|
PRINT » |
|
E-MAIL THIS PAGE » |
|
CLOSE THIS WINDOW » |
Barrel Rescue
Turning used oak barrels into new oak adjuncts for wine maturation
During barrel maturation, the volatile compounds extracted from oak wood can contribute to a wine’s overall aroma and flavor, enhancing its character and complexity.8 These compounds include, but are not limited to, cis- and trans-oak lactone, guaiacol, 4-methylguaiacol, eugenol, vanillin, furfural and 5-methylfurfural. Due to its abundance in oak-aged wine and relatively low detection threshold, cis-oak lactone is perhaps the most important oak volatile. Both cis- and trans-oak lactone impart coconut aroma and flavor characters to wine,5 while guaiacol and 4-methylguaiacol exhibit smoky attributes, and vanillin and eugenol afford distinctive vanilla and clove characters, respectively.9 The furanones are thought to make less significant contributions to aroma due to their considerably higher aroma-detection thresholds, but they may influence the perception of oak lactone.9
The extractable levels of oak volatiles can vary significantly from barrel to barrel, largely due to variation in oak composition associated with species, origin (i.e., French or American oak) and the seasoning and toasting processes of cooperage.3,8
However, barrels have a finite pool of extractable material, and the amounts of oak-derived volatile compounds available for extraction diminish over time.7 As a consequence, barrels are typically decommissioned after five to six years. Used barrels can be rejuvenated by scraping a thin layer (up to 5 mm) from the barrel’s interior to expose fresh, unextracted wood.6 Sometimes the rejuvenated barrel will also be re-toasted. While this practice can prolong the life of a barrel, rejuvenated wood contains reduced amounts of extractable material, so it contributes less flavor than new barrels. Furthermore, any wine that remains in the wood can be caramelized during re-toasting, creating a risk for unpleasant “burnt” characters. The increased microbial load and potential for Brettanomyces spoilage can also discourage barrel rejuvenation. For these reasons, most winemakers favor replacement of old barrels.
A group of Australian scientists recently investigated whether decommissioned barrels represent an untapped source of high-quality oak that can be “reclaimed” for use in wine maturation. The potential for contamination by the spoilage yeast Brettanomyces in reclaimed oak also was investigated.
Flavor potential of reclaimed oak battens
Several used French and American barrels were “reclaimed” (see photos on page 43). This involved barrels being broken down into their individual staves, while the wine-affected portion of oak was removed and discarded. The remaining oak was then split lengthways into battens and processed to remove barrel grooves and expose the fresh grain of the wood. Pressure and far-infrared (FIR) heat were then applied to the resulting oak battens to simultaneously straighten them and generate oak volatiles. In this way, the 2.1 square-meter internal surface area of a 225-liter barrel yields approximately 8 square meters of reclaimed oak battens, substantially increasing the wine volume that can be matured using the same oak wood. To evaluate the flavor potential of the new oak battens, 1mm-thick shavings were taken from a number of samples (24 reclaimed French oak battens and 22 reclaimed American oak battens) and soaked in model wine (20% alcohol by volume) for seven days at room temperature. The extracts were then analyzed by gas chromatography-mass spectrometry to determine oak volatile concentrations.
Comparable levels of important oak-derived volatiles were observed in reclaimed French oak battens and new French oak as well as reclaimed American oak battens and new American oak (see “Concentration of Volatiles in Toasted Oak” on page 45), although the concentrations of oak volatiles differed between French and American oak battens, in agreement with previous studies.1,7,8 Reclaimed French oak in particular contained higher proportions of the trans-isomer of oak lactone rather than the cis-isomer; whereas reclaimed American oak predominantly contained cis-oak lactone. These results demonstrate that reclaimed oak is capable of imparting oak aroma and flavor to wine and is therefore suitable for use in winemaking.
Potential carryover of Brettanomyces spoilage
To investigate the potential carryover of Brettanomyces spoilage, the concentrations of 4-ethylphenol and 4-ethylguaiacol were measured in reclaimed oak. No 4-ethylphenol was detected in any of the reclaimed oak samples, and only trace levels of 4-ethylguaiacol (i.e., less than 5.1 µg/g) were observed (see “Concentrations of 4-ethylguaiacol and 4-ethylphenol in Reclaimed Toasted Oak”). These results indicate there was no carryover of Brettanomyces-derived volatile phenols during the barrel reclamation process; however, this may also reflect the absence of spoilage in the first place. A spoilage trial was therefore undertaken to investigate Brettanomyces viability under FIR toasting conditions.
The trial involved an untoasted, reclaimed oak batten being soaked in yeast peptone dextrose (YPD) medium and inoculated with a Brettanomyces culture for four days. The batten was then subjected to FIR toasting, after which shavings were collected and soaked in YPD media for seven days. The resulting media was streaked onto agar plates and incubated for 14 days. Where yeast growth occurred, the genomic DNA was extracted for molecular identification. The results indicated a 99% match to Hanseniaspora guilliermondii, a strain of yeast likely to be a media contaminant.2 (i.e., There was no evidence to suggest the presence of Brettanomyces yeast following FIR toasting.)
These results suggest the temperatures achieved during FIR toasting (between 200° C inside the batten and 250° C on the surface of the batten), were not only sufficient to generate oak volatiles but to also reduce microbial load, thereby preventing the carryover of spoilage yeast and/or bacteria.
Suitability for winemaking
The barrel reclamation process converts each used barrel into approximately 60 reclaimed oak battens, which can then be added to wine to impart oak characters. The reclaim process costs approximately $2.50 per batten, or $150 per barrel, which is significantly less than a new French or American oak barrel. In this study, reclaimed oak battens were found to contain similar levels of important oak volatiles to those reported in new oak wood, indicating suitability of reclaimed oak for wine maturation. Furthermore, there was no evidence to suggest any risk of carryover of Brettanomyces yeast.
This article previously appeared in Australia & New Zealand Grape Grower & Winemaker magazine and is republished with permission. Authors Kerry Wilkinson, Sijing Li Paul Grbin and Peter Warren are from the University of Adelaide’s School of Agriculture, Food and Wine in South Australia. The full study was published in 2015 in the Journal of Agricultural and Food Chemistry as “Aroma Potential of Oak Battens Prepared From Decommissioned Oak Barrels” with additional authors Anna Crump from the University of Adelaide and Peter Warren and Yoji Hayasaka from the Austrian oak firm Ausvat.
References
1. Alañón, M.E., Díaz-Maroto, M.C. and Pérez-Coello, M.S. (2012) “Analysis of volatile composition of toasted and non-toasted commercial chips by GC-MS after an accelerated solvent extraction method.” Int. J. Food Sci. Technol., 47: 816-826.
2. Barnett, J.A., Payne, R.W. and Yarrow, D. (2000) Yeasts: Characteristics and identification. Cambridge University Press: Cambridge, U.K.
3. Campbell, J.I., Sykes, M., Sefton, M.A. and Pollnitz, A.P. (2005) “The effects of size, temperature and air contact on the outcome of heating oak fragments,” Aust. J. Grape Wine Res., 11: 348-354.
4. Fernández de Simón, B.F., Cadahía, E., Muiño, I., Del Álamo, M. and Nevares, I. (2010) “Volatile composition of toasted oak chips and staves and of red wine aged with them.” Am. J. Enol. Vitic., 61: 157-165.
5. Maga, J.A. (1996) Oak lactones in alcoholic beverages. Food Rev. Int., 12: 105-130.
6. Mosedale, J.R., Puech, J.-L. and Feuillat, F. (1999) The influence on wine flavor of the oak species and natural variation of heartwood components. Am. J. Enol. Vitic., 50: 503-512.
7. Pérez-Prieto, L., López-Roca, J., Martínez-Cutillas, A., Pardo Minguez, F. and Gómez-Plaza, E. (2002) “Maturing wines in oak barrels. Effects of origin, volume, and age of the barrel on the wine volatile composition.” J. Agric. Food Chem., 50: 3272-3276.
8. Rodríguez-Rodríguez, P. and Gómez-Plaza, E. (2011) “Effect of volume and toast level of French oak barrels (Quercus petraea L.) on Cabernet Sauvignon wine characteristics.” Am. J. Enol. Vitic., 62: 359-365.
9. Spillman, P., Sefton, M.A. and Gawel, R. (2004) “The contribution of volatile compounds derived during oak barrel maturation to the aroma of a Chardonnay and Cabernet Sauvignon wine.” Aust. J. Grape Wine Res., 10: 227-235.
|
|
PRINT » |
|
|
E-MAIL THIS ARTICLE » |
|
|
CLOSE THIS WINDOW » |
