LeBrew·
What Afects TDS Readings?
Abstract
The TDS meter has become an essential tool in coffee extraction research and quality control, as it quantifies the proportion of total dissolved solids (TDS) in brewed coffee and provides an objective basis for brewing evaluation. Within the widely recognized Golden Cup Theory, TDS together with extraction yield (EY) serve as the two core indicators for determining whether coffee strength falls within the optimal range. By measuring TDS, baristas and researchers can more accurately identify issues such as over-concentration, under-extraction, or over-extraction, and make data-driven adjustments to brewing parameters.
However, TDS measurement does not fully represent the actual concentration of dissolved solids. Results can be affected by multiple factors, including temperature, coffee oils, and suspended fine particles, which may cause deviations in readings. Therefore, understanding the working principle of the coffee refractometer, maintaining controlled testing conditions, and applying appropriate sample preparation are essential for reliable results. Building on an explanation of TDS measurement principles, this paper further analyzes the key factors influencing TDS values and their mechanisms, while proposing practical improvements to enhance the scientific standardization of coffee extraction.
Fundamentals of TDS Concentration Meter
In coffee research and quality control, Total Dissolved Solids (TDS) is a key indicator for evaluating extraction level and brew strength. TDS measurement is typically based on the principle of refractive index: the refractive index of a liquid is closely related to the concentration of soluble compounds within it. As the amounts of sugars, organic acids, caffeine, and other dissolved solids increase in brewed coffee, the speed of light in the liquid slows down, thereby increasing the refractive index. A coffee refractometer (TDS meter) calculates the percentage of dissolved solids by comparing the refractive index of the sample against pure water at the same temperature.
However, refractive index is influenced not only by true soluble solids but also by other physical and chemical factors. In coffee’s complex colloidal system, elements such as lipids, suspended fine particles, and temperature variations can all affect light propagation, resulting in TDS readings that are higher or lower than the actual value. Understanding these limitations of the method and controlling experimental conditions are therefore essential for using TDS instruments correctly and obtaining reliable data.
What affects the degree of TDS?
It is important to note that, in theory, a TDS meter only reflects the concentration of dissolved solids in coffee. However, brewed coffee is not an ideal homogeneous system. It also contains emulsified or free lipids, suspended fine particles, and carbon dioxide gas released continuously during brewing and extraction. These non-dissolved components alter the optical properties of the liquid and can interfere with the coffee refractometer readings, causing measured TDS values to be either higher or lower than the true dissolved solid concentration.
For this reason, when interpreting coffee concentration measurements, one must consider the sample characteristics. Coffees with high oil content (such as dark roasts) or brewing methods that generate more fines are especially prone to skewed TDS readings. Complementary analysis methods are recommended to ensure results more accurately reflect the true dissolved solids.
The Impact of Temperature on TDS Measurement
Temperature is the most direct factor influencing coffee TDS measurements. Since the refractive index is highly temperature-sensitive, at the same concentration a liquid’s refractive index will be lower at higher temperatures compared to room temperature. This occurs because heating causes thermal expansion, reducing molecular density; as a result, light interacts less with the molecules. Combined with reduced molecular polarizability, light travels faster through the medium, lowering the refractive index.
Consequently, when coffee is measured at high temperatures, the TDS reading often appears lower than the true value. Although most coffee refractometers are equipped with Automatic Temperature Compensation (ATC), issues such as uneven cooling during rapid temperature changes or incomplete stabilization of the sample can still produce unstable readings.
In short, temperature alters the refractive index itself and thereby indirectly impacts TDS measurement results. Practically, it is recommended to allow coffee samples to fully cool to room temperature and rest briefly before testing, ensuring more stable and reliable results.
The Influence of Coffee Oils on TDS Measurements
Coffee oils are another major factor that interferes with TDS measurements. These lipids are insoluble in water and are especially abundant in espresso, which is brewed under high pressure. Coffee oils are dispersed in the liquid in an emulsified form, creating an optical turbidity effect that causes light scattering within the liquid. When this occurs, the coffee refractometer may misinterpret the scattered light as an increase in dissolved solids, resulting in an artificially high TDS value.
In addition, coffee oils tend to adhere to the measuring lens of the refractometer, causing baseline drift and making repeated measurements of the same sample fluctuate. Without regular cleaning and maintenance, the accumulation of oil residues can further amplify these errors, ultimately reducing the stability and reliability of TDS results.
The Impact of Coffee Dust on TDS Measurements
Coffee dust—the micron-sized particles generated during grinding—also plays a significant role in skewing TDS measurements. While these particles remain suspended in liquid rather than fully dissolving, they exhibit colloidal properties in terms of optics. As a result, they alter both the refractive index and the light scattering characteristics of the brew.
In unfiltered beverages such as espresso and Moka pot extractions, coffee dust levels are typically high, often leading to artificially elevated TDS values. Even in pour-over brewing, filter paper cannot completely eliminate these particles due to its porosity, allowing some coffee dust to enter the final cup. Moreover, once suspended in liquid, coffee dust continues to release soluble compounds over time, further increasing the measured TDS level.
It is important to note that such deviations do not represent the actual amount of dissolved solids, but rather stem from the sensitivity of optical measurement methods to suspended particles.
In summary, the combined effects of temperature, coffee oils, and coffee dust mean that TDS should not be interpreted solely as the true percentage of dissolved solids. Instead, it must be understood as a composite optical signal influenced by multiple interacting factors.
How to Minimize Interference in TDS Measurements?
1. Temperature Control
Temperature is the most direct factor influencing coffee TDS readings.
The most effective approach is to measure under stable conditions, ideally between 20–25 °C. Samples should be cooled and allowed to rest before measurement, and multiple readings should be taken and averaged for accuracy. Even with Automatic Temperature Compensation (ATC), immediate measurement at high temperatures should be avoided, as rapid thermal changes can cause unstable values.
2. Oil Interference
Coffee oils are another major source of error. The refractometer lens should be kept clean—wipe away residues after each test and rinse with water to reduce baseline drift.
For high-oil samples such as espresso, pre-treatment is recommended: using a cotton filter to separate oils and coffee dust can make the readings closer to the true dissolved solids content. Without such control, oil-related deviations can exceed 10%.
3. Coffee Dust (Fines)
In pour-over brewing, the impact of coffee dust is relatively minor. Simply cooling the sample quickly and measuring promptly is sufficient.
As soluble compounds from coffee dust typically take more than ten minutes to significantly alter the TDS reading. Within this time window, the coffee brew will also have cooled to an appropriate measurement temperature.
4. Integrated Practice
By reducing these interferences, the accuracy of TDS measurement is improved, and its correlation with extraction yield becomes more reliable. Only under controlled external conditions can TDS truly serve as a valid parameter for evaluating extraction levels and flavor analysis in coffee.
Q&A: Common Questions About Coffee TDS Measurement
Q1: Why are TDS readings often unstable during barista practice?
A: The most common factor is temperature fluctuation. If coffee is measured immediately after extraction while still hot, the TDS reading will be lower than expected. It is recommended to allow the brew to cool to 20–25 °C and rest briefly before testing.
Q2: Are TDS values in espresso reliable?
A: Espresso contains a high amount of coffee oils and coffee dust (fines), which tend to artificially elevate TDS readings. Unless the sample is filtered or pre-treated, espresso TDS should be interpreted as a relative reference, not an exact measure of dissolved solids.
Q3: Is pour-over coffee TDS more accurate than espresso?
A: Generally yes, since pour-over coffee contains less oil and coffee dust, making its TDS values closer to true concentration. However, if the brewed coffee sits for an extended period (about one hour), the TDS will increase significantly as coffee dust continues to release soluble compounds and form colloids after cooling.
Q4: Are there more advanced alternatives to refractometer-based TDS measurement?
A: Currently, the refractive index method remains the industry standard. Although some studies have explored gravimetric or spectroscopic methods, the coffee refractometer continues to dominate due to its portability and rapid measurement capabilities.
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