Measuring Total Dissolved Solids

There is a lot of information already out there about measuring total dissolved solids (TDS) for espresso, but I think it’s worth summarizing some of it here, to lay the groundwork for a couple of upcoming posts.

When we pass water through an espresso puck, some of the coffee material is dissolved into the passing water. We can express this as TDS:

   

If we weigh the dry coffee and the output (solution), we can also calculate extraction yield:

   

TDS is related to the “strength” of the coffee and extraction yield is related to the “flavour” of the coffee, so we can see immediately that the following things might be good to measure:

• Mass of dry coffee
• Mass of solution
• Total dissolved solids

There are a couple of main ways of measuring TDS:

• A digital refractometer. This is the gold standard. A good digital refractometer, like the one from VST, will measure TDS with an accuracy of 0.03%, and will set you back about $650. A cheaper digital refractometer, like the one from Atago, will measure to 0.1% for about $350.
• A traditional refractometer. These use simple optics to measure TDS. With practice, you can read these with an accuracy of about 0.2%. A cheap Brix refractometer can be purchased for about $20.

Robert McKeon Aloe wrote an excellent article which gives a detailed comparison of these two options.

I decided to pick up a Brix refractometer like the one linked above as an inexpensive way to try out this new measurement, then I added TDS measurements to my workflow. I’ve often added measurements speculatively like this—not being completely sure how I will use the data. Often, it’s only when I’ve collected a few measurements and I’m exploring the data that it becomes clear how I can use them.

To measure TDS, I start by taking a sample of my espresso shortly after it’s come out of the machine. I stir with a small spoon to incorporate the crema, and as I’m stirring I take a sample using a transfer pipette. I squeeze the bulb three times, pushing fluid in and out of the pipette, to reduce the effect of any contaminants that might remain inside the pipette.

Once I’ve got the sample, I tip it so the liquid flows into the bulb, then set it aside while I enjoy my espresso and clean up, so that the fluid has time to reach room temperature. This is important, since TDS measurements are affected by the temperature of the sample.

Once the liquid has cooled, I give the pipette a shake, with the bulb end down, to mix the sample, then I squeeze a little of the sample into the sink, put three drops on the refractometer, and close the lid.

I’ve found it’s easiest to read the refractometer if I hold a flashlight so that it illuminates the sample. The boundary between blue and white indicates the measurement.

Looking through the refractometer, the boundary between blue and white is a little blurry. This is where practice comes in. With practice, you can quickly get a feel for where the center of the transition is, and this is what I take as my measurement.

To convert from Brix to TDS, I use the following formula, which is derived from experimental measurements:

   

Some sources recommend filtering the solution before measuring TDS. I’ve chosen to omit this step here for a couple of reasons:

• First, I wanted to keep the procedure as simple as possible. One of my main goals here was just to experiment with adding TDS measurements to my workflow. If the measurements are too fussy, I’m more likely not to do them at all.
• Second, this article by Robert McKeon Aloe looks at the differences between TDS measurements with and without a filter. Ultimately, he finds that filters make a relatively small difference in the TDS measurement. Since I’m using a cheap Brix refractometer, I figured this would not be a major source of error.

In my next post, I’ll look at brewing control charts, which give us a way to visualize the strength and extraction for a given shot, as well as the effect of changing brew ratio and time.

A note on waste: The transfer pipettes I linked above can be cleaned and reused dozens, if not hundreds, of times. After I’ve made a TDS measurement, I pour a small amount of distilled water into a cup, draw it into the pipette, shake vigorously, squeeze the water out of the pipette, and repeat three times. The small amount of residue that’s left in the pipette is unlikely to affect the results of future measurements.