With light to the perfect coffee – refractometer in coffee preparation

VST refractometer

Overview of common coffee refractometers

Several digital refractometers have established themselves in the specialty coffee industry, varying in precision, features, and price. Here's an overview of the three most popular models:

VST LAB Coffee III

The Lab Coffee Refractometer from Voice Systems Technology (VST) is considered the gold standard in the industry. We have three models of this refractometer, and they have been serving us day after day for over 10 years.

The VST refractometer measures coffee strengths from 0.00% to approximately 20% TDS and achieves an outstanding accuracy of ±0.02% (typical). For espresso samples (with higher concentrations), the accuracy is slightly lower at approximately ±0.05% – still very precise. The device offers automatic temperature compensation in the range of 15–40°C, which is useful when the sample is not yet completely cool. High-quality sapphire optics and a 1024-pixel sensor array ensure reliable measurements. VST includes CoffeeTools software for PC/Mac, which enables extraction calculations and data analysis. It's no wonder that VST refractometers have served as official tools at world championships (Brewers Cup, Barista Championship) for years. The only downside is the price – professional quality comes with a professional price (in the four-digit range, including software and filter).

Atago PAL-COFFEE

The PAL-COFFEE from Japanese manufacturer Atago is a handy pocket refractometer and a popular all-rounder. It can display both Brix and TDS (dual-scale versions are available). We've had a PAL-Coffee refractometer since 2018 and use it in brewing courses and competitions.

With a measurement time of only ~3 seconds and automatic temperature compensation up to 100°C, it's even suitable for hot samples directly after brewing. Its accuracy is ±0.15% TDS. Perhaps a bit rough for filter coffee, but sufficient for espresso. Many appreciate the robust, waterproof construction (ideal for the hectic café environment) and the ease of use with just two drops of sample. Practical: Thanks to the Brix scale, it can also be used to measure sugar solutions or fruit juices – but when testing coffee, you should switch to the TDS display to get the percentage of coffee extract directly. The PAL-COFFEE is less expensive than the VST, but still in the upper three-digit range.

DiFluid R2 Extract

The newcomer DiFluid R2 Extract from China has been making waves as a low-cost alternative since 2022. Despite its significantly lower price, it offers impressive accuracy of ±0.02% TDS and a wide measuring range from 0 to approximately 25% (up to 30% TDS according to the manufacturer). The device is small, USB rechargeable, and IP67 waterproof, so you can even pour an espresso over it without immediately giving up the ghost.

The app integration is particularly innovative: The R2 connects via Bluetooth to the DiFluid Café smartphone app, which saves measurements, compiles statistics, and even provides updates for the device. The large display on the device itself shows all important data (including temperature) and simplifies operation with just one button. An aluminum sample tray is included, which ensures rapid temperature stability. In short: The R2 delivers many of the features of high-end devices at an affordable price. An ideal little tool for ambitious home baristas or as a second refractometer to take with you.

Note : Simple analog refractometers for wine or honey, for example, are generally unsuitable for coffee. They are often only designed for high Brix values ​​and are too inaccurate in the low TDS range, or they require manual readings, which are subjective and prone to error. Therefore, anyone serious about measuring coffee should opt for the coffee-compatible devices mentioned above.

Functional principle and correct application

How does a coffee refractometer work? Simply put, the device sends a beam of light through the sample onto a prism and measures the angle at which the light exits the sample. The denser or more concentrated the solution, the more the light is deflected (refracted). Beyond a certain angle of incidence, total internal reflection occurs. The light is then completely reflected back into the medium. This critical angle depends directly on the refractive index (RI) of the liquid. Digital refractometers use this principle: They scan different angles and detect the point at which no more light passes through. From the critical angle, the device calculates the refractive index of the sample.

In coffee, the refractive index is a function of total dissolved solids (TDS), but not a simple linear one. The relationship between RI and coffee concentration is quite complex and not strictly linear, as coffee is composed of hundreds of chemical compounds. To develop its algorithms, VST, for example, analyzed thousands of samples of different coffees, roasts, and brewing methods, determining TDS both refractometrically and by drying. A conversion formula can be derived from such data sets, allowing the refractometer display to directly show % TDS without the user having to do any calculations. Modern coffee refractometers essentially incorporate the empirical results of many laboratory tests as clever curve formulas within their chips, allowing light refraction to be reliably converted into percentages.

In addition to calibration to coffee solutions, temperature compensation plays a crucial role. Heat changes the density of a liquid and thus the refractive index noticeably. A hot sample would therefore produce an incorrect result if left uncorrected. For this reason, all high-quality instruments have automatic temperature compensation (ATC) within a certain range (typically approximately 10–40 °C or more). The electronics measure the sample temperature and convert the RI value as if the sample had been measured at, for example, 20 °C. Nevertheless, for maximum accuracy, it is recommended that the coffee sample be allowed to cool to room temperature , especially with espresso. This reduces measurement errors and also protects the instrument. After all, no one wants to shock the expensive prism assembly with boiling water.

Sample preparation is equally important. Coffee solutions—especially espresso or immersion coffee—always contain extremely fine, insoluble particles (microparticles, fines from the ground coffee) and emulsified oils. This turbidity distorts the refractive index and must be removed for a precise measurement. In practice, filter paper or special syringe filters (e.g., 0.5 μm pore size for espresso) are used for this purpose. When an espresso sample is filtered, the measured TDS usually decreases slightly because the suspended solids no longer contribute to the apparent solids content. At the same time, however, the precision and reproducibility of the measurement increase significantly.

How to use the refractometer

The following step-by-step procedure results in a reliable measurement:

1. Calibration : Before measuring, it's best to check with brewed water whether the refractometer is correctly set to 0.00% (zero calibration). This eliminates measurement drift or residue on the prism.
2. Stir and take the sample : With espresso, you typically take a few ml from the center of the shot (not the crema). I use a pipette or syringe to stir. I draw up the coffee and squirt it back into the container. Stirring ensures we take a well-mixed sample.
3. Filtration : If necessary, filter the sample through a paper or syringe filter to remove any visible suspended particles. This is especially advisable for French press, mocha, or espresso coffees, to prevent particles or oily layers from contaminating the results.
4. Allow to cool : If possible, cool the sample to approximately 20°C. While hot samples can be measured directly by the Atago or DiFluid (they compensate up to 100°C), experience shows that upon cooling, the TDS values ​​rise slightly due to an increase in density or simply some water evaporation. Therefore, it's best to wait a moment or stir the sample to room temperature.
5. Measurement : Place one or two drops of the prepared sample onto the prism, close the lid (if available), and read the value. Modern devices complete the measurement in seconds and directly display the TDS value as a percentage.
6. Cleaning : After each measurement, clean the prism with water and a lint-free cloth. Otherwise, coffee residue could dry on the prism and interfere with the next measurement.

Sounds like a lot of work? Admittedly, it does feel a bit like a laboratory. But soon, the refractometer will be as much a part of the barista's craft as a thermometer is of the chocolatier. With a little practice, the above steps take barely more than a minute. And the reward is precise data about our coffee.

Understanding Coffee Strength (TDS) and Extraction Rate (E)

Once you have determined the TDS value of your coffee, the question arises: Is this good or bad?

In addition to the strength, a second important parameter comes into play here: the extraction yield (often abbreviated as EY). The extraction rate indicates what percentage of the original coffee grounds have been transferred into the solution . In simpler terms: if you brew 20 g of ground coffee and end up with 4 g of it dissolved in the cup, the extraction was 20%. This value is a key indicator of the brewing level: too little extraction results in a thin, acidic taste, while too much extraction leads to bitterness and astringency. Balance and optimum are traditionally found at around 18–22% extraction . In this range, coffee tastes balanced, with good sweetness and without overly dominant acidity or bitterness.

The extraction rate can be calculated from the TDS and the brewing values. The classic formula is:

In words: Multiply the weight of the brewed coffee (in grams) by the measured TDS percentage and divide by the amount of dry coffee used. The result is a percentage of the original coffee mass. For example, 20 g of coffee brewed into 300 g of beverage with a TDS of 1.35% yields:

This would put the bean at ~20.3% dissolved in the water—a value right in the golden zone. In fact, ~18–22% extraction is considered optimal for most brewing methods, as unbalanced flavors tend to become more pronounced outside this range (below 18% is often underdeveloped and sour, above 22% is often dry, astringent, and bitter).

In addition to extraction, the strength (TDS) itself also influences the sensory experience. An espresso with 10% TDS and 20% extraction tastes completely different than a filter coffee with 1.3% TDS and 20% extraction – despite the same extraction rate. The former is ten times more concentrated (hence a smaller, more intense shot), while filter coffee consists of approximately 98.7% water and is significantly "thinner" on the tongue. Typical TDS ranges have therefore been established for different preparation methods:

• Filter coffee (V60, Chemex, filter machine) : approx. 1.2–1.5% TDS, at ~18–22% extraction. Mild to medium-bodied, with clear flavors.
• Espresso : approx. 8–12% TDS, also ~18–22% extraction. Very strong and concentrated, syrupy.
• Moka pot (Bialetti) : approx. 3–5% TDS, extraction ~18–22%. Stronger than filter, but not quite espresso strength—a "little black" with a pronounced body.
• French Press : approx. 1.2–1.7% TDS, extraction ~18–22%. Similar to filter coffee in strength, but more full-bodied, as there is no filter paper to retain oils and fine particles.
• Cold Brew : Typically as a concentrate, 1.4–1.6% TDS with ~18–22% extraction. It has a mild flavor due to cold brewing, although the TDS can be high. Often diluted with water/ice when served.
• Cupping (coffee tasting in the cup): approx. 1.2–1.4% TDS at ~20% extraction.

( Of course, these values ​​can vary depending on the recipe, bean, and taste preference—they are guidelines, not dogmas. ) The exciting finding is that many very different preparations ultimately aim for a similar extraction percentage. The brewing strength (TDS), on the other hand, varies greatly depending on the type of beverage. The Specialty Coffee Association's Brewing Control Chart visualizes precisely this: a coordinate system with extraction on one axis and strength on the other. This contains an "ideal" rectangle (around 18–22% E and 1.15–1.45% TDS for filter coffee) as the target range for balanced flavor. With a refractometer, you can place your brewed coffee precisely on this diagram—and control where the point moves with targeted recipe changes (e.g. grind, dose, amount of water).

Apps and software as helpers

Baristas no longer need to resort to calculators and brewing charts to determine extraction from TDS. There are a number of apps and programs designed specifically for this purpose, often working seamlessly with refractometers:

• VST CoffeeTools: The software developed by VST (for PC/Mac, partly as a mobile app) records the TDS value, coffee dose, and beverage volume, and displays extraction percentage and even graphical evaluations. It is included with the purchase of the VST refractometer and has become a standard tool in the coffee industry for recording and optimizing recipes.
• MoJo To Go: A mobile app originally from VST that allows you to enter and save measurements on the go. "MoJo" became famous as the first barista-oriented tool of its kind – it caused a sensation in 2010–2012 because it ran on the iPod Touch, giving baristas a "toy for extracting coffee" in their pockets. Today, the app is part of the CoffeeTools family and continues to serve as a handy notebook for coffee parameters. I had an iPhone back then, purely because of this app.
• DiFluid Café App: DiFluid offers an app that complements the R2 refractometer and receives data via Bluetooth. It displays live TDS, calculates the extraction rate based on the dose/output, and can save the results to a cloud storage device. The visualization is particularly nice: You can see your brew ratio, TDS, and extraction on graphs—for example, as a point on the brewing chart (SCA standard). This allows you to quickly learn where your brew was (strong/weak, under-/over-extracted).
• SmartRef Coffee Meister: Behind this unwieldy name lies an app from Anton Paar for their digital laboratory refractometer SmartRef. Although the device is a multi-purpose device (for industry, beverages, etc.), it has a dedicated coffee mode with app support. The app calculates TDS, extraction, and allows you to manage results. Anton Paar, otherwise known for high-precision laboratory instruments, is clearly also targeting roasters and laboratories that want to measure coffee quality.

These little digital helpers aren't just gimmicks. They make working with a refractometer much easier , taking the math out of the way, sometimes making adjustments (for example, if you had to dilute espresso to measure it), and presenting the results in an understandable way. Some allow data export, which is useful for roasters or coffee shop chains to monitor brewing standards. In short: apps + refractometer = data-driven coffee enjoyment!

Critical reflection: accuracy, sources of error and benefits

Amid all the euphoria over precise measurements, one should not forget: A refractometer is a tool, not a taste oracle . There are a few points worth critically examining:

• Absolute accuracy vs. practical experience: Manufacturers advertise accuracies of ±0.02% TDS – that's impressive (2 hundredths of 1%). In practice, the true deviation can sometimes be higher, depending on calibration and careful handling. The Atago, for example, has ±0.15%, which means a true TDS of 1.30% could be displayed as 1.15% or 1.45% – a noticeable range. VST and DiFluid are significantly finer, but they also have tolerances (VST ±0.1% in the espresso range according to the specification). It's important to note that for comparative measurements (e.g., recipe A vs. B), all devices are sufficiently accurate, and the trends are clearly visible. However, one should approach the final decimal point with caution. Besides, no palate in the world can taste the difference between 1.37% and 1.40% TDS.
• Sources of error & pitfalls : The biggest enemy of accurate measurements is sloppy handling. Leftover coffee on the prism? -> falsified reading. Sample not mixed properly? -> unrepresentative measurement (e.g., weaker at the top than at the bottom). Not filtered? -> suspended solids appear to increase the TDS. Measurement too hot? -> instrument may not be compensating fully, or water may be evaporating during the measurement. Tip: Always stir samples thoroughly (especially with immersion), pipette cleanly, zero the instrument, and measure within seconds (cover with the supplied lid if necessary; some refractometers have an evaporation protection feature). This will yield very consistent results (repeat measurements should differ by no more than ±0.03%, otherwise it is due to user error). There are also curious peculiarities: Some baristas swear by skimming off the espresso crema before measuring because it is lighter and floats to the top – basically irrelevant as long as you stir the sample, but it goes to show: the devil is in the detail.
• Oils in espresso: One issue that's often discussed is coffee oils. They're hydrophobic and don't really dissolve in water, but they can float around in the espresso as finely dispersed droplets (emulsion). A refractometer actually only measures dissolved substances, but if there are enough oil droplets, they scatter light similarly to solid particles. This could have a minimal impact on the RI. If you filter espresso through fine-pored paper, many oils are trapped. The measured TDS then drops by a few hundredths. Does this mean unfiltered measurements are "incorrect"? Not necessarily—after all, we drink espresso unfiltered, including the oils, and they contribute sensorially to the body. But for comparison purposes, it makes sense to always proceed in the same way (either always filter or always not filter). In competitions or laboratories, filtering is always used to achieve maximum comparability and minimize measurement noise.
• "TDS is not taste": Perhaps the most critical point: A refractometer can measure quantity, not quality . It tells us how much has been dissolved, but nothing about what has been dissolved. Two coffees can have identical TDS and extraction percentage and still taste completely different – ​​because the dissolved substances are different. One recipe might dissolve more acids and less sugar, the other vice versa. Numbers don't lie, but they don't tell the whole story either. An experienced barista will therefore always consider measured values ​​in the context of sensory feedback. The instrumental measurement complements the sensory measurement, but does not replace it. This is precisely where misunderstandings often arise: A high extraction value (e.g., 24%) could indicate overextraction – but this doesn't have to be the case if the coffee still tastes good. Conversely, a coffee with only 17% extraction can be unexpectedly balanced, for example, if the bean was very lightly roasted and less extraction is more digestible. Numbers provide objectivity, but coffee is ultimately a luxury item, not a purity competition.
• Frowns in the community: Indeed, there have been frequent heated debates in the coffee scene about the use of refractometers. Some traditionalists viewed refractometers as technical gimmicks, others as a threat to their sensory skills. Sayings like, "If you don't know TDS and extraction %, you're not a competent barista" were heard. Such statements understandably caused resentment. Fortunately, the discussion is now on a more objective level. Most have recognized that the refractometer is a tool – it can point out errors, ensure consistency, and support training. It demystifies some aspects of brewing (no more voodoo where you don't know why something tastes different today than yesterday). But it doesn't take away creativity or sensory fine-tuning. On the contrary, many top baristas use the device specifically to calibrate their taste impressions. For example, if the coffee tastes watery and the refractometer confirms a TDS of 1.0% (instead of the target ~1.3%), you know objectively: it's too thin – next time, add a little more or grind finer. This way, you learn more quickly to connect cause and effect in the brewing process.

So why a refractometric approach, despite all its limitations? Because it's another building block on the path to consistently great coffee. As one experienced Q-grader put it: "The refractometer doesn't replace your tongue and nose—but it leaves you less in the dark."

Data can be used to test hypotheses (does a finer grind really produce more extraction? Does pre-brewing help increase strength? etc.) and ensure quality. In times when specialty coffee is viewed as a craft on par with wine or culinary art, the use of measuring instruments is quite normal. It's important to use them wisely: Coffee isn't a numerical puzzle to be solved, but a pleasure that can be taken to a new level through knowledge and technology—if you so choose.

Scientific context and new findings

Coffee science is booming, and refractometers often play a key role in quantifying experiments. Here are some recent studies (2021–2024) that are exciting for professional baristas and coffee nerds:

Full immersion, solid extraction

In 2021, a study in Scientific Reports by Liang et al. show that in immersion brewing (e.g. cupping) the extraction rate is fixed within a narrow range - a kind of thermodynamic equilibrium . Interestingly, the TDS could be easily controlled via the brewing ratio (coffee/water ratio), but not the extraction (E), which was always similar at a given temperature. In other words: more water dilutes the coffee (TDS decreases), but does not extract a significantly higher percentage from the ground coffee once saturation is reached. Practical implications? For cuppings between 80 °C and 99 °C, the equilibrium extraction was always around ~20% - regardless of the amount of water. So if you brew very "thin" cupping coffee (lots of water, little coffee), you end up with a weak coffee with ~20% extraction; if you brew very strongly, you get a strong coffee with ~20%. The extraction plateau can be influenced by temperature (higher temperatures dissolve slightly more, lower temperatures slightly less). This finding turns the classic brewing chart on its head in that with immersion, the degree of extraction is less variable than expected – but the strength is very variable, depending on the dilution.

TDS vs. Taste – Cold Brew vs. Hot Brew

In 2022, a research team (Batali et al.) investigated the sensory differences between cold brew coffee and hot brewed coffee, with both subsequently adjusted to the same TDS (by diluting to 2% TDS). This made it possible to isolate how brewing temperature affects the flavor profile, regardless of strength. The result: Even with identical TDS (~2%) and the same extraction, significant sensory differences were evident. Hot brewed coffee tasted consistently more bitter and sour, while cold brewed coffee tasted sweeter and more floral, across all roasts and beans tested. Attributes such as "gumminess" (an off-flavor) were higher in hot brew, and "floral" in cold brew. This refutes the assumption of some skeptics that cold brew is merely "watered down"—no, temperature apparently influences which substances are dissolved and how polymers, acids, etc. are extracted, even if the total amount is the same. For baristas, this means: TDS and extraction percentage alone don't explain the flavor. The method of extraction (in this case, temperature) plays a major role in the aroma profile.

Grinding, fines and extraction dynamics

An exciting study from 2024 ( Smrke et al., Scientific Reports ) examined the influence of coffee particle size distribution —specifically the proportion of fines (ultrafine particles <100 µm)—on espresso extraction (for a better understanding, I recommend our article on " Particle Distribution "). The researchers varied the amount of fines in the coffee puck in a controlled manner and measured the effects. More fines resulted in significantly lower permeability of the puck, the espresso flowed more slowly, and the extraction time increased. While the extraction yield increased slightly, they interestingly found that aroma compounds do not increase linearly with extraction, but rather plateau or even show losses at a certain point. They suspect that while higher extraction releases more aroma, a longer extraction also allows more volatile aromas to escape, even during the extraction process. In other words, an espresso with 25% extraction might have less aroma in the cup than one with 22%, because the longer extraction allows time for aroma evaporation. The study also confirmed sensory analysis that too many fines tend to disrupt the balance—probably by overextracting bitter compounds in areas where the water lingers longer. Practical lesson: A consistent grind with a moderate proportion of fines promotes both flow and flavor balance. The refractometer helps track overall extraction, but detailed analysis (aroma via PTR-MS) shows that more extraction doesn't necessarily mean better.

Taken together, this research demonstrates how important precise measurement is for understanding coffee. Without refractometers (and other measurement methods), many of these findings would not have been possible. In return, science provides exciting insights that professional users can then test in practice ("science-fueled coffee!"). It's a virtuous circle: baristas use scientific methods, and scientists investigate barista questions. Ultimately, all coffee drinkers benefit from better coffee.

Conclusion on the refractometer: Strength and extraction yield

Refractometers have evolved from laboratory instruments to the secret weapon of modern coffee brewing . They allow you to quantitatively measure strength and extraction—two key factors for flavor—and thus control them more consciously. In my daily brewing practice, I rarely use a refractometer.

For me personally, however, it was an important tool, especially in preparation for national Brewers Cup championships, as well as the World Brewers Cup in Rimini in 2024. The refractometer helped me understand brewing filter coffee and espresso much better. It also helped me improve my own sensory perception and challenge myself. Twelve years after buying my first refractometer, I still love the game of tasting with the device. I brew a coffee and try to predict the correct TDS. Even if I'm wrong, I learn something about brewing.

Models like the VST, Atago, or DiFluid offer the right tool for a wide range of needs: from high-end devices with the highest precision to affordable pocket solutions for home use. Combined with apps like VST CoffeeTools or DiFluid Café, monitoring and dialing in recipes becomes a data-driven pleasure. Nevertheless, the following applies: Numbers serve the taste, not the other way around. A refractometer tells us what happened, but whether the result is good is still determined by the tongue and nose. However, when you combine both—sensory expertise and measurement data—you can refine coffee perfection almost alchemically.

Ultimately , despite some limitations, refractometers are a valuable tool for ensuring consistency, supporting learning processes, and further uncovering the secrets of the cup. They've helped transform coffee from a "beverage of feeling" to a beverage of knowledge. And let's be honest: A little nerdiness suits us baristas well, as long as the end result is a delicious cup. With that in mind: May your coffee be delicious and your TDS on point! ☕️📈