Infrared whisky: Testing the water of life

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  • Published: Dec 1, 2011
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Infrared whisky: Testing the water of life

Whisky galore

Near-infrared (NIR) spectroscopy on an optofluidic chip has been developed as a quality monitoring test for single malt Scotch whisky.

Praveen Ashok, Bavishna Praveen and Kishan Dholakia of SUPA (Scottish Universities Physics Alliance), at the School of Physics and Astronomy, at the University of St Andrews, in Fife, Scotland, point out how standardization and quality monitoring of alcoholic beverages is an important issue in the drinks industry. Researchers have developed various analytical methods for quality control. However, the optimal technology would require only a tiny amount of sample for ethanol determination, work rapidly and be simple to use even without high-level specialist training. Moreover, the automated, online use of such a system would be a boon for today's high-speed liquor production processes.

The St Andrews team previously developed a waveguide-based optofluidic device for Raman spectroscopy of bio-analytes and writing in the journal Optics Express now suggest that this chip can also be used for quality monitoring of single malt Scotch whisky using near-infrared (NIR) spectroscopy. The team explains that analysis on this alignment-free, portable chip takes just 2 seconds and requires a sample volume of a mere 20 microlitres. The Waveguide Confined Raman Spectroscopy (WCRS) within an optofluidic chip and adapted for NIR can be up to one-hundred times cheaper than typical fibre-based Raman probes, the team says. They add that partial least square (PLS) calibration is required but this allows them to achieve a margin on ethanol determination to within 1 percent error. Ethanol content is one of the main determinants of whisky quality. Authentic whisky must be 40% ethanol by volume. Another factor that dictates the whisky quality is the presence of organic compounds, congeners, which are formed during fermentation and processing.

Brand awareness

With such precision available, the team was then able to classify different whiskies using principal component analysis (PCA) based upon their aromatic features, age, type and cask. The team tested six different brands of commercially available Scotch whisky. Calibrations were carried out with diluted samples of pure ethanol samples from Sigma Aldrich mixed with de-ionized water. The team avoided experimental bias by creating different sets of samples with ethanol concentrations in the range 36 to 43 percent to overlap with the anticipated concentrations of the whiskies.

They found that, not surprisingly, the spectra of the whisky samples are dominated by Raman peaks of ethanol, which corresponds to a concentration of 40% by volume. While the congener components were of very low volume to give Raman signal within the sensitivity of this device, the fluorescence background in the recorded spectra found to have a correlation with the congener profile of the whisky samples.

"The prospect of implementing an optofluidic analogue of a conventional fibre-based spectroscopic probe allows a rapid analysis of alcoholic beverages with dramatically reduced sample volumes," the researchers say. They add that it is possible to use an additional parameter, the fluorescence decay constant, to classify whiskies that are otherwise indistinguishable through simple analysis, although this step extends the data acquisition time significantly.

The team adds that they tested classification of whisky samples of the same brand, 10-year old Glenmorangie, based on the maturation cask. "The difference in the cask also alters the congener profile," the team says and this was clearly seen in the data.

Strong liquor test

The earlier literature using Raman and ultraviolet spectroscopic techniques coupled with multivariate analyses reveal how powerful a tool they can be. However, these techniques for the authentication of whisky samples and other alcoholic beverages are laboratory based. The advent of NIR spectroscopy in recent years combined with the development of lab-on-a-chip technologies might now allow a powerful analytical technique to be extracted from the laboratory and brought to the production line.

"Currently, we are in discussions with liquor production companies and companies who develop analytical instruments based on spectroscopy in order to find commercialization routes for this technology," Ashok told spectroscopyNOW.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Glenmorangie photo by David Bradley - Near infrared spectroscopic analysis of single malt Scotch whisky on an optofluidic chip
Testing times for malt whisky
Scientists who performed the counterfeit whisky detection study. Left to right: Dholakia, Ashok, Praveen. Inset: the whisky chip

Scientists who performed the counterfeit whisky detection study. Left to right: Dholakia, Ashok, Praveen. Inset: the whisky chip

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