🍷 Refractometer

A refractometer is an instrument that estimates the sugar content of grape juice from its refractive index. A handheld refractometer is often used for a quick test of grape maturity. The juice is squeezed from a sample of berries, a drop is placed on the glass surface of the prism, the prism box is closed and the instrument is held towards a light source. The field of view shows a dark and a light region separated by a boundary, and the reading is taken on the scale where the boundary crosses it, in degrees Brix.

The method is described by the International Organisation of Vine and Wine (OIV): the refractive index of the must at 20 °C, expressed either as an absolute value or as a percentage by mass of sucrose, is converted by a table into the sugar concentration in grams per litre and per kilogram. One degree Brix is one gram of soluble solids per 100 grams of solution. Refractometers are calibrated at 20 °C; if the juice is at another temperature, a correction is needed, and for every degree above or below 20 °C an instrument without automatic temperature compensation needs 0.07 °Brix added or subtracted.

The soluble solids of grape juice are used as an indicator of grape ripeness. Sugars make up 90–94% of the soluble solids, so a refractometer gives only an estimate of the sugar concentration. Brix is not equal to the fermentable sugars (glucose and fructose), although it is typically used to estimate the potential alcohol of the finished wine. The standard grape measurements are Brix, pH and titratable acidity.

Iowa State University Extension says that a refractometer gives a scientific basis for harvest decisions, and that most table wines are harvested between 14 and 25 degrees Brix depending on the cultivar, so the instrument must be able to read in that range.

Why it matters to you. A Brix reading shows sugar only approximately and says nothing about acidity: to judge ripeness, pH and titratable acidity are measured as well, and a representative sample of berries is the first step.