Wine industry requires cheap, effective and fast measurement methods, which have led to the use of near infrared (NIR) spectroscopy as a technique for measuring quality parameters throughout the production process of this industry, from grapes to wine.
Wine is mainly composed of water (70-90%), ethanol (8-20%), sugars (0.1-20%) and acids, although it also has other chemical compounds that greatly affect the sensory properties of the final product. Currently, the main application of NIR spectroscopy in the wine industry is associated to the measurement of alcohol content using two or three relevant wavelengths. However, NIR technology offers much more possibilities, as it is detailed in this post.
First, NIR technology enables the measurement of total soluble solids (a measure of sugars, mainly glucose and fructose) in grapes. Soluble solids content calibrations typically use wavelengths tied to O-H and C-H bonds (980, 1400, 1900, and 2170 nm). NIR and visible spectroscopy are also employed to determine the anthocyanin concentration (linked to the color of the grapes) in homogenized red grapes, although the results are better when the study is restricted to grapes of a single variety and/or geographical area. Acidity of pH are other parameters that are studied. It is also possible to carry out the measurements with grape berries, but the complexity of the measurement system increases exponentially.
Regarding wine, NIR spectroscopy focuses on the study of its chemical composition, being the alcohol content the most studied parameter, which can be determined thanks to the absorption band in the NIR of ethanol. There are also studies that obtain good calibration models for pH, reducing sugars and lactic acid in rose, red and white wines. Fourier transform infrared spectroscopy (FT-IR) is utilized to analyze the authenticity and traceability of wines.
MIR (mid-infrared) spectroscopy or Raman spectroscopy are other techniques used to determine the chemical composition of wine. MIR spectroscopy has better results for the measurement of alcoholic graduation and total acidity. Raman spectroscopy, on the other hand, is considered the most suitable method for total sugar content determination. Raman spectroscopy, as well as NIR and MIR spectroscopy, show good results regarding density, but neither of them offers acceptable precision for volatile acids.
On the other hand, grape spirit (brandy) is obtained by distillation of grape marc, that is, the solid parts of the grape that can be used in the production of wine. The concentration of methanol in the final product must be controlled to comply with food regulations. Two critical parameters that are monitored during the distillation process are methanol and ethanol, which have a characteristic spectrum at NIR and whose concentration can be precisely calibrated.
In conclusion, NIR spectroscopy can be used at different stages of the wine-making process becoming a relevant analysis technique in this field thanks to the development of cheap, fast, non-invasive and portable instrumentation.
Written by J.J. Imas
 Cozzolino, D.; Dambergs, R.G.; Janik, L.; Cynkar, W.U.; Gishen, M. Analysis of Grapes and Wine by near Infrared Spectroscopy, Journal of Near Infrared Spectroscopy2017, 14, 279–289, doi:10.1255/JNIRS.679.
 Teixeira dos Santos, C.A.; Páscoa, R.N.M.J.; Porto, P.A.L.S.; Cerdeira, A.L.; González-Sáiz, J.M.; Pizarro, C.; Lopes, J.A. Raman spectroscopy for wine analyses: A comparison with near and mid infrared spectroscopy. Talanta 2018, 186, 306–314, doi:10.1016/J.TALANTA.2018.04.075.