TDS and the Science of Infusion

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14th of June 2018

What’s makes a good brew? In the process of brewing coffee numerous factors affect the final brew quality. These include the roast freshness, the grind freshness and, of course, the water quality. Water is particularly important as it facilitates the extraction of soluble components from the coffee grounds into the water. Properties such as alkaline level, pH level, water hardness, and the water’s dissolved solids all impact the end result. It’s this last aspect we are going to look at in this article: the role of TDS and extraction.

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To the uninitiated, TDS stands for Total Dissolved Solids. In simple terms, it’s the quantity of soluble solids in a liquid, ranging from organic matter, to salts such as calcium and magnesium. To complicate matters slightly, in coffee making TDS reflects both the level of extraction in the coffee, and also the level of dissolved solids in the water. Coffee extraction is the process of dissolving soluble flavours from coffee grounds in water.

But what is a good TDS and extraction level for coffee, how do you measure them, and why are they important?

Measuring TDS and extraction levels.

 Many coffee practitioners use a device called a refractometer. This tool is designed to measure the angle of light refracted through a liquid. Refractometers can also be used to calculate extraction yield and measure concentration in brewed coffee. This information can then be uploaded and compared to an established index—known as the ‘Refractive Index’ or RI—which tells you the percentage extraction.

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For coffee, the RI is measured by adding a sample of coffee onto the refractometer’s prism. The refractometer shines light against your sample through the prism. The light won’t actually pass through the coffee itself, but instead merely touches the layer between the glass and the liquid then bounces back. The linear detector, receiving the light then sends a signal based on where the light hits, which is used to calculate RI.

When coffee is brewed with more TDS, it will bend the light more. If there’s a lower level of TDS in the brew, it will bend the light less. This means that with a coffee refractometer, the TDS level of a brew can be measured with a great level of accuracy.

What’s a good level of extraction and TDS for coffee?

There is much debate about this within the barista community. Perhaps contrary to expectation, 0 TDS is not a good thing for water as it will leave the water (and thus the coffee) tasting flat. Coffee makers should aim for the highest percentage extraction where the coffee still tastes good. This might sound a little on the subjective side. Broadly speaking, if we were to put this into a percentage, it’d be somewhere between 18-22% extraction. However, each coffee has a different character, so this will vary from blend to blend.

In terms of TDS of the actual water, you are looking at the ratio of solubles to water in a cup. Though (as with all aspects of coffee making) this is largely a matter of personal preference, it is generally thought that 1.15% to 1.35% is an optimal ratio for TDS. In layman’s terms this means that for every 1 soluble there are 99 parts water. The solubles created during extraction will help determine the flavour of the coffee, while the TDS determines the intensity of those flavours. While increasing TDS produces a stronger flavour, increasing it too much can cause certain flavours to become overpowering.

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Why TDS and extraction matter.

TDS measurements are very useful indicators of extraction in coffee brewing. They also have important implications for your source water, and whether you are using the correct type for your brewing. In terms of increasing the consistency of your brews, keeping a close eye on the TDS and extraction levels of your water and brewed coffee is invaluable.

It is also important to remember that tasting should always be seen as an essential technique in refining this process. Certain types of coffee beans may taste better at higher extractions than the guidelines we’ve mentioned above—so have an experiment until you find a balance you think works well for you!

Once again, we can see the importance of using coffee and water which compliment each other. Your coffee brew is only ever going to be as good as the ingredients you use.

 

Pictures from rawpixel on Unsplash, @hynajahoda on Instagram and Iberital