Examining Volatile Aromatic Congeners in Irish Whiskey Using GC–MS and GCO

Whiskey's intricate and diverse flavors are the result of a complex interplay of congeners, a group of chemical compounds that are naturally produced during the fermentation and aging process in whiskey production. Congeners are shaped by the type of cereal used, the mash bill, and the stages of malting, mashing, fermentation, distillation, and cask maturation. These congeners serve as biomarkers for quality and authenticity, and scientists have been using analytical techniques to improve whiskey manufacturing (1).

These compounds contribute to the flavor, aroma, and overall character of the whiskey. Congeners can include a wide range of substances, such as aldehydes, esters, higher alcohols, and phenolic compounds, among others.

However, despite the recent advancements in extraction and analytical techniques, there has been limited research on the extensive profiling of congeners specific to whiskey styles, especially within the rapidly expanding Irish whiskey industry. As technology and chromatographic techniques continue to evolve, there is potential to gain deeper insights into the congeners responsible for distinct flavor profiles and their role in authentication (1).

Professor Kieran N. Kilcawley of Teagasc Food Research Centre and of University College Cork and his colleagues are using gas chromatographic (GC) techniques to explore how to analyze the quality and taste of Irish whiskeys through congener profiles. Research into these congeners is essential for understanding and managing the evolving industry, especially as it rapidly grows (1). LCGC sat down with Prof Kilcawley to learn more about the research he and his team have conducted, the conclusions they’ve drawn from their chromatographic analysis, and the role that chromatography will play going forward in the Irish whiskey industry.

Professor Kieran N. Kilcawley of Teagasc Food Research Centre and of University College Cork | Image Credit: © Kieran Kilcawley.

Can you explain what is a congener and why the congener profile of a whiskey is critically important to its quality in whiskey production?

The term congener is a term used uniquely to describe flavor active compounds in alcoholic beverages. The congener profile of a whiskey is formed from the combination of ingredients, and each part of the process, with fermentation and cask maturation assumed to have the greatest impact on the flavor of the final product. Therefore, a congener profile can potentially be used to differentiate styles of whiskey, brands, and even provide information on cask types. Understanding more about congener formation during the process and their association with sensory quality will provide additional information that could be used to improve quality, or direct flavor. Greater information on congener profiles related to specific types of whiskey can also be used as potential biomarkers for authentication.

What type of congener analysis have you and your team been conducting?

We work on samples diluted to 10% alcohol by volume (ABV), as it helps to standardize different samples for comparative analysis, as the ethanol content of whiskies vary depending upon the type, but also in relation to the strength of new make spirit (post distillation prior to addition the cask), cask strength and in the final bottled product. This is also undertaken to reduce the amount of ethanol introduced into the gas chromatography–mass spectrometry (GC–MS) instrument. The main difficulty with whiskey samples is that they have a very high concentration of ethanol compared to all the congeners. At the moment, we have profiled various new make spirits, single malts, pot still, and grain and blended whiskies. To date, we have optimized and evaluated two volatile extraction methods, headspace solid-phase microextraction (HS-SPME) and HS-SPME Arrow, using both conventional GC–MS and two-dimensional GC time-of-flight MS (GCxGC–TOF-MS).

Can you briefly talk about the four types of Irish whiskey and explain how these whiskey types influenced how you and your team conducted your analysis of the congener profiles?

The four types of Irish whiskey are single malt (made from 100% barley in a pot still), pot still (made from no less than 30% malted barley, 30% green barley and up 5% other cereal sources in a pot still), grain (can be any cereal source but mainly maize, wheat or rye in a column still), and blended whiskey (which consists of mainly grain whiskey with either single malt or pot still added, or both). Each single malt, pot still, and grain whiskey can be cask matured in any type of wooden barrel (mainly oak), but it must be for more than three years. Most bottled whiskies are produced from more than one aged cask (for example, the final product may be an amalgamation of numerous casks, with the choice and ratio dependent upon the skill of the master distiller). A single malt may be a blend of various single malts matured in different casks (virgin oak, ex sherry, port, bourbon casks, to name a few) but from the same distillery. In order for Irish whiskey to be labelled as a spirit drink and of Irish origin production must comply to the Irish Whiskey Technical File (2) and certain legal definitions, under EU Regulations EU2019/787 and 2021/1235 associated with its Geographical Indication Status. The final products are quite complex because of the potential diversity in both the production process and in cask maturation. Grain whiskey tends to have a less complex profile as a column still is used in its production, which can generate higher concentrations of pure ethanol, with less congeners (those less volatile than ethanol). All these factors need to be considered when developing a suitable method to extract and identify volatile congeners. A key issue as previously mentioned is the ratio of ethanol to all the other components, thus the samples need to be diluted and a split injection required to minimize the amount of ethanol introduced to the GC–MS.

a dark and moody atmosphere with a glass of sophisticated whiskey, including ice cubes, placed on a wooden table | Image Credit: © HN Works - stock.adobe.com

In a previous study (3), you and your team investigated how barley variety and its geographical location influence the aromatic profile of whiskey. What were the conclusions that you and your team were able to draw from the analyses conducted?

The study was published in 2021 and was on new make spirit (that is, the spirit post distillation before it goes into the cask). In that study, all other factors other than the cereal (barley) were controlled to the best of our ability. We looked at two different varieties of barley grown at two distinct geographical locations in Ireland, with differing soil types and weather conditions. The location of where the barley was grown had a greater impact on the volatile congeners in the resultant new make spirt than the barley variety. This was a first terroir type study for whiskey and highlighted the potential importance of where the barley was grown. Arguably, such differences in the new make spirit maybe reduced or eliminated by cask maturation, but it does provide food for thought and highlights additional factors in the development of whiskey congeners. Most distilleries utilize barley from a host of farms (geographical locations) and therefore any impact of terroir is lost in such an approach. However, some distilleries are now specifically harnessing this potential diversity in barley and terroir as a unique point of difference.

You and your team used GC–MS and gas chromatography olfactometry (GCO) to determine volatile compounds in whiskey. Why was GC–MS and GCO best suited for the analysis that you were conducting?

GC–MS is ideally suited for the analysis of most whiskey congeners because they are volatile. Even though we can identify more than 100 volatile congeners by HS-SPME GC–MS in a single run, we cannot be sure about their actual impact on sensory perception. We can make estimates based on abundance and referenced odor thresholds, but it is not practical to quantify so many volatiles. Therefore, the best way to identify those most likely to impact sensory perception in our experience is GCO. GCO is a relatively simple way of obtaining information about the aroma activity of individual volatile congeners using the human nose as an additional detector. More information relating to intensity and perception of individual congeners can be achieved using the split valve to dilute the amount of extract going to the column, which has the added advantage of eliminating matrix effects if the sample itself was diluted. GCO is relatively easy to perform, especially if you are regularly using the same personal as trained panelists. Obviously, the main disadvantage of GCO is that odors from the synergistic effects of two or more congeners are not captured. Co-elution issues and distinct aromas without corresponding peaks are typical hurdles, but many can be overcome by modifications in the extraction process or via changes in chromatography.

Your 2021 paper talked about exploring terroir in whiskey, stating that it has been explored in the wine industry, but not for other alcoholic beverages (3). Can you explain what terroir is chemically and how this can impact whiskey quality and production?

Terroir is described as the set of all environmental factors that affect a crop’s phenotype, including unique environmental contexts, farming practices, and the crop’s specific growth habitat. In wine, it represents the quality and the unique sensory characteristics associated with its geographical origin and method of production. Therefore, this context was applied to whiskey in terms of where barley was grown in relation to the production new make spirit. Our study highlighted that the volatile congener profile generated in the new make spirit was impacted by the geographical location of where the barley was grown; that is, the local geographical conditions altered the phenotypic expression, which has also recently been shown in beer production (4). Further work is required to evaluate if these differences influence cask matured whiskey.

What challenges did you and your team have to overcome while conducting whiskey analysis using GC–MS and GCO?

Our main challenges to date relate to the use of GCxGC. GCxGC rapidly generates gigabytes of data and we had to invest in dedicated data storage to support this capability. Data processing is also a significant bottleneck because of the complexity and size of the data sets. Further advances in specific data processing software will help GCxGC to become more mainstream as its benefits our obvious. For example, we have investigated HS-SPME Arrow using conventional GC–MS (mid-polar column) with a single quadrupole MS, and identified around 100 congeners, but have also used HS-SPME Arrow with GCxGC (first dimension non-polar, reverse flow modulation and second dimension mid-polar column) with a TOF-MS instrument and typically see more than 200 congeners in the same sample.

Staying on the topic of challenges, what do you see as the biggest challenges in the Irish whiskey industry, and how do you think chromatographic techniques can meet these challenges?

The number of whiskey distilleries in Ireland has increased significantly in the last decade, with exports having risen from ~€200 million in 2010 to >€1 billion in 2022 (5). This rapid growth has resulted in huge increases in the volume and diversity within each whiskey type, but this increase in popularity has also increased the risk of fraudulent products appearing in the global marketplace. The critical and immediate need to create congener databases to aid in quality control, but also to identify biomarkers for authentication purposes. Chromatography has a significant role for such purposes, but also in tandem with other non-destructive spectroscopic techniques to rapidly identify outliers in production or in the marketplace.

Whiskey distilleries are growing in Ireland as Irish whiskeys grow in popularity. Image Credit: © Peter - stock.adobe.com

What are the next steps in your research?

We are keen to expand the use of GCxGC in whiskey analysis and plan to utilize flow modulation to enhance the least abundant (trace) volatile congeners going into the MS while reducing the volume of the most abundant congeners and ethanol to enhance the number we can positively identify. We would also plan to evaluate high capacity sorptive extraction as a direct immersion technique in whiskey research as we have found it very useful in other products, such as skim milk powder, butter, and beef. This approach is more likely to capture some of the least volatile higher molecular weight congeners because of the direct contact between the sample and the sorbent phase. We are also keen to continue our work using GCO to better identify key volatile congeners associated with the quality of specific types of whiskey. In addition, we are keen to expand our collaboration nationally and internationally with others involved in whiskey or other distilled spirits research.

References

(1) Kelly, T. J.; O’Connor, C.; Kilcawley, K. N. Sources of Volatile Aromatic Congeners in Whiskey. Beverages 2023, 9 (3), 64. DOI: 10.3390/beverages9030064

(2) Food Industry Development Division, Department of Agriculture, Food, and the Marine, Technical File Setting Out The Specifications With Which Irish Whiskey Must Comply [Online]; EU Commission Services, 2019. http://www.marketaccess.agriculture.gov.ie/media/marketaccess/content/IrishWhiskeyUisceBeathaEireannachIrishWhisky030519%20220623.pdf (accessed September 14, 2023)

(3) Kyraleou, M.; Herb, D.; O’Reilly, G.; Conway, N.; Bryan, T.; Kilcawley, K. N. The Impact of Terroir on the Flavour of Single Malt Whisk(e)y New Make Spirit. Foods 2021, 10 (2), 443. DOI: 10.3390/foods10020443

(4) Herb, D., Filichkin, T., Fisk, S., Helgerson,L., Hayes, P., Meints, B, Jennings, R., Monsour, R. Tynan, S., Vinkemeier, K., Romogosa, I., Moscou, M., Carey, D., Thiel, R., Cistue, L., Martens, C., Thomas, W. Effects of Barley 9Hordeum Vulgare L.) Variety and Growing Environment on Beer Flavor. Journal of the American Society of Brewing Chemists, The Science of Beer 2017, 75 (4), 345. DOI: 10.1094/ASBCJ-2017-4860-01

(5) Drinks Ireland Spirits, Irish Spirits Market Report [Online]; Drinks Ireland Ibec, 2022. https://www.ibec.ie/drinksireland/news-insights-and-events/news/2023/08/11/irish-spirits-market-report-2022 (accessed September 14, 2023)