What is TDS?
Put simply, TDS is the total amount of dissolved solids in solution. We shouldn't mistake this with the silt you see at the bottom of a cloth-filtered pour over or a French press - these are undissolved solids and are likely to be husk and skin from the bean. TDS measures the concentration of soluble compounds that have been extracted from the grinds into solution.
Thousands of compounds have been identified and have been found to contribute to the lovely 'coffee' flavour and aroma in our complex pourover or our gooey espresso. TDS is a way to quantify the concentration of the collection of these compounds which relates to the total extraction % by a simple formula, which you will see in the next section.
How we can use TDS
Although we get a concrete number (% TDS), this number alone doesn't tell us how our coffee tastes, our mouth and nose do! So what can we do with this information and how can it be useful for both coffee lovers at home, cafe baristas and roasters alike?
If you like the taste of your brew then you can measure the TDS to see the extraction level and correlate that to how you like your brew. If you don't like it, you can tinker with the extraction using the various methods such as grind size, dose, temperature, agitation etc. to hone in on an extraction level that suits you. It is also worthy of note to mention that the TDS or extraction level of one bean doesn't necessarily mean that the same level of TDS will produce a brew that tastes the same way - different beans will have different compounds. But we can use TDS as a guide within the same bean and roast. Remember, your taste is the ultimate guide, but TDS can show you quantitatively where you are and if your adjustments are leading to an increased level of extraction or not.
Extraction Yield % = Brewed Coffee (g) x TDS (%) / Dose (g)
For example you brew a 2:1 ratio of espresso so this means 18g dose and 36g of brewed coffee. You measure the TDS to be 12%, so EY(%)=(36g * 12%) / 18g = 24%
What devices are out there?
VST Lab Coffee III Refractometer - RRP £550
Let's start with what you shouldn't buy. The VST Coffee Lab III has been the staple in the industry for many years, recommended by aficionados in the field.
But what lets the VST down is its archaic look, its price and its overreaching capabilities. The VST achieves what you need, but also way more than you need, hence the price tag. I'd compare this to using a Vernier calliper to measure the length of a football pitch - not quite that, but you understand the principle, you get the same answer but did you need to know it to the nearest micrometre?
If you have the funds, by all means it is the best you can get. But there are more accessible alternatives...
DiFluid Technology Coffee TDS Refractometer - RRP £139
DiFluid Technology recently released their coffee-focussed TDS meter complete with an app. Addressed face on, it looks modern, clean and does away with the archaic rugged lab-look of the VST. Included are modern comforts such as USB-C charging.
The precision is 0.03%, compared to 0.01% of the VST, but do we need that? Let's remember what we are measuring here and why we use TDS in the first place; it is to track consistency in method and land us in a ball park where we can iterate to a desired cup. It gives us a straight-away answer as to what we can do to improve extraction - it is not a measurement of absolute taste.
1.21 vs 1.19 % TDS is really not a problem that warrants £400 and nor does a cup touting 1.19% vs 1.21% say anything different towards what you would practically do in this situation - you would come out with the same conclusion either way, under extracted or over extracted.
TDS is a measurement of total dissolved solids in a liquid. It can correlate to a measurement of total extraction via a formula.
This measurement of extraction can be used to tinker with your brew method to achieve a more or less extracted cup to your desired taste.
The measurement outcome itself doesn't dictate what your brew will taste like, but when using in a comparative way with brews of the same bean using different methods you can extrapolate how the variables are affecting the brew, and more importantly, how what is affected is changing the way your brew tastes to you.
Use the measurement to iterate to a desired outcome
At Sigma Coffee UK, we chose to stock the DiFluid TDS Refractometer due to its accessibility, value and easy to use app that does all the calculations for you.