Barrel ageing is a key stage in producing great wines. Two major phenomena occur during barrel ageing: one consists of oxidation of certain substances in wines due to oxygen penetrating into the barrel and the other is the diffusion of extractable compounds from oak wood into the wine. These substances modify the wine’s aromas and flavors, adding complexity and hints of “coconut,” “toast,” “vanilla” etc.3
Once the extractable compounds from the wood have been released into the wine, they may be modified by chemical and enzyme reactions.7,9,10,11,2,4 Furthermore, the macromolecular structure of oak wood may absorb certain constituents from the wine.1,8These phenomena result in marked organoleptic changes in the wine.
The complexity of barrel-ageing phenomena and their impact on wine quality make these containers valuable winemaking tools. Winemakers’ objectives change to suit market tastes. The current trend in the winemaking world is to make barrel-aged wines with complex, fruity flavors, and relatively discreet oakiness. This trend is most marked among independent winegrowers, who want their wines to express their terroir, rather than any “universal” taste.6
Some coopers use immersion in hot water to prepare the raised (assembling of the staves in the shape of the barrel before staves are bent and heads are inserted) barrels for bending, but this process is much less widespread than traditional heating over a wood fire.toasted. The only difference from the traditional process is that heating over a fire prior to bending is replaced by immersion in water, and then the process continues with toasting.
The immersion method consists of soaking raised barrels in hot water, then bending the staves rapidly to form the barrel before it is
The first objective of this technique is to reduce stavewood breakage with water immersion before bending, to minimize the waste of broken oak staves occurring from bending following heating over an open fire.
However, this is by no means the most important aspect of the process. The adsorption of water by wood, extraction of wood components, and heat transfer in the wood-water system formed by immersing the barrel in hot water, then toasting wood with a high humidity level drive different kinetics in the production of compounds from toasting and degradation of wood compounds over fire from those that occur after traditional fire bending prior to toasting.
It is noteworthy that during traditional fire toasting coopers spray some water on the inside of the barrel in order to prepare the barrel for bending. However, contrary to water immersion, this operation does not allow any significant changes in wood composition, the moisture content remaining stable.
Wines aged in barrels where the staves have been bent following hot water immersion are empirically perceived to be less marked by oaky aromas. However, to date, very little scientific data was available concerning the impact of this bending technique on wood/wine exchanges, thus making it impossible to explain the sensory differences noted in empirical observations.
This report presents the results of a two-year comparative study of the two stave bending techniques, focusing on the sensory profiles of wines aged in the different types of barrels and their concentrations of volatile molecules.
Material and methodsBarrel production
The barrels were made by Tonnellerie Seguin Moreau (Seguin Moreau Burgundy ZAC du Pré Fleury les Creusottes Nord, Chagny, France) using staves bent either following immersion or by heating over a fire (see Figure 1). All barrels were produced from one batch of French fine grain oak.
In the “traditional process,” raised barrels are preheated (A) for 10 to 15 minutes to a temperature of 100° to 120°C to soften the wood fibers and make them pliable. This process does not cause any major production of compounds arising from toasting; indeed, any modifications are insignificant compared to those that occur during the toasting stage.
In the process employing bending by “immersion,” the preheating stage (A) is replaced by soaking in hot water (B). The temperature of the water does not exceed 100°C; production of toasting products due to heating are negligible.
Toasting – The heating process that causes chemical modifications in the wood was performed according to the standard “medium-long” protocol used in the cooperage. This stage was identical, irrespective of the type of bending process: heating over a fire or by immersion.
Trials occurred on nine sites with nine wines (three white wines and six red wines) from two different vintages: 2007/2008 (Table I). Wines were either barrel-fermented or put into barrel before or after malolactic fermentation. There were three barrels in each wine lot.
Sensory analysis by a tasting panel of Seguin Moreau staff (panel I of 8 to 14 persons), and a trained panel of enology students from the Institut Universitaire de la Vigne et du Vin (panel II of 16 to 18 persons), was done at regular intervals, as described in Table I.
Panelists performed blind quantitative descriptive analysis using a standard list of descriptors including six odor (overall oaky, fruity, sawdust, vanilla, toasty, spicy) and five taste descriptors (acidity, bitterness, tannic intensity, structure, length). The sensory sessions were organized by series with the same wine aged in fire-bent and immersion/fire-bent barrels. The order of presentation was arbitrary and different for different panelists to obtain more objective data.
The tasting sessions were conducted in tasting rooms following the usual practices in wine tasting and were conducted by a session observer. Before the sensory analysis, the bottle was tasted in order to look for possible off-flavors and was rejected if there was an abnormal odor.
Panel members awarded 0 to 10 points according to the intensity of each descriptor (0 – very weak, 10 – very strong).
The values given by each taster were normalized using the average value for that taster over the entire series. The Independent Student’s T-test was applied to the normalized values to identify the descriptors for which there was a variation in intensity between the barrels made using “immersion” and the “traditional process.”
Only those descriptors showing differences with a probability below 5% were retained for further processing.
Analysis of volatile compounds
The following 19 odoriferous volatile molecules from oak wood were assayed in the wines using GC-MS:3
• furan and pyran compounds (furfural, 5-hydroxymethyl-furfural, 5-methyl-furfural, furfuryl alcohol, maltol, and ethyl maltol) – class of compounds characterized by toasty and butterscotch odors in pure state;
• aromatic aldehydes (vanillin and syringaldehyde) – class of compounds characterized by vanilla/woody odors in pure state;
• volatile phenols (guaiacol, 4-methyl-guaiacol, eugenol, isoeugenol, o-cresol, m-cresol, phenol, syringol, and allylsyringol) – class of compounds characterized by smoky, phenol and spicy odors in pure state;• lactones (trans- and cis-whisky-lactones) – class of compounds characterized by coconut and woody odors in pure state.
Under the reducing conditions in wine, furfural tends to be converted into furfural alcohol, so the “total furfural” content was calculated (combined concentrations of furfural and furfural alcohol).
These analyses were carried out at regular intervals, as shown in Table 1.
Preliminary studies had revealed that the wine matrix (variety, wine composition, and use of barrel such as presence of lees, oxygen, etc.) played an important role in determining the concentration of substances extracted from the oak.5 Consequently, it was not possible to apply the standard Student’s T-test to assess the difference in impact between the “traditional” and “immersion” processes.
The Student’s T-test for paired samples (immersion compared to traditional process) was used. The difference between the concentration of molecules released by oak wood in the two types of barrels was calculated for each type of wine. This test, to determine whether the average of these differences was significantly different from 0, consisted of comparing pairs of samples taken from the same wines aged in barrels produced by bending following immersion and the traditional process.
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ResultsSensory analysis of wines
Descriptors with significant differences in intensity (risk threshold less than 5%) are presented in Table II. The “+” sign indicates that the intensity of the descriptor was higher following heating by immersion and the “-” sign that it was higher in the control (fire bending).
One can observe a lack of agreement between panels I and II for wine-1, 2, 3, for some common descriptors found (toasty for wine -1 and sawdust for wine -2). The only wine where students and staff had similar findings was Wine-8. This fact shows the different perception of sensory differences between wines analysed by each panel. That is why the analysis of all of the study data is required in order to establish a common and rigorous conclusion.
Certain descriptors were used very frequently in different tasting sessions to describe various wines. This led to the assumption that heating by immersion produced systematic differences compared to the controls, irrespective of the type of wine or the period when it was tasted by the tasting panel.
To verify this hypothesis, the probability of the significant differences for each descriptor used for all the wines studied were calculated, assuming that the probability that the significant differences for each descriptor given in a specific case (wine X tasting) was 5%, to reflect the risk threshold of 5% chosen
for the sensory analyses. The other assumption was the independence of the events (tasting sessions). The results are in Table III.
Table III shows the frequency of use of descriptors where intensities were higher:
• for fire bending (fire bending line)
• for water bending (water bending line).
• for fire bending (fire bending line)
• for water bending (water bending line).
For example, the occurrence of oaky descriptors in both parts of Table III means that the oaky descriptor was significantly more intense for traditional process in 10 tasting sessions during the entire study, whereas it was more intense for water-immersion in one tasting session. The probability calculated reveals that 10 uses for the first case is an indicator of systematic difference, while one single use for the second case is not.
Key: * = panel 1 by default. “” = intensity of the descriptor was higher following heating by immersion; “” = intensity of the descriptor was higher for the traditional process.
Analysis of these results revealed that the main differences concerned the olfactory descriptors. The “toasty” and “oaky” descriptors were significantly more intense when the raised barrel was preheated over a fire compared to heating by immersion. The other marked difference was that the intensity of the “fruity” descriptor was significantly higher following preheating by immersion.
Differences with a lower confidence level were observed for the “sawdust” and “acidity” descriptors, with slightly higher intensity for “sawdust” after traditional heating.
In contrast, the “acidity” descriptor was also apparently significantly more intense following preheating by immersion in a certain number of sensory analyses. Thus, on the basis of the findings presented in this article, it was not possible to draw any systematic conclusion concerning the difference in intensity of this descriptor, as the results depended on the type of “wine matrix” and the length of ageing time, rather than the bending process used.
Key: positive values indicate that concentrations were higher following preheating over fire, and negative values that they were higher following heating by immersion. n.s. > 0.05; bold type = probabilities below 0.05.
There was no significant difference between the intensity of the other descriptors, especially those describing flavors.
Table IV shows the Student’s T-test results for paired samples (preheating by immersion compared to fire). They include the main differences of concentration of each compound between wine aged in traditional fire-bent barrel and paired wine which was aged in barrel preheated in water before bending. Thus the values of differences could be positive and negative. A positive difference indicates that a higher amount of chemicals is found from the traditional process, whereas a negative difference from water immersion before bending.
Table IV includes the standard deviations of differences between samples and values of paired samples T- test in order to check whether distribution of differences is different from 0. The low T-test p values means that the difference is systematic across all pairs. Such values are presented in bold characters in Table IV.
These results revealed that ageing wines in barrels made by the traditional process resulted in wines with higher concentrations of compounds produced by heating/toasting, particularly furan compounds (furfural, 5-hydroxymethyl-furfural, 5-methyl-furfural), volatile phenols (guaiacol, 4-methyl-guaiacol, isoeugenol, and allylsyringol), aromatic aldehydes (vanillin and syringaldehyde), and maltol.
In contrast, wines aged in barrels heated by immersion had higher cis-whisky-lactone content.
Analyses of the volatile compounds in the wines confirmed that the main differences between the two processes concerned the concentrations of toasting compounds produced by heat/toasting: furan compounds, volatile phenols, and aromatic aldehydes. Wines aged in barrels made by the traditional process had higher concentrations of these compounds. On an olfactory level, wines aged in barrels heated by immersion exhibited less intense “oaky,” “toasty” and, to a lesser extent, “sawdust” aromas. Tasters also found these wines “fruitier.”
Analysis confirmed that the main differences between the two processes concerned the concentrations of toasting compounds. As we shown previously, the production of toasting compounds is an improbable process during the preheating stage (prior to bending) because of low temperature irrespective for fire or water process. That is why the possible explanation of difference consists in the different moisture content, which was higher for water immersion. The evaporation of additional water in the latter case decreases the quantity of calories transmitted to wood during toasting. This “loss of energy” reduces the degree of degradation of wood hemicelluloses and lignin and the production of respectively furan and pyran compounds and aromatic aldehydes and volatile phenols.
n.s. > 0.05; Bold type = probabilities below 0.05.
The authors wish to thank Bérangère Perez, Pauline Lamblin, Diane Gounel, and Morgane Jestin, enology students at the Institut Universitaire de la Vigne et du Vin “Jules Guyot” for their technical assistance in setting up the experiments, and the winemakers and technical managers of the estates where the experiments were carried out.
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