Total Dissolved Solids (TDS)

TDS represents the concentration of dissolved coffee compounds in water. When hot water contacts ground coffee, soluble compounds dissolve into the water. The total mass of these dissolved compounds divided by the total mass of the brewed beverage yields the TDS percentage. The formula is: TDS = (mass of dissolved solids / mass of brew) x 100. TDS is typically measured using a refractometer, which measures the refraction angle of light passing through the coffee. The refractive index correlates linearly with dissolved solid concentration. The relationship between refractive index and TDS was established by Sivetz and later refined by the Coffee Brewing Center. Modern digital refractometers (like the Atago PAL-Coffee) provide readings to 0.01% precision.

Coffee solids dissolved in water include hundreds of organic and inorganic compounds. These include carbohydrates (polysaccharides, sugars), acids (chlorogenic, citric, malic, acetic, quinic), lipids (<a href="/coffee-science/lipids-and-coffee-oil">triglycerides</a>, diterpenes), proteins and amino acids, caffeine, trigonelline, melanoidins, and minerals (potassium, magnesium, calcium). Each compound class has different solubility characteristics and extraction rates. The TDS measurement captures all of these collectively, providing a single number that represents strength but not composition.

The measurement of Total Dissolved Solids (TDS) in coffee primarily utilizes the physics of refractometry, which relies on the principle of light refraction. When light passes from one medium (air) into another (coffee solution), its velocity changes based on the density of the second medium, causing the light to bend at a specific angle. This behavior is described by Snell’s Law (n₁ sin θ₁ = n₂ sin θ₂), where the refractive index (n) of the liquid is determined by the concentration of dissolved organic and inorganic compounds. As the concentration of coffee solids—such as caffeine, organic acids, and sugars—increases, the solution becomes denser, slowing the speed of light and resulting in a higher refractive index. Modern digital refractometers measure the 'critical angle' of total internal reflection between the sample and a high-refractive-index prism (often sapphire). This optical method is preferred over electrical conductivity (EC) because coffee contains many non-ionic molecules that do not conduct electricity but do refract light. Furthermore, the physics of TDS measurement is highly temperature-dependent. Increased temperature leads to thermal expansion, reducing the liquid's density and lowering its refractive index. To maintain accuracy, precision instruments utilize Automatic Temperature Compensation (ATC) to normalize the refractive index to a standard reference temperature, typically 20°C (68°F), ensuring consistency regardless of whether the sample is measured hot or cooled.

TDS is directly related to brew strength. A higher TDS means a stronger, more concentrated cup. However, TDS alone does not indicate quality: a high TDS from over-extraction can taste bitter and astringent, while a low TDS from under-extraction can taste sour and weak. The Specialty Coffee Association (SCA) defines a golden ratio of 1:18 (55g coffee per liter water) producing a TDS of approximately 1.15% to 1.35%. For espresso, the SCA defines TDS of 8% to 12% as ideal. TDS is one of two measurements needed to calculate extraction yield (EY), the other being the brew ratio (dose of coffee to mass of beverage).

TDS = (dissolved solids mass / beverage mass) x 100. Measured via refractometer using refractive index correlation. For pour over: target 1.15 to 1.45%. For espresso: target 8 to 12%. For cold brew concentrate: target 3 to 5%. The dose-to-beverage ratio and TDS together yield extraction yield: EY% = (TDS% x beverage mass) / dose mass. For example: 1.35% TDS x 240g beverage / 15g dose = 21.6% EY. Calibration requires: refractometer, precision scale (0.1g), and consistent sampling protocol (cool sample to room temperature, filter through 0.45 micron syringe filter to remove fines).

Use a refractometer to measure TDS. Combine with your brew ratio to calculate extraction yield. If TDS is too high and the coffee tastes strong, use more water or less coffee. If TDS is too low and the coffee tastes weak, use less water or more coffee. TDS measurements help dial in recipes: for pour over, aim for 1.15% to 1.35%; for espresso, aim for 8% to 12%; for French press, aim for 1.3% to 1.5%.

• •Higher TDS always means better coffee. In reality, very high TDS can indicate over-extraction with bitter, astringent flavors. The ideal TDS depends on the brewing method and personal preference.
• •TDS tells you everything about coffee quality. In reality, TDS measures strength, not quality. Two coffees can have the same TDS but very different flavor profiles depending on which compounds were extracted.
• •You need expensive equipment to measure TDS. While professional refractometers cost $300+, affordable digital refractometers are available for $100 that provide sufficient precision for most users.

• •The SCA Coffee Brewing Chart (originally developed by the Coffee Brewing Center in the 1960s) established the ideal TDS range of 1.15% to 1.35% for drip coffee through consumer preference testing.
• •Research by Earl Lockhart demonstrated the linear relationship between brew ratio, TDS, and extraction yield, forming the basis of modern coffee extraction theory.
• •Studies by the Coffee Science Foundation have shown that refractometer measurements correlate well with gravimetric (evaporation) TDS measurements, validating refractometer use in coffee analysis.
• •The SCA Espresso Extraction standard defines espresso as having a TDS of 8% to 12% and an extraction yield of 18% to 22%.

• •Chapter 5: Extraction Science
• •Chapter 6: Brewing Methods

• •Lockhart, E.E. et al. (1955). 'The Soluble Solids Content of Coffee Beverages.' Journal of Food Science.
• •Frost, S. et al. (2020). 'The Coffee Brewing Control Chart: A Review.' Journal of Coffee Research.
• •SCA (2019). 'Coffee Brewing Standards.' Specialty Coffee Association.
• •Clarke, R.J. (1987). 'Coffee Technology.' Elsevier Applied Science.

• •Specialty Coffee Association
• •Coffee Science Foundation
• •Atago Co. (refractometer documentation)
• •James Hoffmann (World Atlas of Coffee)