Temperature dependence of grape must refractive index and its application to winemaking monitoring

Although winemakers rely on density measurements to supervise the grape must fermentation that results in wine, recent studies show that the refractive index n is also a valid choice for supervision purposes. However, its dependence on the composition of the fermenting must and especially on the temperature of the sample under test makes it difficult to advance the supervision into an automated stage. The lack of correction data necessary to adjust n measurements made at temperatures other than 20 °C (the common reference in winemaking) as a fermentation progresses requires that measurements are made at 20 °C, which is awkward in a real-time monitoring scheme. Aiming at providing a solution to this issue, the present work studies the dependence of n on the temperature and on the must composition during a whole fermentation span. Model solutions composed of fructose, glucose, ethanol and glycerol, along with grape must samples extracted from a fermenter at laboratory scale, were analyzed by refractometry from 15 to 30 °C, an acceptable temperature range for a fermentation to proceed. Finally, the experimental data were fitted to a second-order polynomial, resulting from a k-fold cross validation, to provide a formula that accurately yields temperature-corrected n values. The formula was validated using a portable and low-cost optoelectronic prototype, sensing n from the position of a laser spot with a resolution of 2.3 × 10−5 RIU (refractive index unit), and featuring a temperature controller.