Refraction is the change in direction of a wave due to a change in its transmission medium. Put simply, light bends when it passes through something. The denser the substance (stronger coffee), the more it bends. This phenomenon can be measured very accurately. The result is a refractive index (RI) of the substance. Many industries use RI for precise, repeatable measurements of solids, liquids, and gases — including wine, sugar, soft drink, optical fibre cables, gemstones, and forensic analysis. Refractometry is the analytical method of measuring this RI.
The Coffee Refractometer is a device used to measure the extracted mass of coffee through refractometry. With handheld functionality and an instant reading, it became widely adopted from 2010 onwards after the innovations of the developer, Vince Fedele and his company, VST. When developing the coffee refractometer, VST had to find the relationship between total dissolved solids and refractive index. This might seem simple but there’s a complex relationship between the two. Many factors are at play, all affecting the refractive index of a coffee sample.
Temperature is critical — it directly affects the density of a sample, changing how light travels through it. As an example of a more obvious change in density, think of hot water in a kettle taking up more volume than cold water. Dissolved and undissolved solids are also key. Any undissolved solids (seen as the sediment at the bottom of your cup, but also the submicron particles floating in suspension throughout a coffee) will affect the density of the fluid and must be removed for accurate measurement.
Refractive index and the percentage of dissolved solids in the solution do not follow a linear relationship — there are ebbs and flows. These factors together create a complex multi-dimensional relationship between the amount of coffee solids dissolved in the water, the temperature, and the refractive index of the sample. To ascertain this relationship at all strengths, VST contracted laboratories to make thousands of brews of many different coffees, roast styles, brewing methods, and origins. These samples were weighed then dehydrated,