Measuring strength and extraction

What the instrument reads

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Total dissolved solids, usually written TDS, is the share of a drink by weight that is dissolved coffee. A filter coffee is mostly water; the classic brewing chart places its central region between 1.15 and 1.35 per cent.

The instrument used is a digital refractometer, which measures how much a drop of liquid bends light and converts that to a concentration. The authors of an espresso study describe the limits of this plainly: the reading depends on temperature, it cannot say what the dissolved material is made of, and it has been shown to track the extracted mass. In one published study, the researchers cooled a sample of each brew to room temperature in a sealed container before placing a single drop on the instrument, and reported that drying brewed samples in an oven had confirmed the instrument's readings. That describes their instrument and their coffee; it is good evidence that the method can work, not a calibration of anyone else's device.

The arithmetic

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The same study states the calculation in one line: extraction is the strength multiplied by the weight of the brewed coffee, divided by the weight of the dry grounds.

1. Weigh the dry coffee. The dose in grams, before any water touches it. For example, 20 grams.
2. Weigh the drink, not the water you poured. The liquid that reached the cup or carafe, in grams. Grounds keep some of the water, so this is always less than what went in. For example, 300 grams.
3. Read the strength. The refractometer's figure as a percentage. For example, 1.30 per cent.
4. Multiply and divide. 300 grams at 1.30 per cent holds 3.9 grams of dissolved coffee. Divided by the 20 gram dose, that is 0.195, an extraction yield of 19.5 per cent.

The calculator linked below does exactly this and shows the intermediate step. It is arithmetic; the uncertainty lies entirely in the three numbers fed to it.

Where the error comes from

CoffeeHQ explanation

Using the water poured in place of the drink

The commonest slip. It overstates the result by the weight of water the grounds kept, which is why an implausibly high answer usually means this number is wrong.

A warm or evaporating sample

The researchers quoted above cooled their samples in sealed containers before measuring. A sample left open loses water and reads stronger than the brew was.

Small numbers multiplied

A filter coffee's strength is a little over one per cent. A difference of a few hundredths in the reading moves the calculated extraction by several tenths of a point, so a single reading settles very little.

Coffee left in the grounds

A study of full immersion found that the older method of drying and weighing the spent grounds understates extraction, because the wet grounds still hold brewed coffee. The two methods are not interchangeable.

What a refractometer reading is, strictly

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A refractometer measures how much a liquid bends light. Dissolved material bends it more, so the reading can be converted into a concentration. The conversion is a calibration and not a law: one laboratory describes calibrating its instrument, following a German standard, by matching refractive index against the weight of dried samples. The figure a coffee refractometer displays is the answer that calibration gives, on the assumption that the dissolved material in the sample resembles the material it was calibrated with.

The authors of an espresso study that leaned on the measurement are explicit about its limits. Refractive index depends on temperature. It cannot be used to characterise the composition of the drink, because different molecules bend light to different degrees. It has been shown, they write, to correlate accurately with extracted mass. And their own results show the consequence: two shots at the same extraction yield, one from a fine grind and one from a coarse, cannot have the same chemical composition. The reading counts how much dissolved; it is blind to what.

It also sees only what is dissolved. An espresso carries fine particles, droplets of oil and bubbles of gas, none of which belong in the figure and all of which can disturb the reading. That is why laboratories treat the sample first, and they do not all treat it alike.

Three laboratories, three ways of preparing an espresso sample

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Each of these is the method section of a published espresso study. They are set side by side to show that a dissolved-solids figure for espresso is the product of a procedure, and that figures from different procedures should not be compared to the second decimal place.

Spin, dilute, measure at 20 °C

One group centrifuged each sample, diluted it one part to three with purified water, and read the refractive index at a fixed wavelength and 20 degrees Celsius on a laboratory instrument calibrated against dried samples.

Filter, then read

A second group passed the brew through a filter with pores of 0.45 micrometres and read it on a hand-held refractometer sold for coffee, which is calibrated by its maker to display dissolved solids.

Dilute when too strong

A third read single drops on the same kind of instrument and, for the most concentrated first portions of a shot, diluted them with distilled water to bring them into the range it could measure.

The chart, and what became of it

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The numbers are usually read against the coffee brewing control chart, which the researchers quoted above trace to work published in 1957. It marks a central region, 1.15 to 1.35 per cent strength and 18 to 22 per cent extraction, as ideal, and labels the areas around it weak, strong, bitter and underdeveloped. It is still the basis of trade training.

The researchers who quoted those figures also observed that there was little rigorous sensory evidence behind the brewing temperature the industry specifies. Their group later published a revised chart built from its own tasting-panel and consumer studies of drip coffee, which replaces the single ideal box with a map of which tastes are associated with which region. Its mapping is reported for drip coffee, the coffees brewed and the people who took part.

The revised chart also reports two groups of drinkers with different preferences across the same region, and relabels the old box classic in place of ideal, because, as its authors put it, it is unclear which consumers found those conditions ideal. The upshot is that the famous box is a convention. It is a reasonable place to start and a poor thing to chase.

What a reading cannot do

CoffeeHQ explanation

Two cups with the same strength and the same extraction can taste different, because the numbers say how much dissolved and nothing about what. An average also hides unevenness: a bed half overworked and half untouched can produce the same figure as one extracted evenly.

A reading is most useful as a comparison with your own earlier readings, taken the same way on the same equipment. Used like that it can tell you whether a change moved strength, extraction or neither. Used as a score against a chart, it invites adjusting a cup that tasted fine.

What is outside this page

CoffeeHQ explanation

It names no extraction figure as a goal. It gives no range for espresso. The one espresso study read quotes a trade recommendation of 17 to 23 per cent and then shows two shots at the same figure that cannot be alike. It does not describe how any particular refractometer should be zeroed or cleaned, which is for its maker's instructions. And it does not reproduce the correction for retained liquid that the immersion study derives; that paper should be read directly by anyone who needs it.

What to do next

• Extraction yield calculator — Does the arithmetic above, shows its working, and flags an implausible input.
• Coffee refractometer — What the instrument is, and when owning one starts to be useful.
• Coffee extraction — What strength and extraction mean in the cup, and what to change.