|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
Introductory study on the use of alternative wood maturation methods
Introduction
The current exchange rate pushes up the price of a new 300 L barrel to between R6 000 and R7 000. Prices such as these, and the fact that barrels are mostly used to a third fill only, make barrels the most expensive part of the production cost of wine. For some wines, therefore, barrel maturation has become unaffordable.
Barrel maturation has been intensively researched in the past, but it remains a complex and unpredictable process which is mostly dependent on the relationship among wood, wine and environmental components. Wood contains high concentrations of ellagitannins, which are highly soluble in wine. These compounds contain many hydroxy groups, which may serve as substrates for oxidative polymerisation, and which are, together with oxygen, the reason for accelerated oxidation and polymerisation of barrel matured wine. Quinn and Singleton (1985) clearly showed that if purified tannin (extracted from wood) is added, it has a huge impact on the taste of the resultant wine. omers (1990), on the other hand, said that concentrations of ellagitannins in wine are too low. It is however due to the rapid polymerisation reaction catalysed by ellagitannins (Vivas and Glories, 1997). Ellagitannins quickly form part of oligomers and are therefore not traceable by modern analytical methods. Furthermore they are rapidly bound by proteins and sugars, or oxidised by hydrogen peroxide. The quinones that are formed also promote further chemical polymerisation reactions. These actions promote the mouthfeel of wine. Toasting however also destroys ellagitannin compounds on the surface of the wood (Moutounet et al., 1994). These compounds are therefore extracted from deeper wood in the course of barrel maturation.
The volatile components that are also easily extracted from the wood, multiply intensively during toasting actions. Therefore volatile components form on the toasted surface of the wood. Products such as shavings can be used to impart the flavourful, toasted character to wine, since more toasted surface is exposed to the wine.
Slow oxidation is of course another important aspect of wood maturation. Oxygen oxidises ethanol relatively easily and quickly, forming acetaldehyde and nitrogen peroxide. Both these compounds act as catalysts for polymerisation reactions. There are more and more indications, however, that very little micro-oxidation occurs through the walls of the barrel itself, but rather around the spunge and during filling.
Alternative wood sources (e.g. exogenous tannins) are also available nowadays. This source is a product of the extraction of oak wood tannins using heat in an alcohol-water or water medium only. The tannins are then sold in a powder or liquid form.
In this study different wood treatments (staves, as well as coarse and fine shavings) and exogenous tannin treatments (water and alcohol-water extraction forms) were tested as alternative wood sources in barrel and tank environments on a 2001 Cabernet Sauvignon.
Material and methods
Cabernet Sauvignon (2001) was fermented dry on the skins with Saccharomyces cerevisiae NT 50 at 250C. No sulphur dioxide was added to the juice or wine before malolactic fermentation (MLF) was completed. The total SO2 levels of the wine after MLF were increased to 80 mg/l and a month after the various treatments, the total SO2 was once again increased to 100 mg/l. The various wood treatments were applied strictly in accordance with the recommendations and concentrations of the supplier, as shown in Table 1. The old 300 litre barrels (also French barrels) that were used, were all fifth fill barrels (1996), thoroughly washed and steamed. No wood or microbiological characters were transferred from the barrels to the wine; the barrel environment only was thus imitated. Barrels were filled weekly. The wine was left to mature for 150 days, whereafter the wine was bottled unfiltered directly from the treatment containers. Screw seals were used to prevent any other contamination.
Colour analyses were done according to the methods of Sommers and Evans (1977). The quality of the extract was determined with the pycnometer method. All non-volatile components extracted from the barrel or wood, or oxygen added which was still in the solution, were thus determined. Organoleptic judging was conducted by a panel of five experienced, trained judges. Each judge received a qualitative 20 point tasting chart and was asked to describe the wine in detail. A line chart, on which the mouthfeel (fullness, astringency and bitterness) and volatile oak component were evaluated separately, was also supplied to the panel.
Results and discussion
The results of the colour determinations are indicated in Table 2. Colour density remains relatively constant, indicating that colour precipitation did not have an effect on the wine matured only for a short time. When looking at the percentage of red pigments, polymeric pigments (SO2 resistant) and total red pigments, shavings (fine and coarse) and staves showed better colour quality in the tanks. Compared to new barrels and the controls, the barrel treatments did not lose a lot of colour, and the colour obtained a good mark on the qualitative 20 point tasting chart.
In cases where exogenous tannins were used, better colour was found in the barrel matured wine. AE 1, for example, produced the most total phenols and total red pigment in both the barrel and tank. All exogenous tannins retained good colour, however, except AE 1, where the extra heat treatment of the oak wood possibly destroyed the pigment stabilising compounds (such as ellagitannins).
The extract qualities are described in Table 3. In all treatments the barrels have less extract (g/litre) than the tank matured wines. The reason for this is possibly the precipitation of large polymers in the micro-oxidation conditions of the barrel. This is not necessarily a negative development, since such unstable tannin complexes can later precipitate in the bottle if they are not removed by micro-oxidation. The new barrels had even less extraction, which may also be ascribed to the slow oxidative polymerisation.
The wood structure of new barrels ensures better micro-oxidation conditions through the wood than old barrels, where the settling of wine phenol polymers results in wood compaction.
The qualitative marks obtained by each treatment are shown in Table 3. The Cabernet Sauvignon was a full-bodied wine with a good tannin structure and even the tank control obtained a relatively good mark. As can be expected, the new barrels obtained some of the highest quality marks (14.5). Barrel maturation with wood treatments (staves and shavings) were chosen throughout as the better wines, while the exogenous tannin treatments, on the other hand, benefitted from the tank conditions (AE 1 = 14.1; AE 2 = 14.8; AE 3 = 14.9; AE 4 = 14.6 and WE 1 = 13.8). No big differences were noticed between the tannin extracts and the original maturation techniques. Shavings (fine and coarse) were strictly penalised and judges noticed oxidation in particular. The reason is possibly an excess of oxygen that is drawn into the wine during the treatment. Together with the reduced number of ellagitannins for the regeneration of hydroquinones, oxidative characters form quickly.
Coarse shavings were described as dusty, dry, overwooded and oxidised, while fine shavings were described as overwooded, synthetic wood character and oxidised. The shaving treatments' volatility exceeds the mouthfeel and the wine is therefore not balanced; this is connected with overwooding.
When looking at the results, the various balances among wood character, fullness and overall tannin (astringency) of the various treatments are observed. If the balance is not correct, connotations such as "overwooded" are attached to the wine. This trend was noticed with the shavings (fine and coarse) in the barrels trials, as well as AE 4 in the tank trial. All three the above were described as "overwooded" by the panel of judges. The wine of the new barrel treatments was also described as very volatile, but together with this the body of the wine also increased, thus keeping the wine balanced. None of the judges described the new barrels as "overwooded". It should, however, be possible to imitate the taste profile of a new barrel by making use of combinations of alternative wood sources. Bitterness was never a problem, and even fine shavings in the tank, which obtained the highest degree of bitterness, were not mentioned in the wine descriptions as a problem.
Wood treatments in the barrel maturation environment retained more fullness, overall tannin and volatile oak character than the corresponding tank treatments (Table 4). From the exogenous tannin trials it is clear, on the other hand, that tank environments are more beneficial than the corresponding barrel treatments. The difference between AE 1 and AE 2 is that the former was made of toasted oak wood, while AE 2 was not. The volatility curve shows the increased oak character, but AE 1 clearly imparted less fullness and tannin character to the wine. This is ascribed to the reduced amount of ellagitannins in toasted oak wood.
Conclusion
Barrels will have to be used in a more specialised fashion, which will possibly exclude certain wines from the traditional means of barrel maturation. In this study it is clear that wood treatments in barrel circumstances are still better and that the slight colour loss that the winemaker will sacrifice is minimal. The opposite is true, however, for the exogenous tannins, where the tank environment was beneficial to the maturation process.
The importance of ellagitannin extraction from the wood was once again proven. Shavings (fine and coarse) and even AE 1 clearly have too little ellagitannin reserves to release in the wine. Together with this, large quantities of oxygen are drawn into the wine with the shavings, causing rapid oxidation and resulting in oxidative characters, colour loss and colour instabilities. It should be noted that barrel maturation also has a stabilising effect on wine. Wine stability after tank maturation on wood or exogenous tannins must still be investigated.
The importance of balance in the taste profile imparted by wood to wine is clearly noticeable. New barrels produced a unique taste profile, while other treatments contributed too much or too little of certain characteristics to the wine. However, combinations may be good at imitating new barrels.
In the experiment even the control barrel gave good results and the influence of old barrels should possibly not be underestimated. It should be noted, however, that the wine used in the study was a full-bodied wine with good colour. Wood contact does not have the ability to save a bad wine; it can only enhance a good wine.
Acknowledgements
I would like to thank the following persons and bodies for their input and support of this study: Dr. Wolf Katz, Michael Liedtke (Laffort), Dirk Wiese (Columbit) and Peter Peck (CDS).
References
Moutounet, M. et al., 1989. Analysis by HPLC of extractable substances in oak wood: application in a Chardonnay wine. Sciense Aliments 8:35-51.
Quinn, M. K. & Singleton, V. L., 1985. Isolation and identification of ellagitannins from white oak and an estimation of their roles in wine. American Journal of Enology and Viticulture 36:429-434.
Somers, T. C., & Evans, M.E., 1977. Spectral evaluation of young red wines: antocyanin equilibra, total phenolics, free and molecular SO2, "chemical age". Journal of Science and Food Agriculture 28:279-287.
Somers, T. C., 1990. An assessment of the "oak factor" in current winemaking practice. The Australian Wine Research Institute Technical Review 67:3-10.
Vivas, N., & Glories, Y., 1996. Role of oak ellagitannins in the oxidation process of red wines during aging. American Journal of Enology and Viticulture 47:103-107.
|
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
P van Rensburg and J Joubert
| UP |
COPYRIGHT (C) 2000 WineLand |