Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
email: Office@practicalwinery.com
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WINTER 2013
WINEMAKING
It is known that the face exposed to wine is the least penetrable (the direction is perpendicular to the radial cut). This leads to a slow extraction of wood compounds during barrel ageing. The quantity of whisky-lactones in the moistened layers was examined to be about 35% to 50% (in comparison to new oak) after two years of barrel using.5
This means that the migratory flow of wood compounds can quite accurately be described by the law of mass transfer and this flow is directly proportional to the wine-wood contact surface.
Figure 2. Balance of wood extractives in wine and losses due to transformation.
Is it the same as far as staves are concerned? The use of staves involves their immersion into wine. Yet it is well-known that wood permeability to liquids depends on the direction of penetration. It reaches its maximum in the longitudinal direction, which corresponds to the direction of the sap circulation in a living tree.
In contrast, it is very small in the direction perpendicular to the radial cut (barrel stave cut) – the face exposed to wine when using barrels. Therefore the different stave surfaces (end, length, etc) immersed in wine are not equivalent in terms of penetration depth. When using staves of different geometry, we will lose the proportionality of extraction rate in relation to the contact area.
For example, compare a thin and a thick stave (twice as thick), both of equal length and width. If we calculate the total surface of both staves, the values are similar, since the only difference is the surface of the stave extremities (the contact area will be slightly bigger with the thicker stave).
In order to calculate the percentage of extraction over the contact surface, the wine must penetrate into the whole surface of the stave at a similar shallow depth. Taking in consideration the anisotropy of wood, this is probably not the case. On the contrary, the wine will probably penetrate deeper into the wood in the longitudinal direction. The simple observation of staves used for wine ageing cut lengthwise shows that wine penetrates into the heart of wood (Figure 1).
In this case, the quantity of extracted wood compounds will be much higher from a thick stave than from a thin stave. If total penetration is considered, a thick stave will produce twice as many extracted compounds as a thin stave because the mass of a thick stave is double that of a thin stave.
In the current study, we are trying to answer the question about the penetration rate of wine into a stave and its interaction with wood. If it is low, the extraction should be proportional to the surface, whereas if it is high, the extraction should be proportional to the mass of wood.
Equipment and Methods
Staves of 7 mm and 18 mm (Seguin Moreau, Cognac, France) were used in the study. Both types were made of French oak with medium toasting. The staves were macerated in wine (Domaine de Chiroulet, Vin de Pays des Côtes de Gascogne, Tannat, 2011 vintage) separately (two individual tanks) for 7 mm and 18 mm, the dose being 10 g/L of stave in both cases (two staves per hectoliter for 18 mm and five staves per hectoliter for 7 mm). The staves were immersed in wine in April and removed in November (total contact time of seven months).
4-cm pieces were sawn off each stave, the samples were mixed together, and ground down to a 0.5mm powder. This procedure was required in order to obtain an average sample and reduce the effects of a possible toasting heterogeneity.
The wood powder was soaked by shaking (300 tours per minute) by the model wine solution (12% vol. alc, pH = 3.5) for 24 hours. The extracts were analyzed by HP-SPME-GC-MS according to the Carrillo method for main compounds created from wood toasting: furfural, furfuryl alcohol, 5-methylfurfural, guaiacol, and vanillin.1
After wine maceration (seven months), the staves were removed from the wine. A 4cm long piece was sawn from the central part of each stave. This time, we did not use the extremities since they might have been too exhausted in extractive compounds with the rest of the stave because of the lengthwise penetration of wine into the wood. This way we avoided the artifact risk of the trial.
The wood pieces were ground down, extracted, and analyzed following the method described above. The wines obtained during trials were also analyzed using the same protocol.
Results
We reviewed the compounds distributed between the liquid (wine) and the solid phases (wood after maceration) and compared them with the quantity of wood compounds before maceration. Given the absence of furfuryl alcohol in wood and its appearance in wine through the reduction mechanism for furfural, we defined the total furfural as a sum of furfural and furfuryl alcohol. Its concentration was compared with furfural amount in wood.
The balance of compounds shows us that the sum of compounds after maceration (wine and wood) is lower than their initial amount in wood before maceration. This fact could be explained by the transformation of such compounds in the wine medium and corresponds to the conclusions made by P. Spillman,6,7 about the evolution of furfural and vanillin, and also to the phenomena reported by M.F. Nonier, which pointed out the interaction between wood aldehydes and wine phenolics.4
The amount of wood extractives (after extraction) in wine, in wood, and losses due to transformation are shown in Figure 2 (example of a 7 mm stave). All amounts were expressed in % of initial concentration in wood before maceration.
One can see that the rate of loss of compounds in the wine medium is spectacular, except for guaiacol, which seems to be stable. The furfural, 5-methyl-furfural are very affected (loss of about 90%) by the wine medium, while there is just 50% of loss for vanillin.
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