The distinctive and complex taste of Scotch Whisky originates from the thousands of compounds that can be found in each bottle. Each phase of the production process, from maturation in oak barrels to fermentation and distillation, are essential for building its unique flavor. Scotch Whisky is vulnerable to counterfeiting due to its expensive price point, which means chemical analysis is vital for its authentication.
Authentication is usually carried out using liquid or gas chromatography combined with mass or ultraviolet-visible spectrometry as highly reproducible techniques where quantitative data can be acquired.
These techniques are restricted by the requirement for method development, the smaller range of compounds that can be positively identified, and their long run times.
NMR can be used to analyze alcoholic beverages
Previous studies have demonstrated the efficacy of nuclear magnetic resonance (NMR) for the analysis of alcoholic beverages along with different food products like fruit juices and honey.
This led researchers in a recent study to combine NMR with a range of statistical analysis methods. A range of methods (orthogonal partial least squares-discriminant analysis (OPLS-DA), principal components analysis (PCA), independent component analysis (ICA), and statistical total correlation spectroscopy (STOCSY)) were used to differentiate between 180 Scotch Whisky samples and to determine counterfeit products.
A sodium acetate/acetic acid buffer in D2O containing DSS was used to mix all samples, and a 600 MHz Bruker Avance III spectrometer was used to perform NMR analysis.
Identification of compounds that exist in multiple forms
25 chemical structures in total were positively identified, and all samples were determined as sharing common features between 0.8 ppm and 10 ppm, such as carbohydrates, aromatic cask extractives, and higher alcohols. Multiple compounds were more challenging to characterize as they are found in several forms.
The carbohydrates fructose and glucose, which can be present as α and β furanose or pyranose structures, were effectively identified using NMR even though ethanol and pH strength can impede characterization.
Acetaldehyde can be challenging to identify as it produces hemiacetals in ethanol and water but is an essential compound in the maturation pathway. The two hemiacetal forms of acetaldehyde could be determined through the use of diffusion ordered spectroscopy (DOSY).
Signal overlap can impede the quantification and characterization of Scotch Whisky as a result of the high quantity of compounds in the sample. STOCSY was carried out to gain additional understanding about the origins of and relationships between each compound. This model was chosen as it can be utilized to establish correlations between signals from compounds with shared origins or from the same compound.
ICA analysis was carried out along with this, which can break NMR spectra down into separate compound spectra. The spectra of three distinct compounds could be isolated employing this technique.
Distinguishing characteristics of Scotch Whisky samples
Alternative chemometric techniques combined with NMR were also used by the group to differentiate between a range of characteristics in the Scotch Whisky samples. A distinct separation between malt and blended whiskies could be shown by PCA.
Compared to the malt whiskies, the blended whiskies were more tightly clustered, showing that this group had less variation. It was also found that there were two blended whiskies grouped with the malts, but a high proportion of malt spirit was found in their blend.
OPLS-DA could also effectively differentiate between malt and blended whiskies. Two premium blends that were incorrectly characterized by PCA were correctly characterized by this method.
The samples could also be approximately divided according to their alcohol strength utilizing the PCA model as it could identify slight variations in the relative composition of the compounds and chemical shifts that arise when changing alcohol strength.
Peated and unpeated samples could be differentiated by the team using OPLS-DA, but it was not understood how this model attained this differentiation. The type of barrels that were employed in the maturation stage of the whiskies could also be differentiated using OPLS-DA.
There were four separate barrel types that were compared, which were ale, sherry, bourbon, and bourbon and sherry. This model could determine the significant difference between whiskies matured in ale barrels in comparison to the other three types. It was not possible to gather additional differentiation between the sherry, bourbon, and bourbon and sherry matured whiskies.
Authentication of Scotch Whisky
The team could also effectively isolate counterfeit whisky samples by combining 1H NMR with chemometric techniques. The key variations were a greater level of carbohydrate in the counterfeits and the presence of higher alcohols in the genuine samples. Detectable amounts of glycerol were found in the counterfeit samples, which was not identified in any of the genuine samples.
Some differentiation between the genuine and counterfeit whiskies was demonstrated by the PCA model, but due to the large difference within the counterfeit sample set and significant difference between both sample sets, this characterization was not statistically significant.
OPLS-DA models were effective in confidently identifying the genuine samples from counterfeit samples either by utilizing whole NMR spectra or the region between 6 and 10 ppm.
This study highlights the proof-of-principle evidence that NMR spectroscopy combined with chemometric methods delivers quantitative, untargeted insights into the diverse chemistry of Scotch Whisky.
By using this technique in combination with various chemometric methods, more detailed characterization can be gained, and most crucially, counterfeit samples can be successfully identified.
This technique overall provides a quantitative, routine, and efficient alternative to other analytical methods for use in the characterization of Scotch Whisky.
References and further reading
- 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.
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