March 11, 2016

Coffee Science: What Is TDS and Why Should You Care?


Visit any specialty coffee forum and chances are you’ll find a long discussion on extraction percentages and TDS – without much of an explanation of what they actually mean. Numbers like 23% extraction are bandied around without anyone actually saying why it should, or shouldn’t, be at that level.

On top of that, TDS is a controversial topic. Some hail it as a total game changer; others consider it a useless, or even dangerous, distraction.

So what does TDS actually mean? What levels are good? And ultimately, is it useful? Read on as we answer all those questions.

Lee este artículo en español Ciencia del Café: ¿Qué es el TDS y Por Qué Debes Conocerlo?


Measuring TDS with a refractometer. Credit: Gonçalo B. Duarte

What is TDS?

TDS stands for Total Dissolved Solids. Put simply, it’s the amount of “stuff” or soluble solids in a liquid, from organic matter to inorganic salts such as magnesium and calcium. It can be a good thing or a bad thing, depending on what the liquid is and how high the TDS is.

In coffee, TDS reflects the level of extraction of the coffee, as well as how many dissolved solids there are in the water. Contrary to what you might expect, 0 TDS isn’t a good thing for water – it leaves it tasting “flat”.

The most common way to measure TDS is with a refractometer. This clever little device measures the degree to which light is refracted by the liquid. The app then takes this information and compares it to an established index, which gives us a percentage extraction. Simple. (Kind of.)

SEE ALSO: Coffee Science: How Can We Identify & Improve Cherry Ripeness?

Why Is It Important?

TDS provides easy-to-analyse, concrete data that can help roasters and baristas measure, and therefore control, extraction. This should improve the taste, mouth feel, and consistency of your shots and brews – ensuring that, every single time, you brew a well-balanced coffee with a good level of complexity and sweetness. What’s not to like?

How Does It Impact Coffee?

The first use of TDS tech in coffee was for analysing water prior to  brewing. Then it started to be used on brewed coffee, and that’s when things started to get really interesting.

beer brewing

A craft beer brewer analyses his work. Credit: @jenkemkingbrewing

A company called VST decided to really explore the role TDS could have in analysing coffee, and so developed two things: the VST Refractometer and VST Coffee Tools. The latter is software designed to be used together with the coffee refractometer.

So why do you need both a refractometer and some software? Well, you use the refractometer to measure the TDS. You also input all the relevant data into Coffee Tools (dry dose of coffee, amount of water, desired brew strength, etc.). The software then analyses all this data and lets you know how well you hit your mark. Cool, right?

What’s a Good Level To Aim For?

We should be aiming for the highest percentage extraction at which the coffee still tastes good. This tends to be between 18-22% extraction, yet each coffee is individual and has a different character. Merely aiming for this golden range, without evaluating its impact on the coffee’s taste, will leave you doomed to failure.

It’s also worth mentioning that an inability to hit a reasonable extraction percentage could indicate roast issues, a problem with the water or the brewing equipment, such as the filter basket, the grinder/burrs or even the pressure and brew water temperature. However, in most cases, TDS just serves as another source of information helping you to dial in your coffee.

Does TDS Have any Limitations?

Measuring the TDS of a beverage can have a serious pitfall. Focusing only on the TDS reading makes it easy to ignore all the other important factors.

Take analysing water. The SCAA recommend a set criteria for the TDS reading of brewing water and, while it’s too technical to go into here, it provides a good and measurable figure to make sure your water is ok. The issue is that this needs to also take into account things such as calcium hardness, alkalinity, chlorine, and sodium levels. The brewer could reach the required TDS for water just by dissolving salt in distilled water – but that won’t make it good for brewing coffee.


Examining TDS – useful science, but there are other things we also need to consider. Credit: Dominic Ascanio / Hatchet Coffee 

Exactly the same is true when brewing. And so, when dialling in, the palate must remain the primary tool. Baristas and roasters alike must remember that their real goal is not to hit an extraction percentage, but to create delicious coffee.

Used correctly, TDS can be a great asset to any coffee professional – but only when used correctly.

Feature Photo Credit: @ems_rod

All views within this opinion piece belong to the guest writer, and do not reflect Perfect Daily Grind’s stance. Perfect Daily Grind believes in furthering debate over topical issues within the industry, and so seeks to represent the views of all sides.