The Influence of Water and Temperature on the Volatile Compounds of Oak Barrel Staves

Many of you already know that using oak barrels during winemaking and aging increases the complexity of the finished wine, and often increases the overall quality of the wine. Using oak changes the aroma and color, as well as the stability of the finished wines. The type of aromas and flavors imparted into the wine depends upon a variety of factors, including the type of grape, the type of oak, and even where in the forest from which the oak tree was harvested. When making the oak barrels, heat treatments are frequently employed to help the wood become more pliable and thus able to be bent into the curved position of the barrel.

These heat treatments, referred to as “toasting”, alter the flavors and aromas imparted by the oak into the barrel, though the exact behavior of volatile compound concentration changes in wine is not known due to differing results in the literature. Traditionally, all of the research so far has focused on how toasting or heat treatments affect the aromatic and volatile compounds of wine

when dry wood is used during cooperage, though none have examined getting the wood wet first prior to toasting. Soaking the wood staves prior to heat treatment could have a significant impact on the aroma and flavor of the finished wine, though to date, no studies have examined this until now.

The goal of the study presented today was to examine 6 different aromatic compounds in wood samples that were either wet prior to heat treatment or not, to determine what effect, if any, soaking the wood has on the volatile composition of the wood (and thus potential volatile composition of the finished wine). This study examined several different temperatures and 2 different heat treatment exposure lengths.

Methods

Wood samples originated from one 400-year-old Quercus petreae tree from the “forêt des beaux Monts” in Oise, France. The staves were given by Tonnellerie Seguin Moreau and had been naturally seasoned for two years prior to the experiment. Staves were cut into samples of 70mm x 25mm x 3mm.

Heat treatments were performed in triplicate. Five temperatures were tested (90, 120, 160, 200, and 240 oC) and two treatment time periods were tested (10 and 25 minutes). For the soaked wood treatment, cut stave pieces were soaked in 90 oC hot water for 20 minutes. Unheated samples were used as controls.

After heat treatments, stave pieces were broken down and homogenized into sawdust in order to extract the volatile compounds from the wood. Volatile compounds were analyzed using HS-SPME GS-MS analysis.

Results

• Guaiacol:
o No significant differences in guaiacol levels were found between wet and dry woods for temperatures up to 200  oC.
o Guaiacol was 5x higher in dry woods than wet woods at the 240  oC treatment temperature (significant difference) and 10 minute treatment, and 2x higher for the 25 min treatment at this temperature.
o Guaiacol levels in dry wood at 240  oC for 10 minutes were not significantly different than the levels in wet wood at 240  oC for 25 minutes.
• Eugenol:
o Eugenol values were constant in woods for all temperatures, though were slightly higher at the 25 minute treatment compared with the 10 minute treatment.
o At the 240  oC temperature and the 25 minute duration, eugenol values in dry wood significantly decreased to levels found at the 240  oC temperature and 10 minute duration treatment.
• Furfural:
o Furfural levels in dry wood significantly increased at the 160  oC and 200  oC temperature treatments, and significantly increased further at the 240  oC temperature treatment.
o Furfural levels in wet wood significantly increased at the 200  oC temperature and peaked at the 240  oC temperature treatment.
• Vanillin:
o In all treatments at the 10 minute duration, vanillin levels were similar, with the exception of 240  oC temperature which showed increased vanillin levels in dry wood.
o For the 25 minute duration, there was a significant increase in vanillin in dry wood at 200  oC and a significant decrease in vanillin at 240  oC.
• Cis-whiskey lactone:
o Cis-whiskey lactone levels remained constant in dry wood for all temperatures except for the 240  oC treatment which showed a significant decrease in cis-whiskey lactone levels.
o Cis-whiskey lactone levels were significantly lower in soaked wood compared with dry wood at the 160  oC treatment temperature, similar at 90, 120, and 200  oC, and significantly higher at the 240  oC treatment temperature.
• Trans-whiskey lactone:
o Trans-whiskey lactone levels were significantly lower in wet wood at 90 and 160  oC, similar at 120 and 200  oC, and significantly higher than dry wood at 240  oC.
• General Trends:
o Lower temperatures were not correlated and in some cases negatively correlated with furfural, vanillin, guaiacol, and trans-whiskey lactone in woods.
o Higher temperatures were positively correlated with furfural, vanillin, guaiacol, and trans-whiskey lactone in woods.
o Higher temperatures were negatively correlated with cis-whiskey lactone and eugenol.
o Lower temperatures (particularly in the 25 minute duration treatments) were positively correlated with cis-whiskey lactone and eugenol.
o There was no significant influence of the temperatures 90, 120, and 160  oC on wood volatile compounds.
o Increased temperatures led to greater correlations with furfural, vanillin, and guaiacol and to weak correlations with cis-whiskey lactone, eugenol, and trans-whiskey lactone.

Conclusions

Overall, the results of this study indicated that the temperature of the heat treatment greatly influenced the concentrations of furfural and vanillin, though also had minor impacts on the concentrations of eugenol, cis-, and trans-whiskey lactone. According to the results, furfural was the volatile compound most influenced by the experimental treatments. Also, treating the wood with water prior to the heat treatment appeared to have a significant influence on the concentrations of all the oak volatile compounds studied with the exception of eugenol. The authors concluded that the formation of these volatile compounds may be a combination of the heat treatment influencing the production of the volatile precursors as well as the degradation of the volatile compounds.

After undergoing a wet treatment, it was found that the wood samples in general showed lower concentrations of volatile compounds than the dry wood samples. The authors concluded, and I tend to agree, that the absorption of water by the

wood may have some sort of protective effect against the degradation of the volatile compounds, therefore reducing the overall concentration of the compounds found in the homogenized samples. In a way, I would think the water is having some sort of cooling effect, thus delaying the extraction of volatile compounds from the wood.

I would have liked to have seen the authors take this a step further, and actually produce a wine made from barrels undergoing these temperature and water treatments. Do the increases and decreases in volatile compounds noted in the wood change the volatile composition of the wine in the same manner? Or are there other mechanisms involved that result in a different volatile composition of the finished wine? How do these treatments alter the aromatic and volatile composition of different kinds of wine? Do wines made from these types of treatment barrels taste quality and possess higher quality than untreated barrels? All of these questions would make for a great follow-up paper.

What about you all? How did you interpret these results? What experiments would you have liked to have seen done in addition to what was presented here? Any other comments or questions? Please feel free to comment!

Source: Duval, C.J., Sok, N., Laroche, J., Gourrat, K., Prida, A., Lequin, S., Chassagne, D., and Gougeon, R.D. 2013. Dry vs soaked wood: Modulating the volatile extractible fraction of oak wood by heat treatments. Food Chemistry 138: 270-277.