Identification of Volatile Compounds with a “Toasty” Aroma in Heated Oak Used in Barrelmaking

Toasting wood to be used in barrels for aging wine and brandy produces a great number of volatile and odiferous compounds. Some of these have a “toasty caramel” aroma. Analysis by high-performance gas chromatography of toasted oak extracts, combined with olfactory detection, enabled various chromatographic peaks with these specific aromas to be isolated. These same odors were simultaneously studied by heating glucose both with and without proline. Aromatic compounds of interest were identified thanks to a combination of gas chromatography and both mass and infrared spectrometry. In addition to already-known substances such as 2-hydroxy-3-methyl-2-cyclopenten-1-one (cyclotene) (1) and 3-hydroxy-2-methyl-4H-pyran-4-one (maltol) (2), we identified, for the first time, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) (3), 4-hydroxy-2,5-dimethyl-3(2H)-furan-3-one (furaneol) (4), and 2,3-dihydro-5-hydroxy-6-methyl-4H-pyran-4-one (dihydromaltol) (5) as compounds which actively contribute to the “toasty caramel” aroma of heated oak. Their role in the aroma of wines and spirits is thus well worth studying. These molecules were absent from the glucose pyrolysis but were relatively plentiful after heating in the presence of proline. Therefore, Maillard reactions were seen to be the cause of these substances in the heated oak wood. Keywords: Oak; thermal degradation; volatile compounds; “toasty” aroma; Maillard reactions