Optoelectronic sensor for measuring ethanol content during grape must fermentation using NIR spectroscopy

The evolution of sugar, alcohol, released CO₂ and microbiological activity in fermenting grape must is a matter of the utmost importance for the winemaking industry. Supervising the fermentation process is an awkward and non-comprehensive task, especially in wine cellars where production rates are massive, and enologists usually measure the density of the extracted samples from each fermentation tank manually twice a day, since density is an indicator of the total amount of sugars, ethanol and glycerol. This work shows the design of a fast, low-cost, portable and reliable optoelectronic sensor for measuring ethanol concentration in fermenting grape must samples. Different sets of model solutions, which contain ethanol, fructose, glucose and glycerol dissolved in water and emulate the grape must composition at different stages of the fermentation, were prepared both for calibration and validation. The absorption characteristics of these model solutions were measured by a commercial spectrophotometer in the near-infrared region. Spectra were analyzed using principal component analysis, in order to identify key wavelengths from which valuable information regarding the sample composition can be extracted. Finally, an optoelectronic prototype, based on absorbance measurements at three identified wavelengths, was designed, fabricated and successfully tested both with model solutions and samples from two real fermentations. The system, featuring three LED lamps and their corresponding paired photodiodes operating at 1.20, 1.30 and 1.72 µm, provides the ethanol content by applying a multiple linear regression.