Whisky is a complex and variable material, the quality of which is affected by the raw materials and processing.
Luisa Pereira, Thermo Fisher Scientific
Whisky is a complex and variable material, the quality of which is affected by the raw materials and processing. The overall flavor and quality results from the combination of levels of higher alcohols and other flavor components generated as a result of the fermentation process.
There are two basic types of whisky, pure malt and grain. Single malt is considered the best quality with premium brands claiming individual, unique flavors. Grain whisky is not commonly available and many commercial whisky brands are a blend of malt and grain being labelled "blended whisky."
The compounds of interest in the analysis of whisky quality and for whisky differentiation are:
Acetaldehyde, Methanol, n-Propanol, isobutanol (used for brand differentiation), Ethyl acetate, and 3-Methylbutanol and 2-Methylbutanol (not present in grain whiskies, indicate quality of blend).
Whisky analysis has traditionally been carried out using a polar, wax type column, however this has some limitations. Firstly, the very volatile components, especially those eluting prior to the ethanol peak are difficult to separate on a wax column due to their short residence time on the stationary phase. Secondly, the wax phase in not able to resolve 2- and 3-methylbutanol (amyl alcohols). The use of a nonpolar phase enables separation of the 2 critical amyl alcohols as well as shifting the elution of some volatiles to positions where they are easily measured.
A Thermo Scientific TRACE™ GC ULTRA, fitted with a FID detector, split/splitless injector and Helium carrier gas was used. All injections were 1 μL dispensed by a Thermo Scientific TriPlus™ autosampler.
Thermo Scientific TRACE TR-WaxMS 30 m × 0.25 mm × 0.25 μm; Thermo Scientific TRACE TR-V1 30 m × 0.25 mm × 1.4 μm.
The method was optimised for each column (1).
A set of standards was run on each column to establish the position of each alcohol peak. Whisky samples (commercially available whiskies) were injected neat.
Figure 1 shows the analysis of The Balvenie® 10-year-old Scottish whisky, a Speyside single malt brand on the two columns.
Figure 1: Speyside whisky analysis on (a) TR WaxMS and (b) TR V1. Compounds: 1. Acetaldehyde, 2. Methanol, 3. Ethanol, 4. n-Propanol, 5. Ethyl acetate, 6. Isobutanol, 7. n-Butanol, 8. Acetal, 9. 3-Methylbutanol, 10. 2-Methylbutanol. Experimental conditions in reference 2.
The profile for the polar TR-WaxMS column shows the close elution of ethyl acetate and acetaldehyde diethyl acetal (peaks 5 and 8) and the two amyl alcohols (peaks 9 and 10) are unresolved.
The polar TR-V1 column gives a better separation of all the required compounds. The 3-methyl butanol and 2-methyl butanol are separated, making it possible to better assess the quality of the whisky. Some tailing occurs but statistical analysis of 10 injections confirmed good reproducibility for measured amyl alcohols.
The advantage of the wax type column is better peak shape and being well establish in this type of analysis. The disadvantage is the co-elution of the low polarity compounds prior to the ethanol peak which may be required as a measurement of the wash quality. The TR-V1 column (cyanopropylphenyl) gave better resolution for all compounds. This column is also suitable for detailed analysis including characteristic flavor and phenolic compounds.
(1) Whisky, Technology, Production and Marketing, Pub. Academic Press. ISBN 0-12-669202-5.
(2) J. Warden, L. Pereira, Thermo Scientific Poster PO20373_E02/07.
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