Struck flint, struck match and gun smoke… linked compounds with a potentially large impact

Dr. Carien Coetzee
Basic Wine
30 July 2023

The term “minerality” is commonly used by wine critics and enthusiasts to describe a range of sensory experiences in wine, however, the concept of minerality in wines is ill-defined¹. Minerality in wine is usually a positive quality that adds complexity and depth to the wine². The terms typically associated with minerality include aroma attributes such as “earthy”, “flinty”, “stony”, “iodine”, and “salty”, among others².

Overall, the latest research on minerality in wine suggests that the topic is complex and multifaceted, with many factors influencing the perception of mineral character (see other blog posts on minerality). While the precise chemical compounds responsible for perceived minerality are still being identified, two polyfunctional thiols have been linked to aroma attributes “struck flint”, “struck match” and “gun smoke” which is commonly associated with minerality^5,7.

1. Benzenemethanethiol (BMT)

• Other names: phenylmethanethiol (PMT) and benzyl mercaptan (BM)
• Aroma detection threshold³: 0.3 ng/L
• Formation: It is commonly believed that BMT is formed from the reaction of benzaldehyde with hydrogen sulfide, however, results from a study⁴ do not support the theory leaving unanswered questions about how BMT is formed.
• BMT in Sauvignon blanc vs non-Sauvignon blanc varietals⁵:

2. 2-Furylmethanethiol (2FMT)

• Other names: (furan-2-yl)-methanethiol, 2-furanmethanethiol and 2-furfurylthiol (FFT)
• Aroma detection threshold³: 0.4 ng/L
• 2FMT in Sauvignon blanc vs non-Sauvignon blanc varietals⁵:

• Formation: 2-Furylmethanethiol has previously been shown to form in white wines during alcoholic fermentation in the barrel from the furan-2-carbaldehyde (furfural) released by toasted oak staves reacting with the hydrogen sulfide produced by yeast⁶.

Studies showed that both of these compounds are present in white wines, and specifically in Sauvignon blanc wines, at concentrations of sensory significance^5,7.  2FMT played a major role in the sensory perception of “flint”-type aromas while these sensory attributes were modestly related to BMT⁷. Even though 2FMT is reported to contribute a roasted coffee aroma in certain wines⁸, no evidence of this aromatic contribution was found in the study conducted by Espinase Nandorfy et al., 2023⁷. The sensory panels that evaluated wines spiked with both compounds described and rated wines high in 2FMT and BMT as high in “flint”, “struck match” and “mineral” aroma rather than any “roasted coffee”-related attribute. Higher levels of 2FMT did, however, contribute a character leaning towards “sulfurous/burnt”. It seems that the context set by other wine volatiles may affect the odour percept conferred⁷.

Sensory suppression effects

Sensory tests showed that 2FMT suppressed the perception of attributes “apple/pear”, “peach”, “floral”, “pineapple” and “citrus”⁷. The presence of 2FMT could thus result in a wine with lower fruit- and floral-driven characteristics. BMT also exerted some suppressive effects, however to a lesser extent when compared to 2FMT. It could be that the masking effect on fruity and floral notes enhances the perception of minerality. These suppressive effects might be concentration-dependent as other studies⁵ reported the contribution of 2FMT to “fruity”, “fresh” and “green” notes of white wines⁵ when present at low concentrations.

Conclusion

Aroma attributes “struck flint”, “struck match”, “gun smoke” and “mineral” are considered desirable in some styles of wines. It seems that the two compounds, benzenemethanethiol and 2-furylmethanethiol contribute significantly to these attributes, however, it is still unclear to what extent other compounds and sensory interactions could contribute to the perceived minerality of a wine. The impact of winemaking techniques on the concentration of these compounds also needs further investigation.

References

(1)           Ballester, J.; Mihnea, M.; Peyron, D.; Valentin, D. Exploring Minerality of Burgundy Chardonnay Wines: A Sensory Approach with Wine Experts and Trained Panellists. Aust J Grape Wine Res 2013, 19 (2), 140–152. https://doi.org/10.1111/ajgw.12024.

(2)           O’Kennedy, K. Minerality in Wine – Understanding the Concept and Factors Involved. Wineland Technical 2023.

(3)           Gambetta, J. M.; Bastian, S. E. P.; Cozzolino, D.; Jeffery, D. W. Factors Influencing the Aroma Composition of Chardonnay Wines. J Agric Food Chem 2014, 62 (28), 6512–6534. https://doi.org/10.1021/jf501945s.

(4)           Smith, P. Formation and Fate of Positive and Negative Sulfur Compounds; 2017. https://www.awri.com.au/research_and_development/rde-plan/projects/project-3-5-3/ (accessed 2023-08-15).

(5)           Mateo-Vivaracho, L.; Zapata, J.; Cacho, J.; Ferreira, V. Analysis, Occurrence, and Potential Sensory Significance of Five Polyfunctional Mercaptans in White Wines. J Agric Food Chem 2010, 58 (18), 10184–10194. https://doi.org/10.1021/jf101095a.

(6)           Siebert, T. E.; Espinase Nandorfy, D.; Cordente, A. G.; Pisaniello, L.; Watson, F. T.; Barter, S. R.; Likos, D.; Kulcsar, A. C.; Francis, I. L.; Bekker, M. Z. Struck Flint Aroma in Chardonnay Wines: What Causes It and How Much Is Too Much? In IVES Conference Series; 2022.

(7)           Espinase Nandorfy, D.; Siebert, T.; Bilogrevic, E.; Likos, D.; Watson, F.; Barter, S.; Pisaniello, L.; Kulcsar, A.; Shellie, R. A.; Keast, R.; Francis, L.; Bekker, M. The Role of Potent Thiols in “Empyreumatic” Flint/Struck-Match/Mineral Odours in Chardonnay Wine. Aust J Grape Wine Res 2023, 2023, 1–17. https://doi.org/10.1155/2023/8847476.

(8)           Tominaga, T.; Blanchard, L.; Darriet, P.; Dubourdieu, D. A Powerful Aromatic Volatile Thiol, 2-Furanmethanethiol, Exhibiting Roast Coffee Aroma in Wines Made from Several Vitis Vinifera Grape Varieties. J Agric Food Chem 2000, 48 (5), 1799–1802. https://doi.org/10.1021/jf990660r.

image

​