Investigating Off-Odours in Wine Using GC–O

The aroma of a wine is an important part of the wine tasting experience. Angela Lopez-Pinar from the Friedrich-Alexander University Erlangen-Nürnberg in Germany, has been investigating off-odours in wine. She recently spoke to us about this research.

Q. What led your group to investigate the presence of 2-methylisoborneol (2-MIB) in aged wines? Was the presence of this substance a surprise to you?

A: We found that the literature on 2-MIB gives conflicting reports and that the role of this compound as an off-odour component in wine remains unclear. On the one hand, 2-MIB has been described in numerous investigations as one of the most relevant substances responsible for earthy odours in wine (1–5), yet on the other it has been reported to be unstable in wine as a result of acidic conditions and therefore might not be expected to remain present for long (6). Together with our collaborator, Philippe Darriet from INRA, Institut des Sciences de la Vigne et du Vin (Université de Bordeaux), we set about trying to clarify whether this substance is sufficiently stable in wine to contribute to off-odours, or if the earthy odour notes previously detected were perhaps not from 2-MIB but from another (potentially related) substance(s) that would be stable in wine.

As such, we were surprised to clearly identify 2-MIB as an odorous constituent in older wine samples, thus making us reconsider whether this substance is really as unstable as previously assumed.

Q. Why is there such confusion over the role of 2-MIB as an off-odour compound in wine?

A: The confusion can be mostly blamed on the compound 2,4,6-trichloroanisole (2,4,6-TCA), which is the main contributor to the cork taint that is indisputably the most important musty fault in wine. 2,4,6-TCA has hitherto been the main focus of research on musty defects, and to a lesser extent other haloanisoles, so the role of other compounds on this off-odour has not yet been fully resolved. In addition, substances belonging to other groups, for example 2-MIB and geosmin, appear to be caused by fungal flora rather than the cork.

Indeed, most previous studies on 2-MIB have focused on optimizing and validating analytical methods for the detection of this compound, rather than exploring its cause in wine. Typically, in such studies, the wine samples are spiked with 2-MIB as a reference substance to achieve sufficient sensitivity in its detection, rather than it being detected in real samples, the latter being reported in only a very few cases.

Q. Were there any analytical challenges that you encountered and if so how did you overcome them?

A: Yes, the analysis of 2-MIB is very challenging because it is an extremely potent odour compound that is typically present in wine at only very low concentrations. Such trace concentrations make it exceedingly difficult to detect the compound using mass spectrometery (MS) during conventional gas chromatographic (GC) analysis, despite its clear detection by the human nose during GC–olfactometry (GC–O). Nevertheless, we were able to overcome this problem by applying two-dimensional GC–MS coupled with olfactometry. In this method, also known as “heart cut” GC–MS, a selected fraction of the sample gas eluting from the first GC column is transferred to a second capillary column: this makes it possible to remove interferences from other substances eluting at a similar time in the first dimension, thereby overcoming the problem of coelution.

Q. Aroma in wine is very important. It is one of the main quality parameters of a vintage. Which kinds of aromas are more palatable to the drinker?

A: This is not a straightforward question to answer as there are no universal preferences across different drinkers, with both individual liking and palatability varying widely amongst consumers. Wine preference is multifaceted and is developed by both exposure and training. Cognitive psychological research has shown that the judgement and preferences of wine-tasters are formed based on their personal experiences, and that these are driven at an unconscious level. In certain cultures, wine education in the family setting during upbringing directs preferences towards a particular type of wine or wine from a certain region. Likewise, complex aromas that are associated with specific, privileged terroirs can drive preference towards wines with these characteristics. In addition, when accompanying a meal, typically a wine that complements the meal itself is intentionally sought. Cultural influences also play an important role, for example, Australian and American consumers usually prefer much fruitier wines than European consumers. In short, there is no single, universal model of wine preferences, but rather different typologies according to the experience of each consumer.

Q. What are the factors that need to be considered when selecting an analytical method for wine analysis? Why is gas chromatography–olfactometry (GC–O) your method of choice?

A: Our study aimed to investigate odorous constituents of wine aroma, with a special focus on earthy-musty notes. GC–O is a technique that enables the human assessor to smell the chromatographic effluent and thereby distinguish between odour-active and odourless volatile compounds. This method offers a targeted analysis on substances of potential relevance for specific aroma impressions in wine.

Q. What is your group working on next?

A: Our current research on wine focuses on the effects of fungal diseases, namely bunch rot and powdery mildew, on the aroma of grape must and wine.

However, our research is not limited to wine, but broadly speaking encompasses all types of food and their aroma in relation to raw materials, processing, packaging, and storage. We investigate the formation, degradation, and release of odorants over the course of chemical, biochemical, technological, and physicochemical processes, as well as addressing the impacting factors that lead to high flavour quality or potential defects in food products.

In addition to food, our group is also active in the analysis of odours of non-food products such as household products, cosmetics, and body care products, to name but a few. Clothes, footwear, leather, wood, paper, and cardboard products are other areas covered by our non-food research activities on smell. There is currently a strong demand in the characterization of unintended or disturbing smell emanations from articles of daily use, such as children’s products, leisure, and sports equipment, as well as furniture and home textiles. We work closely with our partner institute, the Fraunhofer Institute of Process Engineering and Packaging IVV, with the head of our aroma research group, Professor Buettner, also heading the Department of Sensory Analytics there.

References

1. G. Bianco, G. Novario, R. Zianni, and T.R.I. Cataldi, Anal. Bioanal. Chem.393(8), 2019–2027 (2009).
2. S. Boutou and P. Chatonnet, J. Chromatogr. A1141(1), 1–9 (2007).
3. R.M. Callejón, C. Ubeda, R. Ríos-Reina, M.L. Morales, and A.M. Troncoso, J. Chromatogr. A1428, 72–85 (2015).
4. C. Cortada, L. Vidal, and A. Canals, J. Chromatogr. A1218(1), 17–22 (2011).
5. N. Sadoughi, L.M. Schmidtke, G. Antalick, J.W. Blackman, and C.C. Steel, J. Agric. Food Chem.63(11), 2877–2885 (2015).
6. S. La Guerche, B. Dauphin, M. Pons, D. Blancard, and P. Darriet, J. Agric. Food Chem. 54, 9193–9200 (2006).

Angela Lopez-Pinar studied food science and technology at the University of Valencia in Spain. She is currently in the final phase of her Ph.D. thesis on the clarification of off-odours in wine, which she is undertaking at the Department of Chemistry and Pharmacy, Emil Fischer Center, Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany. The studies on the impact of 2-MIB on wine were jointly supervised by Professor Dr. Philippe Darriet, Université de Bordeaux and Professor Dr. Andrea Buettner, Universität Erlangen-Nürnberg and Fraunhofer IVV.
E-mail: angela.lopez.pinar@fau.de
Website: http://www.lebensmittelchemie.nat.uni-erlangen.de/agbuettner/
https://www.ivv.fraunhofer.de/de/forschung/analytische-sensorik.html