The taste of coffee can be quantified with numerical values. This may surprise some, as taste is often considered a subjective experience. However, in the context of coffee, there are specific parameters used to measure it: TDS and the extraction level. Let's delve into what these terms mean.
TDS (Total Dissolved Solids) refers to the amount of dissolved particles in the brewed coffee, essentially its strength. On the other hand, the extraction level represents the percentage of all soluble substances that we have successfully extracted from the coffee beans during brewing.
Understanding Extraction Level
Coffee beans are composed of approximately 50% cellulose, a substance that is entirely insoluble. The remaining 36% consists of soluble substances, which is the maximum that can be extracted under laboratory conditions. When brewing coffee with water, typically between 14% and 26% of these substances are extracted from the beans. Importantly, these substances do not dissolve simultaneously; acids are extracted first, followed by sugars, and finally, bitter compounds.
Coffee achieves its ideal flavor when these substances are balanced with one another. If too many or too few substances are extracted, the coffee’s flavor can be compromised. The challenge, therefore, lies in finding the precise balance where the coffee is strong yet smooth, with the extraction level remaining optimal.
The Specialty Coffee Association has established standards indicating that for a balanced cup of coffee, the optimal extraction level should fall within the range of 18% to 22%. These figures were determined through extensive experimentation. It was found that if extraction is below 18%, the taste is dominated by chlorogenic acidity and grassy, bitter notes. Conversely, if the extraction exceeds 22%, the flavor acquires a medicinal bitterness due to the high concentration of caffeine and tannins, and the acidity decreases significantly.
Understanding TDS
TDS (Total Dissolved Solids) refers to the percentage of dissolved substances in the beverage relative to its total weight. For example, a TDS of 1.35% means that the beverage consists of 98.65% water and 1.35% of substances extracted from the coffee. The higher the TDS, the more robust the coffee tends to taste.
The Specialty Coffee Association has also conducted research on people's preferences regarding coffee strength, revealing that the desired richness of the final beverage should fall within the range of 1.15% to 1.45% TDS.
The Relationship Between Extraction and TDS
There is a certain correlation between TDS and the extraction level, but they can also vary independently. To illustrate this, let's consider a few examples:
If we use more coffee than necessary but brew it too quickly or at a low temperature, the result will be a strong coffee with a high TDS, but the extraction level will be low. This coffee will be rich but under-extracted, with the flavor dominated by grassy, bitter notes and excessive acidity.
The opposite situation can also occur: if we use less coffee than necessary but brew it for an extended period or at too high a temperature, we will end up with a weak, watery coffee with a low TDS. However, due to over-extraction, it will taste bitter because too many tannins and melanoidins have been extracted from the beans. This flavor is reminiscent of medicinal bitterness.
The TDS level is primarily influenced by the coffee-to-water ratio, while the extraction level is affected by a multitude of other factors, which will be discussed below. Importantly, TDS can vary widely depending on personal taste preferences. For instance, coffee brewed using a pour-over method may have a TDS of 1.15%–1.45%, whereas espresso can range from 8% to 12% (ten times higher). However, the extraction percentage consistently remains within the range of 18% to 24%.
To achieve optimal levels of both TDS and extraction, an ideal coffee-to-water ratio has been established for alternative brewing methods (drip coffee maker, pour-over, French press): 60 grams of coffee per liter of water. Espresso, on the other hand, typically uses a different ratio—1:2, meaning 1 gram of coffee for every 2 grams of the final beverage. For example, 18 grams of coffee for 36 grams of espresso.
This ratio facilitates achieving an acceptable TDS level, leaving only the task of fine-tuning the extraction to obtain the desired flavor profile.
How to Measure TDS and Extraction Level
TDS in coffee is measured using a refractometer, an instrument that gauges the refraction of light.
The extraction level can then be determined using a simple formula after measuring the TDS:
Extraction Level (%) = Weight of the Finished Beverage (g) x TDS (%) / Weight of Ground Coffee (g)
To calculate the extraction level, you need to weigh the finished beverage after brewing. For instance, if you pour 250 grams of water into a dripper, you may end up with approximately 220 grams of coffee, as some water will be absorbed by the ground coffee.
You would then multiply the weight of the finished beverage (220g) by the TDS measured with the refractometer (1.35%) and divide by the weight of the ground coffee (15g). This yields an extraction level of 19.8%.
For convenience, the Specialty Coffee Association has also developed a specialized chart. On the horizontal axis, the extraction percentages are marked, and on the vertical axis, the TDS is displayed. By finding the intersection of your beverage's extraction level and TDS, you can determine its flavor profile:
Espresso values differ due to its density, with the ideal TDS range being 8–12%.
Using these charts allows you to evaluate and understand what adjustments are necessary to improve the taste of your coffee. For example, if the TDS of a pour-over coffee were 1.05%, the extraction level would be 15.4%, which is quite low, resulting in an underdeveloped flavor. To enhance the results, you would need to increase the extraction level.
To craft an optimal brewing recipe, you should first aim to achieve the desired TDS and then fine-tune the extraction level accordingly.
How to Control TDS
TDS is primarily influenced by two key factors:
- Ratio: The coffee-to-water ratio: The more coffee you use, the higher the TDS, and vice versa.
- Grind Size: The finer the grind, the higher the TDS.
How to Control Extraction Level
The extraction level is mainly affected by four key factors. By adjusting these, you can control the extraction level:
- Brewing Time: The longer the brewing time, the higher the extraction level.
- Water Temperature: The higher the temperature, the higher the extraction level.
- Grind Size: The finer the grind, the higher the extraction level.
- Turbulence: Any stirring or pouring water in small increments increases extraction.
Secondarily, factors such as roast level, water mineralization, and the coffee bean variety influence the extraction level. The darker the roast, the more soluble solids it contains, resulting in faster extraction. We explored the impact of water on extraction and flavor in detail in our article about water.
Different coffee varieties extract at different rates due to variations in cellular structure, density, and chemical composition. A common example is the SL28 and SL34 varieties often found in Kenya, which extract rapidly due to their high sugar content and unique cellular structure. Therefore, if you brew Kenyan coffee and, for example, Brazilian coffee using the same recipe, the Kenyan coffee is likely to be more heavily extracted.
Key Takeaways
This topic may seem complex, but in practice, it’s quite straightforward. Using a refractometer can make the coffee brewing process more deliberate and transparent. With "data" in hand, you can more accurately assess how well you have unlocked the coffee’s potential and how to achieve the best results.
However, refractometers are not commonly used, even in coffee shops, let alone in home brewing. Therefore, the first thing to focus on is the correct coffee-to-water ratio. This will help you achieve the proper TDS. From there, you can rely on taste and adjust the recipe according to the outcome.
And, of course, it's essential to follow general coffee brewing guidelines.