Methodological issues of grape and wine water evaluation by quantitative nuclear magnetic resonance deuterium spectroscopy (2H(D)-qNMR)

Abstract

The new methodological approach of rapid evaluation of intracellular grape and wine water without sample preparation, based on the quantitative nuclear magnetic deuterium spectroscopy (2H(D)-qNMR) is suggested and developed under the research program carried out under the scientific grant of the International Organization of Vine and Wine (O.I.V.). The approach provides a quantitative high precision measurement of the deuterium content in water based on the direct dependence of the areas of NMR signals on the number of nuclei responsible for these signals. The developed approach uses similar to the well-known SNIF-NMR method with an internal reference substance (IRS) - dimethyl sulfoxide (DMSO). The IRS signals in the NMR spectrum do not overlap with the signals of main components of the analyte (e.g., grape must/juice, wine). In addition, it is possible to change the deuterium content using an available NMR solvent - DMSO-d6. At the same time, the amount of deuterium in the IRS-DMSO is increased by adding DMSO-d6 so that the proportion of the standard, equal to 10-15% by volume, gives a signal commensurate in its intensity with the signal of water contained in the analyte. The exact amount of deuterium in the IRS is determined by comparing the integral intensities of signals in the 2H(D)-NMR spectrum in a standard water sample (VSMOW) with a known deuterium content. After calibration of the deuterium content in IRS, the analyte and DMSO in quantities of 0.5 and 0.075 ml, respectively, are placed in a standard tube, then the 2H(D)-NMR spectrum is measured in the following conditions: 90° pulse, 2.5 sec acquisition time, 3 sec delay relaxation, 1000 scans. Additionally, to assess the amount of water in the analyte, the 1H-NMR spectrum is recorded, from which the contents of main sample components, for example, ethanol, glycerol, sugars, etc., are determined. The calculation of the deuterium content is carried out by quantifying the integral intensities of the IRS signals and the water contained in the sample. The developed methodological approach has been validated in experimental tests to study the natural content of deuterium in intracellular water of grapes of different seasons, as well as to study changes in the quantitative balance of deuterium in the case of adding extension water to musts before and after fermentation.