Welcome to day 11 of 30.
Today I am going to go into what seems like a taboo topic on TDS meters and refractometers. I find them over rated and quite useless when it comes to true practical use. Of course I can't just go into a full trash talk session since (as usual) there are some reasons they can be useful.
It seems like everywhere I go these days someone is slowly filtering their coffee through a syringe with the intent of turning coffee into some sort of voodoo magic. With the wholesale adoption of TDS meters and refractometers, a new metric has emerged for baristas and coffee professionals to use. Hearing questions like "What is the TDS?" or "Did you hit 20%?" is not out of the question these days. Unfortunately the focus on these metrics has led to a less intuitive understanding of coffee for newer baristas.
A refractometer is basically a small light beam that determines how much of the light is being refracted through a solution of coffee (*note* there is a lot more to refractometers than I will be talking about here, so don't think I'm ignoring the way they work). There are other ways to determine TDS in coffee, but the refractometer has become the tool of choice for coffee measurements. Through the use of a refractometer and some simple math and measurements, the Total Dissolved Solids, Brew strength, and Extraction yield can be determined. These measurements can help you understand how much of the coffee has been extracted into the coffee solution. So it makes sense to think that since we can measure small variances in the brewed coffee, we can create perfectly repeatable results.
The truth on the other hand is that we still cannot.
The nature of using a refractometer is that you must make the brew first and then measure the results. This is the definition of a reactionary approach to brewing. You brew, measure, brew again to compensate, and measure again. Granted we have charts that show where our brew lines up as far as "ideal" ranges in extraction, but those are actually very large targets to hit. Additionally, we must understand that we are currently overlooking some things about brewed coffee, specifically suspended particles.
The most common refractometer used for coffee measurements recommends using a syringe filter to get only the small (ideally 2 microns or less) particles which are dissolved in the liquid. But if we look at standard paper filter brewed coffee, the paper tends to allow particles about 20 microns and smaller through, which is considerably more range than just the dissolved sub 2 micron particles. So with a filter brewed coffee we have larger suspended particles that are not being measured by our standard measurement. Of course we don't have to filter the coffee sample through a syringe filter, but some theories indicate that it can help reduce "noise" in the sample. Noise would be particles that are not translucent and therefore block some of the light rather than refracting it.
The common term for the full combination of particles in coffee liquid is "Total Brew Solids", which I think makes sense to measure. The problem is that this requires some proper lab equipment, including a dehydrator and scale the can measure tiny quantities, as well as a lot of time. Ultimately (at least right now) it is not feasible to do a Total Brew Solids measurement in any active cafe or standard preparation environment.
Beyond paper filter brewed coffee, we get "settleable solids" when we make coffee with metal filters like those used in a French Press or espresso. So now we have a combination of <2 micron dissolved particles, 2-20 micron suspended/dissolved particles, and temporarily suspended particles >20 microns depending on if we are talking paper filter or espresso/metal extraction methods.
Holy crap this just got complicated!
The next thing to consider is simply how much of which particles are being extracted into the coffee solution. Are you getting primarily the ones which are being measured? Are you filtering the liquid? Are you accounting for variability if you are not filtering? Are you getting different amounts of the particles you are not measuring each time? What could be affecting these variables?
I am writing a book that touches on these things, so you will have to wait for a little more research.
For now though, let's consider the failings of our lovely little refractometer. If we filter the coffee we are not measuring the liquid that we would actually serve. It simply does not account for a potentially large amount of the total brew and it has no reference for flavor components which are favorable vs unfavorable. And if you are not filtering the liquid a refractometer is not truly measuring the particles you left in, especially settleable solids. These problems alone make me leave the refractometer on the shelf most of the time. However, they can be used to determine a couple of useful pieces of information.
A refractometer can help you know if you extracted too much or too little of a coffee. Based on the math and numbers you can determine if you extracted 15% of your coffee, or if you got to 24%. These percentages can clue you in to where a problem may be arising, and if used properly you can brew more efficiently and potentially get a better flavor (maybe). Of course if you are more experienced, you can probably tell the brew is off simply by tasting the coffee.
A refractometer can also help you know how soluble a coffee is. Is it easily extracted or hard to extract? These are often determinations that can help a roasting program adjust depending on what is desired. Again, experienced baristas or roasters tend to make these adjustments naturally but a TDS meter may help give specific numbers to the equation.
In my opinion the best use for a refractometer in a cafe is simply to determine if the tastiest brew is in fact within the desired range for TDS, yield, and strength. They can work this way, but let's please stop acting as if we have the ultimate brew because our yield numbers say it is right. Let's use our palates, and senses.
Lastly, our TDS and brew numbers are often a relative measurement, which means that you need to compare numbers on your own equipment, water, and systems for what works. It really doesn't work to compare your numbers to someone else's because even something as small as grinder burr degradation will change how and why your measurements are showing up. (This can open another whole can of worms talking about grind geometry and total surface area...)
I have done a number of experiments measuring TDS, and I have created brews with identical TDS, yield, and strength, yet massively different flavor experiences. Basically the road to a specific extraction profile can be achieved in multiple ways, and not all of them are delicious. There are numerous variables that impact the extraction of our coffee, and we must understand them properly in order to make the best coffee possible.
Until we have feasible ways to measure total brew solids or other specific components of the coffee extraction on the fly, you will need to use your sense of taste whether you use a TDS meter or not.
That's all for today. Thanks for reading as always,
Pete