfiltered samples and those taken with filtered samples. You can see that unfiltered samples will on average seem 0.38% too high in concentration, but that difference is rather unpredictable – this difference itself varies by about 0.11%, so it is not possible to just correct your unfiltered measurement by subtracting 0.38% to it.

Please have a look at his blog post for much more detailed results, and a very detailed description of his experiment.

Another perk from Mitch: he had the very ingenious idea to add a full glossary section to his website, which I will now link to in the Resources menu of my blog. This way, every time you encounter a weird geeky word that you’re not sure about, you can consult his glossary !

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