Partial Dissolved Solids: Evaluating an Extraction Curve

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By Mike Strumpf

Nothing gets me excited about the technical aspects of brewing quite like the Brewers Cup competition, and attending the U.S. Coffee Championship Qualifying Events got me thinking:

What would it look like if we created an Extraction Curve that shows dissolved solids levels throughout extraction that is similar to how a roast curve shows bean temperature throughout roasting? How would we do this? What language would we use to evaluate such a curve?

We ran an experiment to make this Extraction Curve idea a reality, and the method and results can be seen below. Each evaluation point in the extraction cycle gives us a snapshot of the final dissolved solids of the cup of coffee, which are being referred to as partial dissolved solids.

It is known that decaffeinated coffee extracts differently from non-decaffeinated coffee, though I have never fully understood how. I decided to map the Extraction Curve of the same coffee before and after decaffeination to both test out Extraction Curves and to learn about the differences in extraction between the two coffees. Here’s what we know already about brewing decaffeinated coffee:

  • In past research with Mahlkonig, we found that there is no significant difference in grind particle distribution of decaffeinated and non-decaffeinated coffees.
  • Decaffeinated coffee starts out with fewer solids available than non-decaffeinated coffee (due to cleaning and removal of caffeine). This leads to a lower TDS of decaffeinated coffee than of non-decaffeinated coffee using the same coffee to water ratio.
  • Tasty coffee has an extraction percentage between 18 and 22% (weight of brewed coffee*TDS percentage/weight of coffee grounds).

It has been anecdotally known that decaffeinated coffee is quick to give up its solids (often referred to as more soluble), but I had never seen proof of this. To find out what is happening, we ran an experiment which consisted of brewing the before decaffeination and after decaffeination versions of the same coffee on a Fetco CBS2021 using the same brew profile. While the coffee was brewing, samples were taken from the exit stream of the brew basket every 15 seconds and measured the TDS using a VST LAB Coffee III. We confirmed that our decaffeinated coffee releases solids faster than non-decaffeinated coffee, and now I can sleep better at night.

The final TDS of the decaffeinated coffee was 1.2% and the TDS of the non-decaffeinated coffee was 1.44%. The graph below shows the readings we got during the experiment, normalized to the final TDS of each brew. This normalization means that the TDS readings shown on the graph are the difference between TDS (at the time on the X-Axis) and the final TDS of the brew. My analysis of the graph is that the decaffeinated coffee (shown in red) quickly solids in the beginning of the brew cycle, and then has few solids left to release in the second half of the brew cycle.

Before and After Decaffeination Extraction Curve

Once I knew for sure that decaffeinated coffee released its solids uniquely, I pondered which brewing variables should be altered to compensate for this knowledge. I couldn’t think of anyone better to get advice from than coffee brewing competitors! Here are some of the responses to my question of “What would you modify in your brewing process to adjust for a coffee that releases soluble solids quickly?”

“My first instinct would be to coarsen my grind in order to slow the release and not over extract my cup. With a coarser grind I would typically updose second. With solubility changes from one coffee to another I would change one variable, document my changes, then taste, taste, taste. Change the next variable and repeat. Depending on how it’s brewing, I may also increase my water ppm as a final variable to change and taste test. With a goal of just making that coffee oh so yummy.” – Todd Goldsworthy, Klatch Coffee, 2014 and 2016 US Brewers Cup Champion

“When working with a coffee that releases soluble solids quickly I want to be more gentle in my approach. That can come in many forms. With espresso that can be starting with a pre-infusion at a lower pressure to saturate the bed before applying full pressure. With brews that can be using fewer pulses at a more gentle pace. This can also be reducing the bloom time to help shorten the overall brew time.” – Sarah Anderson, Intelligentsia Coffee, 2015 US Brewers Cup Champion

“The main adjustment I make to adjust for solubility is grind size, and consequentially extraction time, increasing size relative to solubility. I will increase turbulence with less soluble coffees on a programmable batch brewer, but prefer to not change too much technique when manual brewing. As a general rule, I start with water at 200 degrees F. I will add heat to boost acidity, floral, and fruit notes, lower temperatures if I want to bring out lower tones and mute acidity. I find that within a narrow range, temperature has a greater impact on what is extracted than on how quickly the extraction occurs.” – Ben Jones, Batdorf and Bronson Coffee Roasters, 2016 U.S. AeroPress Champion

“For a more soluble coffee, I would shoot for less contact time, and less water to avoid over extraction. I would use a tighter ratio of coffee to water, and a slightly larger grind size. (I was curious so I actually tried this out at my cafe, and the decaf tasted amazing. My recipe was a 1/14 ratio, finishing at 2:45 total brew time.)” – Blair Smith, Augie’s Coffee Roasters, US Brewers Cup competitor and judge

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The common suggestion given across the board was to increase the grind size and speed up the extraction time. Another suggestion was to shorten the bloom time, which makes sense with decaffeinated coffee since it degasses faster than non-decaffeinated coffee. Using a coarsely ground coffee can lead to low strength, and the suggestion to increase the coffee to water ratio is a good method for increasing both the strength (soluble concentration) and extraction (soluble yield) of your cup of coffee.

In researching the concept of an Extraction Curve, I came across a great article from Pilot Coffee titled “The Fix: Coffee Solubility and Age.” Pilot came to the same conclusions about how to compensate for highly soluble coffee. But don’t take our two experiments as the end of research on this subject! Try mapping out the Extraction Curves of your own coffees and brew methods and you’ll be impressed by how much you can learn.

Mike Strumpf is the Director of Coffee at the Swiss Water Decaffeinated Coffee Company. He is a licensed Q Grader, WBC Head Judge, and likes coffee.

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