An alternative method for TDS measurement
Nowadays, coffee Total Dissolved Solids (TDS) measurement is becoming a standard for every roaster, coffee producer, competition, coffee enthusiast or researcher. Everywhere, everyone is talking about TDS, Extraction yields, and this is becoming one of the most common methods for extraction quality control. TDS is present to every coffee recipe and without it, it is considered a downside or unprofessional. Unfortunately, a refractometer for coffee can be somewhat expensive with prices starting from 200€ and reaching up to 1000€. So, this fancy instrument is not always affordable by nonprofessionals. In some countries, even the cheapest refractometer is more than a minimum monthly salary, and this is creating a huge gap between baristas from these countries and rich countries or professionals and coffee enthusiasts. For me, this is a huge negative that is pulling back people from the possibility to improve the industry.
What is TDS?
TDS is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular (colloidal) suspended form (1). I’ll make a note that suspended particles can’t be measured using a standard refractometer. TDS is simply the amount of “stuff” or soluble solids in a liquid, mostly organic matter and inorganic salts. It reflects the extraction of the coffee and correlates with the coffee extraction yield.
Why am I critical to TDS meters?
Don’t get me wrong, TDS meters are a good method for simple quality control, but this isn’t a complete solution for the preparation of good coffee. TDS meters have a lot of limitations and I recommend using it as a guideline, but also keep in mind that it can’t be 100% reliable. As a service engineer working with analytical and measurement instruments for more than 11 years, I have a more critical opinion about these simple refractometers and the results acquired by them. Believe me, I know the difference between refractometer for 200€ and for 20,000€. There is also a tendency I’m observing that more of the bloggers and YouTube vloggers have no idea how to correctly use the TDS refractometer. And this is a huge problem in the coffee field, as many believe blindly on the results they get from the measurement. There is a great post from Coffee Ad Astra on how to improve TDS measurement precision up to 0.01%. But for me, the problem is also inaccuracy of the TDS meters. The difference between accuracy and precision is shown in the picture below.
How can you be sure that your measurement of 1.5% (just an example) is 1.5% without any calibration? Sorry, but Zero-point calibration isn’t a calibration and you’ll never see it used in GLP or GMP (Good Laboratory Practice or Good Method Practice). This provoked me to find a different way of measuring TDS and compare the result received from the refractometer. And while researching I found an easy, precise, and accurate method that is also standardized (2). And not in the last place this is a cheap method that anyone can replicate at home with available equipment.
Total Dissolved Solids by Gravimetric determination.
This method is developed for the determination of TDS in water and wastewater. Total dissolved solids are a measure of the dissolved matter in water that remains after all the water has been evaporated. It uses a simple procedure that can be easily replicated. The main idea is to evaporate the entire water and measure the weight of the remaining residue. A known volume of a well-mixed sample is filtered through a standard filter and the filtrate collected. The filtrate is evaporated to a constant weight condition in an oven maintained at a temperature of 120°C-130°C to remove mechanically occluded water. The mass of the dried sample’s dissolved solids is determined and used to calculate the concentration of total dissolved solids in the sample. Here is the simple procedure:
Wash filter paper;
Dry evaporating dish and weigh;
Stir sample;
Pipette 50ml while stirring;
Filter and wash three times;
Transfer filtrate to an evaporating dish and dry;
Cool and weigh;
Calculate in mg/L;
Calculating of TDS concentration is easy:
A = weight of dried residue + dish, mg;
B = weight of the dish, mg;
This will measure the total dissolved solids removing the undissolved particles that could end in the coffee. If you want to measure total solids (dissolved and not dissolved) you can use the following procedure:
Weigh evaporating dish;
Stir sample;
Pipette 50ml while stirring into an evaporating dish and dry;
Cool and weigh evaporating dish;
Calculate in mg/L;
A = weight of dried residue + dish, mg;
B = weight of the dish, mg;
How can you adjust for measurement at home?
You can use the filter that you generally use with V60 or equivalent. If you don’t have the proper equipment for the volumetric transfer of 50ml or you want to calculate the result in ppm or % (not mg/L) you can transfer 50g of coffee beverage (by weight) to the evaporating dish. The final formula should look like:
A = weight of dried residue + dish, mg;
B = weight of the dish, mg;
I recommend and prefer the second method without filtering the undissolved matter. Evaporation can be conducted in a standard kitchen oven at 120°C -130°C with temperature control. I recommend performing three repetitions per sample for a more accurate result. The most important in this measurement is the scale. It should be capable of measuring at least the second digit after the decimal point (0.01g). There are plenty of cheap options for such a precision scale available online for around 10-15€. More precise balances that could measure to 0.001g are more expensive but still available at prices starting from 80-100€ but I don’t think it is worth buying one. For those familiar with VST: WTF article from Bartista Hustle I have to say that I strongly disagree with the necessity of having a scale with 0.0001g precision. It is easier, faster and more precise to measure it with such a scale but the result will still be roughly the same. After all, it is meaningless to record TDS value after the first or the second digit after the decimal point as this is irrelevant for the coffee beverage evaluation. The difference with the more precise balance will come from the possibility to use a smaller initial volume that is making the next step, evaporation much easier and faster. The second thing I disagree BH is that TDS refractometers are accurate. I already spoke about that at the beginning but there will be another post specifically on this subject.
Conclusions
TDS measurement is still a great method for quick quantitative evaluation for extraction repeatability or comparison. But you always must keep in mind it’s limitations and never trust it completely.
With this post, I wanted to give an alternative to TDS refractometry measurement that could be performed without expensive equipment in the home environment. The method is both accurate and precise, but with comparison to refractometer – much slower. Still, it is an opportunity for those without the possibility to invest in equipment to feel more professional and to give them the chance to experiment.
(1) "Salinity and drinking water: SA Health". www.sahealth.sa.gov.au. Retrieved 2020-02-22.
(2) Standard Methods: for the Examination of Water and Wastewater, APHA, AWWA, WEF/1995. APHA Publication and Chemistry for Environmental Engineering, C.N. Sawyer, P.L. McCarty and C.F. Parkin. McGraw-Hill, 1994