What is TDS?
To make a great cup of coffee you not only want the best coffee beans available but also the best water. One of the most important aspects to look at when rating the quality of water is the TDS. TDS stands for Total Dissolved Solids. A TDS measurement represents the total concentration of dissolved substances in the water, which can include minerals, salts, and other solids (1). The amount and type of solids that are dissolved in the water will affect its flavor. Because coffee is about 96 percent water, the TDS of water used in the brewing process will greatly affect the quality of the finished product.
TDS readings vary greatly between different kinds of water. Most distilled water has a TDS of 0 ppm (parts per million). During the distillation process steam is condensed from boiling water and the result is pure water with no dissolved solids. Spring water on the other hand has a relatively high TDS, which can range from 50 to 450 ppm (2). This is because the water picks up different minerals and salts on its journey through underground rock passages and cracks in the earth on its way to the spring. The result is water with a high level of dissolved solids. Tap water is somewhere in between these two. Ideal tap water ranges from 100-150 ppm while average tap water can range from 100-400 ppm (2). So what does this all have to do with coffee?
The TDS of water not only can affect the initial flavor of a cup of coffee but it can also affect the extraction process. The idea is that low TDS waters tend to over extract coffee. There are little to no solids dissolved in these waters so they have a greater ability to absorb coffee material from the ground beans. This will lead to a coffee that is bitter and dry. On the other hand, high TDS waters often have high mineral contents and tend to under extract coffee. These waters already have a high level of solids dissolved in them and will have less capacity to absorb coffee material from the coffee grounds. This may lead to a coffee that is sour or lacking sweetness. According to the Specialty Coffee Association of America (SCAA) the ideal TDS range for the water used to brew coffee is 75-250 ppm. The target TDS is 150 ppm (3). The target TDS of 150 ppm should lead to a properly extracted cup of coffee with balanced flavors and acidity.
While the SCAA sets a target TDS range of 75-250 ppm and an ideal value of 150 ppm these are simply guidelines. It is important to note that while two waters may have the exact same TDS values they can have totally different flavor profiles. An extreme example would be if we dissolved 10 grams of salt into a liter of water and 10 grams of sugar into another liter of water. These waters might end up having the same TDS values but they would obviously taste completely different.The idea is that the TDS alone affects the extraction process. Low TDS waters tend to over extract coffee. There are little to no solids dissolved in these waters so they have a greater ability to absorb coffee material from the ground beans. This will lead to a coffee that is bitter and dry. On the other hand, high TDS waters often have high mineral contents and tend to under extract coffee. These waters already have a high level of solids dissolved in them and will have less capacity to absorb coffee material from the coffee grounds. This may lead to a coffee that is sour or lacking sweetness.
This being said, what exactly is dissolved in the water will definitely affect its flavor and the flavor of a cup of coffee brewed with that water. This is why we saw differences in coffee brewed with each type of water. Even thought the city waters were fairly close in TDS values (R-120, CH-125, D-132), they each presented different aromas and flavor profiles because of the different kinds of solids dissolved in each water.
We were very surprised at the differences in flavor between coffee made with water from Raleigh, Durham, and Chapel Hill. The coffees brewed with the bottled waters also presented a wide variety of flavors and aromas compared to each other. We would like to point out that we did use the exact same brewing methods for each water during our tasting at HQ Raleigh. We mention this because while this method may have brought out certain qualities in the Durham or Chapel Hill waters we could have easily tweaked some aspects of the brewing to dial in anyone of the waters. So in this way the TDS serves as a useful guideline that a barista or coffee connoisseur can use to help dial in their particular cup of coffee.
source : https://www.raleighcoffeecompany.com