The Number That Explains the Taste

A barista pulls a shot, takes a sip, and frowns. Something is off. The espresso tastes sharp, almost vinegary, but the grind looks right, the dose is correct, and the timer showed 28 seconds. Without a way to measure what actually happened inside that puck, the next adjustment is a guess.

Refractometers offer a way out. These optical instruments measure how much light bends as it passes through a brewed coffee sample, converting that reading into a total dissolved solids (TDS) percentage. TDS tells you how much of the liquid is dissolved coffee material rather than water. Filter coffee typically lands between 1.15% and 1.45% TDS, while espresso runs far more concentrated, usually 8% to 12%.

For Sofia's growing specialty coffee community, understanding extraction science is becoming part of the conversation. Refractometers remain relatively uncommon even among serious baristas here, but as the local scene matures, the question shifts from whether these tools exist to whether they actually help.

What the Numbers Actually Mean

TDS alone tells only half the story. Baristas combine it with the coffee dose and beverage weight to calculate extraction yield:

Extraction yield (%) = TDS × beverage weight ÷ coffee dose

TDS tells us how concentrated the coffee is, while extraction yield tells us how much of the coffee was extracted into the cup.

Shaun Liew, co-founder of Curate Coffee Roasters and ONO Specialty Coffee & Matcha, Malaysia Brewers Cup champion

A concrete example: brew 20g of coffee, end up with 300g of beverage at 1.30% TDS, and the extraction yield comes to roughly 19.5%. Most specialty guidelines treat 18% to 22% as balanced territory. Below 18%, coffee tends toward sour, under-extracted flavours. Above 22%, bitterness and astringency take over.

The critical insight is that TDS and extraction yield measure different things. A high TDS with a low extraction yield usually points to a strong but sour brew. A low TDS with a high extraction yield often produces a thin, bitter cup.

I don't look at TDS and extraction as two separate measurements. I use them together to understand what's happening in the cup.

Shaun Liew

Why Accurate Measurement Is Harder Than It Looks

Taking a reliable reading requires more care than most baristas expect.

Temperature-related issues are one of the biggest factors in incorrect measurements across the industry.

Ben Barnard, Program Manager at DiFluid

Many refractometers require samples to cool to room temperature before reading, adding friction to a busy workflow. Other sources of error compound the problem: undissolved solids distort results, inconsistent zeroing throws off baselines, and inaccurate dose weighing feeds directly into the extraction yield calculation.

A few habits improve reliability:

• Filter samples before measuring
• Zero the device with distilled water for comparable figures, or with your brewing water to isolate what the coffee added
• Weigh the dose accurately
• Measure the same recipe several times to understand what normal readings look like

This teaches you what your numbers normally look like and helps you understand the relationship between your recipe, your sensory experience and your extraction.

Shaun Liew

Where Refractometers Fit in Real Practice

Refractometers appear most often in cafés focused on quality, training, or competition. The barrier is not cost. It is ease of use and understanding.

Emerging technology is removing friction. DiFluid's RS Extract reads samples between 4°C and 65°C without cooling, eliminates the need for a pipette, and pairs with scales via Bluetooth to log data automatically. But the tool itself is not the point.

Don't start by chasing a number. Start by learning how to taste. First brew coffee, taste it, and form your own opinion about what you like. Then use the refractometer to understand why you are getting that result.

Shaun Liew

In his shops, the refractometer serves as a quality-control and training tool, not something used for every cup. When developing a brew recipe, the team tastes first. If the flavour is right, they measure TDS and extraction to create a reference point. When something changes, they measure again to see whether the brewing parameters shifted.

The device also supports staff training.

Instead of telling my team, 'This extraction is too low,' we can taste the coffee together, measure it, and connect the sensory experience to the number. Over time, that helps baristas develop their palate and understand cause and effect.

Shaun Liew

Taste first, measure consistently, use data to guide small changes. The refractometer cannot replace a trained palate, but it can explain why the palate noticed something was off.