TDS or Total BS?

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Wine, Coffee, and Science

Wine is a 300 billion dollar a year industry. From farming to consumption, coopers, oenologists, viticulturists, sommeliers, and countless others have devoted their lives to studying wine to an exact science, all for the purpose of improving quality and taste. Throughout history, championed as one of the most complex substances, wine is known to contain around 300 different aromatic and volatile compounds contributing to its wide range of flavors. In comparison, coffee has been identified to contain around 900 of these organic compounds. With all the effort, money, and science dedicated to wine, it should be no surprise that coffee is gaining more attention as a “specialty” beverage. Besides, after a late night of over-indulging, why shouldn’t the quality of your morning coffee rival that of the wine you had the night before?

The Science of Refractometery

Like wine, the fusion of coffee and science is becoming more prevalent, and at times, is highly controversial. Among the new technologies devoted to improving the quality of coffee, are light refractometers. These devices measure the angle light bends as it passes through a liquid, in order to calculate brew strength by the percentage of coffee soluble material in your drink, commonly known as Total-Dissolved Solid (TDS) percentage. TDS can also be used to calculate extraction percentage, a signifier of brewing efficiency. Through years of research, it has been estimated that the overwhelming majority of coffee drinkers prefer their espresso within the range of 10-13% TDS, and unpressurized brewed coffees with a TDS between 1.25% and 1.45%, as well as an extraction percentage for both methods between 17.5% and 20.5%.

Though light refractometry has been received as a highly valuable practice by well-respected coffee elites, such as Matt Perger and Scott Rao, there are a vast number of experienced members within the coffee community who disagree. Many argue that measuring TDS removes the human element of brewing coffee, placing value in numbers above the palate of an experienced coffee professional, and if that were truly the case, I would oppose the use of refractometers as well.

Refractometry for the Beginner Barista

Over the past few months I have had the opportunity to use a light refractometer, and before I state my views, I must preclude them with this: I have no credibility in the global coffee community, and that I have been working in specialty coffee for less than a year. However, with my lack of experience, I possess something unique to those experienced baristas who deny the benefits of refractometry, and that is the luxury of utilizing numbers while learning to brew coffee as well as dial-in espresso. I must admit, it has helped me immensely.

Refractometry and Extraction

Taste is subjective. For any barista in-training, trying to identify flavor, mouth-feel, and various degrees of over- and under-extraction from what is being described by a trainer with a differing palate, can be quite difficult. As someone learning to dial-in espresso, the ability to identify distinct flavors and mouth feel with a number, allows for trainees to understand exactly what they are tasting and why. After my trials of playing with a refractometer, I now associate saltier tasting shots that lack finish with the number 16 (i.e. 16% extraction), and I understand that by grinding finer, my shots will become sweeter and I can attain a higher extraction. If I taste a shot that is highly astringent, with a finish that lingers, I associate it with a number above 19, suggesting my shot contains grounds that are much too fine and are highly over extracted, which can be corrected by coarsening my grind. This is not to say espresso cannot taste good at a 16% or above a 19% extraction, it will still depend on the grinder you are using, the coffee, the recipe, the overall extraction itself, and a myriad of other variables, which is why refractometers are not, and should not serve as a replacement for a palate. As once stated by Matt Perger, “Refractometers don’t dictate what you should serve, they just help you track and understand what you’re serving. If your coffees aren’t tasting great and a refractometer indicates your strength and extraction are lower than when it was previously delicious; is that the refractometer telling you to increase them? No. It’s telling you what’s different, and you’ve learnt why.”

Refractometry and Strength

Aside from gaining an understanding of the basic principles of grind size and its effects on over and under extraction of espresso, using numbers has allowed me to fully understand other variables of brewing. Using a refractometer and recording shot information has allowed me to witness and taste the inverse correlation between shot strength and extraction and their dependence on brewed coffee yield. Understanding this “yield compromise”, allows me to predict that if I increase my shot yield, extraction will increase, however, the strength of my espresso will decrease, as demonstrated by the figure below.

Figure 1: Matt Perger, Espresso Recipes with Barista Hustle

% Extraction= Extracted Mass/Dry Mass. By increasing yield, extracted mass increases, however, there will be more water in your brewed coffee, thus lowering brew strength.


For the Experienced Barista

Even for experienced baristas, refractometry can be of great use when developing new brewing recipes, as well as gaining a deeper understanding of each step of the brewing process. Whether it is a new single origin on espresso, or developing a new house recipe for Aeropress, measuring TDS% gives you quantitative proof of how a minor tweak in a recipe affects extraction. Even though one slight change to an Aeropress recipe alone may only increase an extraction by a few tenths of a percent, combining and improving multiple variables one by one may increase the extraction of your coffee by a couple percent, which, even for an inexperienced taster, can make for a noticeably more enjoyable cup of coffee.

Refractometry for Equipment Maintenance

Light refractometers can also be used as a diagnostic tool in assessing the general health of brewing equipment. During the last couple months I noticed that while using one specific grinder, I was unable to pull a shot of espresso with an extraction higher than ~17% without any unpalatable astringent aftertaste. Understanding that 17% was a relatively low extraction percentage, while still having an over extracted taste, I was able to reason that something may be wrong the grinders’ burrs. Over time, the grooves in burrs become less sharp, and the overall variance in sharpness between each groove will become greater and greater, ultimately producing coffee grounds of vastly different sizes (i.e. increasing particle distribution size), with the smallest of grounds, producing the highly undesirable flavor of over extraction, an occurrence demonstrated in the figure below. Sure enough, the burrs were worn and chipped. After installing new burrs, I was able to pull sweeter tasting shots with more clarity, with as high as 19% extractions.

Figure 2: MPE Chicago

Although it is an oversimplification of the particle size distribution (psd) produced by coffee grinders, the chart accurately portrays the general increase in psd as burrs age. (Actual psd distribution curves display a two-peaked curve, not a single-peak as shown above.)

A Reflection on Refraction

Coffee is a complex beverage, and as baristas and coffee professionals, it is our job to understand all of the intricacies involved in improving the ways we prepare it. The use of science and technology is incapable of, and is not meant to cheapen the process of creating excellent coffee. If anything, science and technology allow us to ask deeper questions, and gain a richer understanding of what we do, and how we can progress. Coffee farmers and producers abroad are constantly experimenting with new technologies like measuring water activity, soil pH, as well as altering the ways coffee is grown, processed, and sorted. On our end, the least we can do is experiment and open our minds to any new tools that allow us to make coffee better.