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Alternative oak treatments in South Africa (Part 1)
Establishing a correct stave dosage
Wessel du Toit, Department of Viticulture & Oenology, Stellenbosch University, Stellenbosch
The cost of oak wood barrels has increased the past few years and now forms one of the main production costs of many red wines. Oak barrels can normally only be used three to four times due to the leaching of flavour compounds from the wood to the wine during the ageing process. The use of alternative oak treatments, such as oak chips, blocks and staves are thus becoming more popular. A previous study by Du Toit et al., 2006 concluded that micro-oxygenation can be used in conjunction with oak staves to simulate an oak barrel, but the type of wood used is very important.
These two articles will focus on some experiments performed recently at the Department of Viticulture and Oenology, Stellenbosch University on using oak staves as an alternative oak ageing method. The first article will look at establishing appropriate dosages which will simulate to certain extend the extraction obtained with a new oak barrel. These experiments were performed in conjunction with micro-oxygenation to further simulate conditions normally to be found in an oak barrel.
MATERIALS AND METHODS
A 2006 Cabernet Sauvignon wine from the Paarl area, prepared with standard winemaking were used for this experiment and the treatment started just after the completion of malolactic fermentation. American oak wood, each time from the same batch, was used in this experiment for both staves used as alternative oak treatments and to produce barrels from. We used a toasting method, in conjunction with a major SA cooper (Radoux), to toast oak wood destined for oak barrels or as staves in roughly the same manner. This entailed toasting the staves on both sides as opposed to that used for a barrel, where it is only toasted on one side. This is due to the wine being in contact with both sides when staves are used as alternative oak treatments.
We used a 225 L oak barrel as a standard to calculate oak treatments. The internal surface of a 225 L oak barrel is around 1.92 m2. To two 1 000 L tanks staves at 40% of the equivalent of the internal surface area of a 225 L barrel was added to, while to two other 1 000 L tanks this dosage was increased to 100% of the internal surface area of a 225 L barrel. To one each of the 40% and 100% stave treatments micro-oxygenation was also applied at 4 mg O2/L/month, with a Parsec micro-oxygenation apparatus from Reines trading. The same wine was also matured in an old third fill 225 L barrel.
During the course of the experiment a wide array of colour and phenolic characteristics were monitored with a spectrophotometer, as well as HPLC. These included colour density, modified colour density (where the bleaching effect of SO2 is negated by acetaldehyde addition), total red pigment, SO2 resistant pigments total tannins, total anthocyanins, total phenolics, catechin, cinnamic acid derivates, individual anthocyanins, dimers B1 and B2, polymeric pigments, polymeric phenolics etc. Certain volatile oak compounds, such as eugenol, lactones, furfural etc. were also analysed with GC-MS, as well as standard wine analyses such as SO2, alcohol, VA etc.
RESULTS AND DISCUSSION
The colour density (CD) of the wine after 4 months can be seen in Fig. 1. It is clear that only after 4 months of ageing with oak staves and micro-oxygenation an increase in the CD was observed. Wines matured in older barrels 2nd fill and 3rd fill barrels showed lower CD, probably due to less oxygen diffusing into the wine and less hydrolysable tannins being available for indirect acetaldehyde induced polymerization.
After 6 months this difference was even more pronounced, as can be seen in Fig. 2. Again the wine exposed to micro-oxygenation and higher oak staves dosage had the highest CD. The CD of the 40% oak treatment was roughly the same as that of a new barrel after 180 days of treatment. Surprisingly enough the modified colour densities (MCD) were also higher in the wines receiving oxygen and stored in barrels. In the MCD measurement acetaldehyde is added to a small sample of the wine before measurement. This is done to negate the bleaching effect of SO2 on the colour. The difference in the CD and MCD was smaller in the wines receiving oxygen and in the new barrels, due to the oxygen addition and higher concentrations of hydrolysable tannins being released into the wine, which catalyses pigment formation. These pigments are normally more resistant towards the bleaching effect of SO2. Polymerisation reactions between free anthocyanins and catechin moieties form these pigments, which obviously leads to a decrease in free anthocyanin concentration. This was also reflected in the anthocyanin concentration of the wines (Fig. 3), where the wines receiving higher concentrations of oak contact and especially oxygen, had a lower value of anthocyanins, with the old barrel having the highest concentration of anthocyanins. The so-called Boulton analyses also reflected this, with wines receiving higher oxygen added and higher concentrations of staves having lower free anthocyanins and higher polymeric pigments and co-pigments (Fig. 4) after 180 days.
The total red pigments were also lower in the stave and new barrel treated wines compared to that stored in an old barrel, but the parentage of pigments in the red form were higher in these wines (Table 1), explaining the higher CD of these wines. This is typical of what happens with red wine in a new barrel, where colourless anthocyanins are incorporated into coloured pigments. However, some of these pigments can precipitate over time, but these wines CD is normally still higher due to the higher % of colour in the red-brown form. The addition of oak staves and oxygen thus not only leads to polymerisation, but also to the transformation of previously colourless compounds into red-brown pigments. This percentage was however higher in the oxygen treated wines than in the wines matured in the oak barrel, which could be due to slightly too high oxygen addition added to this wine. This highlights the need for further research regarding the role and addition of oxygen in wine.
Colour hue did not differ significantly between all the treatments (results not shown). This was due to a parallel increase in 420 and 520 nm (colour hue is measured by 420 nm/520 nm).
In Table 2 the HPLC analyses of certain individual phenolic compounds can be seen. Micro-oxygenation and higher oak stave additions seems to have led to lower levels of the dimmers B1 and B2 and certain anthocyanins such as malvidin-3-glucoside and delphinidin-3-glucoside. Micro-oxygenation increased the polymeric pigment and phenol content, which can be explained by the Bayer reaction, where oxygen addition to phenolics leads to the production of small amounts of acetaldehyde, which can form a bridge between phenolic compounds. However, it was surprisingly the lowest in this case in the wine matured in the new barrel. It is believed that aldehydes originating from oak can also have this polymerisation induced function, explaining why the same happened to certain extend in the control wines with a 100% oak contact receiving no oxygen.
Free SO2 concentrations in the 2006 Cabernet Sauvignon can also be seen in Table 3. It is clear that free SO2 levels dropped quicker where higher stave dosages and oxygen was added. Oxygen coming into contact with wine leads to the oxidation of phenolic compounds, originating from both the grapes and oak. This leads to the formation of H2O2, which can further oxidize ethanol to form acetaldehyde. The latter can form a bridge between anthocyanin and catechin moieties to form pigments, as mentioned before. SO2 reacts with the H2O2, thus preventing further oxidation by this potent oxidant. One can thus ask the question how oxidation still occurs when SO2 is present in the wine. The reason for this is that SO2 occurs at much lower concentrations in wine than ethanol, so part of the H2O2 can still react with the latter if the SO2 concentration is not too high. Resent research showed that when total SO2 levels during micro-oxygenation exceed 100 mg/L a large part of the oxygen treatment is negated. We tried to keep the free SO2 levels at about 20 mg/L during the treatment.
In Table 4 the concentration of some oak derived volatiles after 200 days can be seen. Guaiacol, which contributes to the smoky flavour of red wines, was higher in the wines treated with staves than with the barrel, which could be due to slight differences in toasting. The concentration of guaiacol was also higher than the odour threshold value (OTV, the minimum concentration where people normally start to perceive it) of 10 µg/L. Trans oak lactone was much lower than the OTV, which was also often found by other researchers. However, the cis-oak lactone, one of the most important flavours compounds deriving from oak showed interesting results. Those in the wine matured in the barrel was on par with those receiving 40% oak wood contact, which was also the case with eugenol, which contributes to the spiciness of red wine. Both these compounds occurred at levels higher than their respective OTV. The two Brettanomyces derived compounds; 4-ethylguaiacol and 4-ethylphenol showed similar values and were both lower than their OTVs.
Standard wine analyses, such as alcohol, TA, pH and VA did not show any significant differences between the wines.
A tasting panel could distinguish in general between the 100% oak contact and that of the barrel treatments after 180 days, but not between the 40% oak contact and the new barrel, giving another indication that 40% is closer to the dosage simulating the barrel to the largest degree. The panel also in general preferred the barrel and 40% oak contact wines over the 100% oak contact wines, stating that the latter wines were over wooded.
These results give preliminary indications that 40% oak wood stave contact with micro-oxygenation gives much closer results to a new barrel than a 100% oak wood contact. The reasons for this are not clear, but could be due to a larger extraction at the end of staves. It is thus important that winemakers rather use area/hL than mass/hL when deciding on how much staves to add to a tank. A rough recommendation can be that if an intense oak flavour is required about 30 - 40% of the internal surface of a barrel be added to the wine, while if a more subdued oak flavour is required only 10 - 15% should be added. However, this should be done in conjunction with a reliable cooper, as products can also change from supplier to supplier. One should also keep in mind that staves are not always produced from the same quality wood as barrels and the next article will address this aspect further.
ACKNOWLEDGEMENTS
Winetech and Thrip for funding of this project, Reines Trading for supplying the micro-oxygenation equipment, Radoux for toasting the staves and KWV for the oak volatile analyses.
REFERENCES
Cano-López, M., Pardo-Minguez, F., López-Roca, J.M., Gómez-Plaza, E. (2006). Effect of micro-oxygenation on anthocyanin and derived pigment content and chromatic characteristics of red wines. Am J Enol Vitic 57, 325 - 331. |
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