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NMR can be Used to Characterize the Complex Composition of Scotch Whisky

Scotch Whisky is made using three stages: fermentation, distillation, and maturation in oak barrels. Each step contributes to the complex composition of higher alcohols, carbohydrates, and aromatic cask extractives that create the distinctive taste of this spirit. Characterization of Scotch Whisky has typically been performed by gas or liquid chromatography coupled to ultraviolet-visible or mass spectrometry. These techniques are highly reproducible and offer quantitative data but are limited by long run times and the need for extensive method development.

A recent study has demonstrated that NMR in combination with chemometric techniques can yield a large amount of information about the complex chemical composition of Scotch Whisky.

Using a 600 MHz Bruker Avance III spectrometer 180 Scotch Whisky samples were analyzed, and all were found to share common features between 0.8 ppm and 10 ppm. Compounds that exist in multiple forms, including α and β epimers of glucose and fructose and acetaldehyde in its hemiacetal form, were also successfully identified, though diffusion ordered spectroscopy (DOSY) was required to characterize the multiple forms of acetaldehyde.

Statistical total correlation spectroscopy (STOCSY) and independent component analysis (ICA) was then performed, which enabled correlations between signals from the same compound or compounds with similar origins to be determined and decomposition of the NMR spectra into individual spectra of three different compounds, respectively.

Principal components analysis (PCA) was able to approximately separate the samples based on their alcohol strength, due to the subtle changes in the relative composition of the compounds and chemical shifts when this parameter changes.

PCA models also showed a clear separation between the blended and malt whisky samples, with the blended whiskies being more tightly clustered indicating less variation in the group. Orthogonal partial least squares-discriminant analysis (OPLS-DA) was able to perform this differentiation as well and, in addition, was able to correctly classify two high-end blends that were incorrectly characterized by PCA.

Furthermore, OPLS-DA models could distinguish between peated and unpeated samples and determine whether the samples had been placed in ale casks for maturation compared to bourbon or sherry casks.

PCA and OPLS-DA models were both able to distinguish between authentic and counterfeit Scotch Whisky. The authentic samples contained the presence of higher alcohols, while the counterfeit samples had higher levels of carbohydrates as well as detectable levels of glycerol, which was not observed in any of the authentic samples. Similarly, to the differentiation of blended and malt whiskies, the analysis with OPLS-DA models was statistically significant when comparing the whole NMR spectra or the region between 6 and 10 ppm, while the PCA data was not as reliable.

Overall, this study demonstrates proof-of-principle evidence that NMR spectroscopy in combination with chemometric techniques fast, untargeted, and quantitative analysis of the complex composition of Scotch Whisky.

References

  1. Kew W., et al. (2019). Analysis of Scotch Whisky by 1H NMR and Chemometrics Yields Insights into its Complex Chemistry. Food Chemistry. https://doi.org/10.1016/j.foodchem.2019.125052.