The 'Gazette'
The sensory complexity of a great whisky is largely determined by the molecules it extracts from the oak wood as it ages. There are many such molecules, formed at different stages during the toasting process.
Pyrolysis and thermolysis happen during the wood’s thermal treatment, leading to a degradation of polysaccharides (cellulose and hemicellulose), lignins and polyphenols (ellagitannins). This results in the generation of new aromatic compounds absent from unburned wood, and an increase of existing aromatic substances.
The furan aldehydes: The release of these compounds is naturally promoted by the production of acetic acid during wood pyrolysis. These compounds are characterised by toasted almond and caramel scents, but due to their high perception threshold their aromatic impact remains limited.
Tabanone : Characterised by notes of tobacco, incense and spices, it is formed when heating the wood by thermal degradation of a galloyglucoside precursor.
Whisky lactone : Key compound of the coconut aroma of fresh wood, it increases with toasting, up to a certain toasting intensity. The whisky lactone’s cis isomer, being the most aromatic and dominant in fresh wood, is also dominant in toasted wood. Whisky lactone is the key aromatic compound of American oak.
The toasting step promotes the Maillard reaction. This complex chemical reaction has been thoroughly studied, particularly in the food industry, as it is responsible among other things for the formation of certain aromas whilst cooking food. After a condensation reaction between a reducing sugar and an α-amino acid and followed by several rearrangements and cyclisations, the formation of aromatic compounds.
Maillard reaction forms furanic compounds nd heterocyclic nitrogen, such as pyrazines and pyrroles. Therefore, in addition to aromas of fresh wood there are now toasted, spicy, roasted and smoky aromas that complexify the aromatic signature of the oak wood.
Whilst the toasting step favours the formation and accumulation of aromatic compounds, it is important to underline that depending on the intensity of the toasting, their levels may decrease or increase, changing the balance of aromas. We’ll look at this subject in the next article.
Chatonnet, P., Cutzach, I., Pons, M., & Dubourdieu, D. (1999). Monitoring toasting intensity of barrels by chromatographic analysis of volatile compounds from toasted oak wood. Journal of Agricultural and Food Chemistry, 47(10), 4310-4318
Vivas, N & Bourgeois, G. (1998). La maîtrise de la chauffe traditionnelle en tonnellerie. Acte du colloque Sciences et Techniques de la Tonnellerie, 29-35
Slaghenaufi, D., Marchand-Marion, S., Richard, T., Waffo-Teguo, P., Bisson, J., Monti, J. P., … & de Revel, G. (2013). Centrifugal partition chromatography applied to the isolation of oak wood aroma precursors. Food chemistry, 141(3), 2238 2245
Farrell, R. R., Wellinger, M., Gloess, A. N., Nichols, D. S., Breadmore, M. C., Shellie, R. A., & Yeretzian, C. (2015). Real-time mass spectrometry monitoring of oak wood toasting: Elucidating aroma development relevant to oak-aged wine quality. Scientific reports, 5, 17334
Hayasaka, Y., Wilkinson, K. L., Elsey, G. M., Raunkjær, M., & Sefton, M. A. (2007). Identification of natural oak lactone precursors in extracts of American and French oak woods by liquid chromatography–tandem mass spectrometry. Journal of agricultural and food chemistry, 55(22), 9195-9201
Masson E., Baumes R., Le Guernevé C., Puech J.L. Identification of a precursor of β- methyl- γ-octalactone in the wood os Sessile oak (Quercus petrae (Matt.) Liebl.). J. Agric. Food Chem. 2000, 48, 4306-4309
Wilkinson, K. L., Elsey, G. M., Prager, R. H., Tanaka, T., & Sefton, M. A. (2004). Precursors to oak lactone. Part 2: Synthesis, separation and cleavage of several β-D-glucopyranosides of 3-methyl-4-hydroxyoctanoic acid. Tetrahedron, 60(29), 6091-6100
Maga, J. A. (1987). The flavor chemistry of wood smoke. Food Reviews International, 3(1-2), 139-183
Picard, M., Garrouste, C., Absalon, C., Nonier, M. F., Vivas, N., & Vivas, N. (2019). Development of a Solid-Phase Microextraction-Gas Chromatography/Mass Spectrometry Method for Quantifying Nitrogen-Heterocyclic Volatile Aroma Compounds: Application to Spirit and Wood Matrices. Journal of agricultural and food chemistry, 67(49), 13694-13705
Maturation | April 2025 | Dr. Magali Picard