Refractometry and Espresso Quality

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Republished with permission from David Schomer. View Original.

There has been some good work done on using digital refractometry to asses total dissolved solids in coffee.  Naturally I was curious to see if a refractometry index (RI) reading could actually be a use ful number to asses the quality of espresso coffee.

I bought the Atago Pal 1 and read up on it.  Refractometery is the measurement of how much light is absorbed by your sample.  For wine making  they measure grape juice  and a high number correlates to a high sugar content, measured in “brix” units.  Hence the term, “brix meter”.   However, that has no bearing on the instruments usefulness to measure sugar content in coffee.  For example, if you measured a sample of India ink it would read 100 because it is very dense and blocks all the light from passing through the sample.

I went to work measuring the RI of many espresso shots, both excellent pours  and highly flawed shots.  Remember that excellent to me means that the shot preserves the fragrance of the ground coffee through the brewing process to be enjoyed as a flavor/aroma sensation.  Here is the test data:

  1. Espresso Dolce,  ristretto pour prepared with 17 grams of freshly ground coffee and extraction time of 25 to 27 seconds.  Flavor profile is caramel, leather, toast, dark chocolate cocoa powder, with blueberry notes. RI= 32.2  (average of about five shots, high 34.0  low 30.2)

  2. Stale Espresso Dolce, same pouring parameters.  Flavor profile metallic/ vinegary.   RI= 31.0 (two shots 28 and 34)

  3. Espresso Dolce ristretto prepared on a flat burr Mazzer (all other work on conical/flat DRM grinders)  Flavor profile, thinner but similar to #1.    RI= 28.0 (two shots, 26 and 30)

  4. Espresso Dolce ristretto , prepared with 200 degree brewing water instead of 203. Flavor profile sour/astringent.  RI= 28.0 one sample

  5. Indian varietal  ristretto, Northern Italian roast (my usual roast).  Flavor profile, very sweet caramel. RI= 31.0 one sample

  6. Indian varietal same as above, ristretto, very dark roast, oily beans. Flavor profile, burnt rubber, bitter.  RI= 30.6 one sample

  7. Espresso Dolce lungo pour 3.5 oz. in 25 seconds. Flavor profile, thin metallic, slightly sour.  RI= 20.7 one sample

It is easy to conclude from this brief study that flavor development and integrity to the fragrance are not measured in any way using digital refractometry.  The only interesting finding is that a coarser grind, yielding 3.5 oz liquid in 25 seconds, reduces the measured RI.  This tentatively  supports the assertion that digital refractometry might be a useful way to measure total dissolved solids in a brewing method.  So perhaps it is useful as a measure of brewing efficiency but will not tell you anything about the flavor profile.