What is TDS in coffee and how to measure strength of coffee?

How do we measure strength?

Just saying something is strong isn’t that scientific or precise and we need a measurement for strength so that we can talk about same things. In coffee the strength is measured in Total Dissolved Solids (TDS) and usually it is marked in percentages (%). When we measure TDS, we are evaluating soluble concentration so how much from coffee’s solubles have concentrated into the cup. Put simply, how concentrated (strong) or watery (mild) the coffee is.

It is quite impossible to say what is drink's TDS  just by looking at it so to measure it we need a refractometer. Refractometer measures the angle of lights’ refraction through a liquid and by that it can tell how thick or thin the liquid is. Refractometers are usually used in laboratories to test e.g. plasma protein in blood or to identify the materials in a gemstone. It works really well for coffee as well! To measure TDS of coffee with a refractometer you should follow a certain procedure:

1. Reset the refractometer to zero with distilled water (not necessary before every reading, only once a day)
2. Stir beverage REALLY WELL (coffee stratifies really fast)
3.  Take a sample into pipette
4. Pipette coffee on the refractometer glass so that coffee covers the glass
5. Wait 10-20 seconds
6. Close the refractometer’s lid and press “GO” until you see the same reading 3 times in a row
7. Dry the glass with a tissue
8. Wipe the glass with alcohol pad or paper towel with alcohol

NOTE! This procedure works if you are measuring filter coffee. When making measurements from espresso, you need to filter the sample with specific filters (like these) to remove co2 from the sample. Then you take a sample from the espresso with a syringe, insert the filter to the syringe and push the sample through the filter. Otherwise the procedure for TDS measurement is the same.

How to use TDS?

One way to use TDS is to define different coffee drinks. Espresso is an espresso when its TDS is something between 8-12 %. Have you heard about ristretto? There million definitions how to make a ristretto but I think the most precise way to define it is to say that ristretto’s TDS should be something more than 12 %. So it is just a stronger espresso (and maybe under-extracted?). Same way works lungo, its TDS should be something between 2-8 %. For filter coffee correct TDS should be something between 1,15 % and 1,35 % (SCA Golden Cup) but I think that is quite mild for light roasts so I’m willing to go up to 1,9 %. 

RISTRETTO
recipe: 20 g in, 20 g out, 19 sec
TDS 13.65 %, Extraction Yield 14.16 %

ESPRESSO
recipe: 20 g in, 39.7 g out, 28 sec
TDS 9.77 %, Extraction Yield 20.12 %

LUNGO
recipe: 20 g, 81.2 g out, 48 sec
TDS 5.43 %, Extraction Yield 22.87 %

When we know the TDS, we are able to count the Extraction Yield % which tells us more about the flavour. Read more about extraction from my blog. As you can see from the calculations above, low TDS doesn’t necessary mean low extraction, or strong coffee high extraction. The lungo that I made, was the lowest in TDS (mildest) but highest in extraction. On the other hand the ristretto was really strong (highest in TDS) but extremely under-extracted (sour).

Table for different drinks

RISTRETTO > 12 %
ESPRESSO 8-12 %
LUNGO 2-8 %
AMERICANO 1-2 %
FILTER COFFEE 1,15 – 1,35 %

Above we have three espressos that are a bit different in volume but they all have pretty much the same TDS. Recipes starting from the left:

• 15 g in, 30,4 g out, 21 sec, TDS 9.40 %, Extraction Yield 19.76 %
• 20 g in, 39,7 g out, 28 sec, TDS 9.77 %, Extraction Yield 20.12 %
• 25 g in, 49,5 g out, 39 sec, TDS 9.87 %, Extraction Yield 20.27 %

As their Extraction Yield % are quite the same, they would taste pretty much the same as well.

How to manipulate TDS?

TDS works pretty much the same way as extraction. You are able to increase TDS with these ways:

Brew ratio -> more coffee in relation to water -> higher TDS
This is pretty basic and quite logical for everyone: If you use more coffee in relation to water, you will end up with stronger coffee (higher TDS).

Finer grind -> more surface area to extract from -> higher TDS
Grinding finer means that you have more surface area where the water can extract things. If you cut a coffee bean in half, you will double its surface area and it will be twice as easy to extract things from it. As it is easier to extract, the strength will also increase. 

As you grind finer it will easier to extract but also you will increase the brew time at the same time which will also increase extraction. The more water and coffee spend time together, the more coffee will give for itself to the water.

Higher temperature in brewing water
You will have to have energy to extract the coffee. The most common and easiest energy source for extracting is water’s thermal energy. More temperature, more extraction, more strength.

Turbulence (stirring) when making filter coffee
Turbulence means some kind of movement in the coffee bed when making filter coffee. Moving the grounds and water will expose more surface area to the water so that it is possible to extract more. More turbulence, more extraction, more strength.

Appropriate brewing practices -> avoiding channeling
Channelling means that water is running through the coffee bed unevenly. There are numerous reasons for channelling such as poor dripping/water distribution in drip filter coffee or cracked coffee puck in espresso making.