The entire coffee industry is being changed by a device that can fit in the palm of your hand. A refractometer measures the refractive index of a solution. In fields as varied as veterinary medicine, brewing, and beekeeping that refractive data can be used to acquire valuable information about a sample. Although refractometers have been around since the late 19th century, only recently have they been widely used by coffee professionals.

Devices such as a VST LAB Coffee III  Refractometer can be used to find the total dissolved solids (TDS) in a brew. In other words, you can measure the exact percentage of your coffee that is dissolved coffee solids and not water. Armed with the TDS, the brew weight, and the beverage weight, you can find the extraction percentage, i.e. the percentage of the ground coffee that you extracted during brewing. Software such as VST’s CoffeeTools makes the equation as easy as plugging in the data.

Although this data might be intellectually fascinating, the pragmatist is likely to think, “Who cares? What does it taste like?” To be sure, refractometers do not replace the role of a skilled barista dialing in a coffee by taste. But they do provide an empirical reference point. This is useful because even skilled coffee tasters can be led astray by their senses. For example, a barista might taste a brew and determine that it is weak and under-extracted. Before the prevalence of refractometers, a common response was to increase the dose. Although this will result in a stronger (i.e. more concentrated) cup of coffee, it actually lowers the extraction percentage. The final brew is stronger, but less of the coffee used was extracted.

So how are coffee professionals using refractometers? Roasters are testing the solubility of their roasts to ensure they are achieving even roast development. Cafés are using refractometers to help set their brewing parameters, whether it’s autodrip or espresso. Even some home baristas are using refractometers to help them get the most out of their brews. Although it’s not a silver bullet, refractometers are a valuable tool for any coffee professional’s toolkit.