Alternative oak treatments in South Africa (Part 2): Investigating the effect of different staves in simulating an oak barrel

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Alternative oak treatments in South Africa (Part 2):
Investigating the effect of different staves in simulating an oak barrel

Wessel du Toit

Wessel du Toit, Department of Viticulture & Oenology, Stellenbosch University, Stellenbosch
A previous article in this series looked at establishing a stave dosage which could simulate the extraction from an oak barrel. We established using the staves of a major South African cooper that around 40% staves contact of a 225 L barrel gives similar results to a new barrel. We thus also decided to investigate the effect of using different staves in combination and ascertain its effect on red wine compared to the same wine matured in new oak barrels. The aim was thus to ascertain the effect of staves produced from wood normally destined for barrel production, as well as those normally used for alternative oak product production.
MATERIALS AND METHODS
A 2007 Pinotage were used for this experiment. The wine was prepared from standard practices and treatments started soon after the completion of malolactic fermentation. American oak wood was used in this experiment. The Department of Viticulture and Oenology, Stellenbosch University developed a method, in conjunction with a major SA cooper (Radoux), to toast oak wood destined for oak barrels or as staves with roughly the same manner. This entailed toasting the staves made from oak wood normally used for barrel production on both sides with an open fire, as the wine in a barrel are in contact with the toasted sides of the barrel staves. Staves (B) were thus produced from oak wood normally used for barrel production and toasted in roughly a similar manner as new barrels (NB) made from the same batch of wood. Staves (N) produced from wood normally used for alternative oak products were also toasted with an oven. All oak used in these experiments were American oak.
In this experiment four 1 000 L tanks had a stave dosage of 40%. The internal surface area of a 225 L barrel was used as a standard to determine what surface area of oak staves be added to the wine (refer to the previous article). To two of the tanks micro-oxygenation was applied at 4 mg O2/L/month and to two none. One micro-oxygenation tank and one control tank received staves produced and toasted from wood normally used to produce alternative oak products. The other two tanks received staves made from the same batch of wood used to produce the new oak barrel used in the experiment.
During the treatment of the wine its colour and other phenolic characteristics were analysed with a spectrophotometer. 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 and total phenolics. 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. A blind tasting was also performed with a triangular and preference tasting after 180 days of storage.
RESULTS AND DISCUSSION
Different colour analyses on this wine revealed that wines made with the B treatment and receiving micro-oxygenation, yielded slightly higher colour characteristics, but these differences were very small (Table 1). Micro-oxygenation per se led to slightly higher colour density, but it was very small in general winemaking terms, which correlates with work previous findings that it generally lead to small increases in colour density when micro-oxygenation is used on a wine with an already high colour density (>18). Colour densities between the different oak treatments also did not differ dramatically.

These small differences were also reflected in the SO₂ resistant pigments, total red pigments and % of colour in the red form (Table 1 and 2). Total red pigments did not differ dramatically between the different treatments and very little difference was observed in the degree of red colour between the different treatments, although it increased from 25% to around 45% from day 45 to 180 in all the treatments. This is due to colourless anthocyanin-tannin complexes being transformed into coloured pigmented complexes. Colour hue did not differ significantly between the treatments (results not shown). In Fig. 1 the evolution of anthocyanins during the course of the treatments can be seen. It is clear that micro-oxygenation led to some lower anthocyanins levels, probably due to polymerisation between anthocyanins and other phenolic compounds induced by the production of acetaldehyde. Again little difference was observed between the different oak treatments, but the new barrel treatment’s anthocyanins concentration was slightly higher than that of the micro-oxygenation treated wines. The Pinotage was also analysed after 180 days for phenolic compounds with HPLC, but no clear tendency could be observed, except for the micro-oxygenation and oak treatments leading to lower anthocyanin and gallic acid levels (Table 3).

In Table 4 the free SO₂ concentrations of the 2007 Pinotage can be seen. As expected, micro-oxygenation led to lower SO₂ levels in the wine. At day 115 the free SO₂ level in the N mox, B mox and NB treatments were all below 20 mg/L and were adjusted to 25 mg/L free. In a new barrel more O2 comes into contact with the wine, compared to an old barrel, which was also reflected in these lower SO₂ values. This is due to the pores in new barrels being less blocked as well as more hydrolysable tannins being released into the wine, which induces polymerisation. This could all lead to lower free SO₂ levels. It was thus clear that in this specific experiment the addition of different oak staves and oxygen did not lead to large differences in colour and other phenolic characteristics. However, the colour in the wines treated with oxygen was slightly more which was also reflected in them having lower anthocyanin concentrations and higher SO₂ resistant pigments.

In Tables 5 and 6 the concentrations of some important oak volatiles can be seen in the 2007 Pinotage after 60 and 180 days of ageing can be seen. It is clear that the concentration of these compounds increased over time. The concentrations of furfural stayed below the odour threshold value (the minimum concentration where people start to perceive it in wine) of 5 800 µg/L. Guaiacol, which gives the typical smoky flavour to certain red wines were roughly the same after 60 days, but after 180 days were similar in the B and NB treated wines. Trans-oak lactone values were also lower than the ODV throughout the experiment. This was probably due to its relative high OTV of 380 µg/L, when compared to that of the cis-oak lactone. Cis-oak lactone is a potent oak volatile, which is responsible for the typical oak wood or coconut flavour of red wines. It is clear that the concentration of this compound were closest to that of the wine matured in the new barrel (NB) when staves normally used for barrel production (B) were used in the experiment. Micro-oxygenation did not seem had a definitely effect on the concentrations of these compounds. Eugenol (spicy, clove like flavour) levels were similar in all the stave treated wines, but higher than the ODV. Low levels of 4-ethylphenol and 4-ethylguaiacol indicated little or no Brettanomyces activity in these wines.

These differences can be explained due to the fact that not the same wood are used for alternative oak products (such as staves) and barrel production. Although we also tried to toast the staves in the B treatment in the same manner as those used for new barrel (NB) production, small differences in toasting intensities could also contribute to the differences observed. The staves used in the N treatment were toasted in an oven, which can lead to differences from wood staves toasted over a fire. The same wine was also tasted after 180 days. A tasting panel could significantly distinguish the N treated wines from the B and NB treated wines. However, they could not distinguish the B and NB treated wines from each other, irrespective if micro-oxygenation was applied or not. The panel also preferred the wines matured in the B and NB treatment over that matured in the N treatment.
These results indicate the importance of using a good quality stave when a barrel needs to be simulated with stave addition and micro-oxygenation applications. It thus seems that combining good quality staves at the right dosage with controlled micro-oxygenation can simulate, but probably not completely replace, a barrel.
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.
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