Tasting coffee is difficult, but one thing people always try is how strong the coffee is. We can all agree that espresso coffee is stronger and therefore has more flavor than filter coffee. But let’s delve deeper into the world of coffee strength again!
HOW DO WE MEASURE FORCE?
Just saying something is strong is not that scientific or accurate and we need a measure of strength before we can talk about the same things. In coffee the content is measured in Total Dissolved Solids (TDS) and is usually marked in percentages (%). When we measure TDS, we are evaluating the soluble concentration, that is, how much of the soluble coffee is concentrated in the cup. Simply put, how concentrated (strong) or watered down (mild) the coffee is.
It's almost impossible to tell what the TDS of a drink is just by looking at it, so to measure it we need a refractometer. The refractometer measures the angle of refraction of light through a liquid and can thus tell how thick or thin the liquid is. Refractometers are generally used in laboratories for testing, for example. plasma protein in the blood or to identify the materials of a gemstone. Works great for coffee too!
To measure the TDS of coffee with a refractometer you must follow a certain procedure:
1-Reset the refractometer with distilled water (not necessary before each reading, just once a day)
2-Stir the drink VERY WELL (the coffee stratifies very quickly)
3-Place a sample in a pipette
4-Pipe the coffee into the refractometer glass so that the coffee covers the glass
5-Wait 10-20 seconds
6-Close the refractometer lid and press “GO” until you see the same reading 3 times in a row
7-Dry the glass with a tissue
8-Clean the glass with cotton wool soaked in alcohol or paper towel with alcohol.
OBSERVATION! This procedure works if you are measuring filtered coffee. When measuring espresso coffee, you need to filter the sample with specific filters (like the one in the photo below) to remove CO2 from the sample. Then you take a sample of the espresso with a syringe, insert the filter into the syringe and push the sample through the filter. Otherwise, the procedure for measuring TDS 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 between 8-12%. The TDS of the ristretto should be something above 12%. So it's just a stronger (and perhaps under-extracted) espresso. The same way lungo works, your TDS should be between 2-8%. For filter coffee, the correct TDS should be somewhere between 1.15% and 1.35% (SCA Golden Cup), but I think it's quite mild for light roasts, so I believe it could go as low as 1.9%.
Comparison of Ristreto, Espresso and Lungo based on an experience with recipes using the same amount of powder, but with different amounts of water to evaluate TDS:
RISTRETO
recipe: 20 g input, 20 g output, 19 sec.
TDS 13.65%, extraction yield 14.16%
ESPRESSO
recipe: 20 g input, 39.7 g output, 28 sec.
TDS 9.77%, extraction yield 20.12%
LUNGO
recipe: 20 g, 81.2 g output, 48 sec.
TDS 5.43%, extraction yield 22.87%
When we know the TDS, we can count the % extraction yield which tells us more about the flavor. As you can see from the calculations above, low TDS does not necessarily mean low extraction, or strong coffee, high extraction. The lungo was lowest in TDS (mildest) but highest in extraction. On the other hand, the ristretto was very strong (higher in TDS) but extremely under-extracted (sour).
TDS table for different drinks:
RISTRETTO > 12%
ESPRESSO 8-12%
LUNGO 2-8%
AMERICAN 1-2 %
FILTERED 1.15 – 1.35%
Above we have three espressos with slightly different volumes, but they all have practically the same TDS. Recipes starting from the left:
15 g input, 30.4 g output, 21 sec, TDS 9.40%, extraction yield 19.76%
20 g input, 39.7 g output, 28 sec, TDS 9.77%, extraction yield 20.12%
25 g input, 49.5 g output, 39 sec, TDS 9.87%, extraction yield 20.27%
Since the % extraction yield is practically the same, they would also taste the same.
HOW TO HANDLE TDS?
TDS works in much the same way as extraction. You can increase TDS in these ways:
Brewing ratio -> more coffee to water -> higher TDS
This is very 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 -> higher TDS
Grinding finer means you have more surface area where water can extract things. If you cut a coffee bean in half, its surface area will double and it will be twice as easy to extract from it. As it is easier to extract, the resistance will also increase.
As you grind finer, it will be easier to extract, but at the same time you will increase the brewing time, which will also increase extraction. The more water and coffee pass together, the more coffee will give way to the water.
Higher temperature in extraction water
You will have to have energy to extract the coffee. The most common and easiest to extract energy source is the thermal energy of water. More temperature, more extraction, more strength.
Turbulence (agitation) when making filter coffee
Turbulence means some type of movement in the coffee bed during the preparation of filtered coffee. Moving soil and water will expose more surface area to water so more can be extracted. More turbulence, more extraction, more strength.
Appropriate practices in preparing the drink -> avoiding plumbing
Channeling means that the water runs unevenly through the coffee bed. There are numerous reasons for plumbing, such as poor drip/water distribution in filtered coffee or a cracked coffee disc when preparing espresso coffee.
Now that you know how to evaluate the TDS of your coffee, tell me what your preference is, higher or lower TDS?
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