Barrel toasting

Barrel toasting is of fundamental importance in the leaching of volatile components into the wine and in many cases may mask the attributes intrinsic to the species or geographical region of origin of the oak (Francis et al. 1992). 

Figure 16. Development of enolic compounds during barrel toasting...

Bouquet can now be obtained by exploring all the different options available for toasting the oak barrels. Toasting can be further categorized by different intensities from light to... 

Chatonnet, P. (1999). Discrimination and control of toasting intensity and quality of oak wood barrels. Am. J. Enol. Vitic. 50,479 94. 

Blanchard, L., Tominaga, T, Dubourdieu, D. (2001). Formation of furfurylthiol exhibiting a strong coffee aroma during oak barrel fermentation from furfural released by toasted staves. J. Agric. Eood Chem., 49, 4833 835. 

Other volatile compounds present in oak wood can transmit unpleasant aromas to the wine, such as the sawdust aroma of dry wood that is perceptible in some wines aged in new barrels. The substances that cause these aromas have been identified in both American and European oak wood, for instance, ( )-2-nonenal, 3-octen-l-one, ( )-2-octenal, and 1-decanal. Their connection with the sawdust aroma has been established by olfactometry, and they have been identified in wines suffering from this flaw, though toasting the wood (Chatonnet and Dubourdieu 1998). 

Heating also produces a series of other substances as a result of Maillard reactions. These include cyclotene, maltol and its derivatives (hydroxymaltol, dihydro-maltol), 2,3-dihydro-5-hydroxy-2-methyl4(/7) pyranone (DHM) and its 5-hydroxy derivative (DDMP) and furaneol. The sensory impact of these components can play an important role in developing the toasted , burnt , or caramel aromas of some barrel-aged wines (Cutzach et al. 1997,1999). 

Various aldehydes are encountered in wine. The most abundant is acetaldehyde which is both a product of yeast metabolism and an oxidation product of ethanol. Glyoxylic acid, resulting from oxidation of tartaric acid, especially catalyzed by metal ions (Fe, Cu) or ascorbic acid, can also be present. Other aldehydes reported to participate in these reactions include furfural and 5-hydroxymethylfurfural that are degradation products of sugar and can be extracted from barrels (Es-Safi et al. 2000), vanillin which also results from oak toasting, isovaleraldehyde, benzaldehyde, pro-pionaldehyde, isobutyraldehyde, formaldehyde and 2-methylbutyraldehyde which are present in the spirits used to produce fortified wines (Pissara et al. 2003). 

More potent yeasts responsible for spoilages of wines with volatile phenols are of Brettanomyces/Dekkerra type that can produce the vinyl phenols and ethyl phenols of hydroxyl cinnamic acids. Ribdreau-Gayon et al. (2000) give an overview on this topic. Another source of volatile phenols is the oak wood of barrel and oak chips. These phenols are characterized by smoked and toasted aromas. Important volatile phenols in wine are depicted in Fig. 9C.4. 

Table II. Incidence of the origin of wood and intensity of toasting on the volatile compounds of a white wine aged nine months in new oak barrels...

Table El shows the amounts of some of the previously identified molecules in the same red wine, either barrel-aged or not. Some of the samples were aged in barrels of different origins, toasted to varying degrees.

Chatonnet, P., Cutzach, I., Pons, M., and Dubourdieu, D. (1999). Monitoring toasting intensity of barrels by chromatographic analysis of volatile compounds from toasted oak wood, J. Agric. Food. Chem., 47,4310-4318. 

The twelve new and twelve used barrels used in this experiment were made of American white oak (Quercus alba, fine grain) or French oak (Quercus petraea from the Allier forest in France), and were obtained from the same cooperage firm in Spain with the same specifications (medium toast level). The used barrels had been used three times for the aging of Monastrell wine and they were sanitatied before use by burning sulphur inside. The following barrels were used in the experiment 220 liter French oak barrel (3 new barrels and 3 used barrels) 220 liter American oak barrels (3 new barrels and 3 used barrels) 500 liter American oak barrels (3 new barrels and 3 used barrels) and 1000 liter American oak barrels (3 new barrels and 3 used barrels). 

When wines are aged in new oak barrels, the toasting of the wood involved in barrel... 

Glyoxylic acid is formed by the oxidation of tartaric acid (Fulcrand et al., 1997). Furfural is present in rotten grapes and may also be released by barrels, depending on the toasting level. 

The presence of furfural and hydroxymethyl-furfural, released from grapes affected by Botry-tis cinerea as well as from toasted oak in barrels (Table 13.20, Section 13.8.3), leads to the formation of orange-yellow and brick-red compounds. These xanthylium structures are formed by condensation of the aldehydes with catechin and anthocyanins (malvidin and cyanidin) (Es-Safi et al, 2000 and 2002). Oxidation of tartaric acid results in glyoxylic acid, which then... 

The types and concentrations of odoriferous substances released into white wines according to the origin of the wood and degree of toasting of the barrels is described elsewhere (Volume 1, Section 13.8, Table 13.19). 

Once the stave wood is considered to be dry and seasoned, a cooper makes it into staves that are assembled (in groups of 18-25) with metal hoops to form barrels. The oak is then subjected to heating and toasting, both fundamental stages in barrel manufacture. The two stages can be summarized as follows ... 

The second operation, toasting, gives the barrel its final shape, while at the same time modifying the oak s structure and composition. 

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