The replacement of traditional oak barrels with alternative oak products such as staves, chips, shavings, powder and liquid extracts was mainly financially driven.

In contrast with the use of oak barrels, very little is known about the impact that the use of these alternative products may have on the flavour of wine and even less is probably known about the processing methods to be applied to these alternative products in order to obtain specific preferences for the treated wines.

The influence of oak on the flavour and taste of wine is due mainly to the extraction of volatile components from the wood. Some of the most important components with their corresponding characteristics are the following:

Guaiacol: Smoky.
4-Methyl guaiacol: Smoky.
Cis-oak lactone: Coconut, oak, vanilla, fruit, wood and caramel.
Trans-oak lactone: Coconut, oak, weed, wood, hay and jasmine.
Vanillin: Vanilla.
Some of these components, such as the cis-oak lactone, are present in significant concentrations in untreated oak, while others, such as vanillin guaiacol, 4-methyl guaiacol and furfural are mostly derived from oak polymers that are broken down during the toasting process by heat, hydrolysis and the acids in wines, or a combination of both. Alternative wood products are often treated with heat so that the influence thereof on wine will correspond with barrel treatment.

Apart from the origin and composition of raw oak products, the sensorial influence of alternative wood products may also be influenced by the size of the chips, the duration of the heating process, the heating temperature, the stage of application in the vinification process, the wine contact period, the prior treatment of the products with sulphur dioxide and the shelf life thereof.

The influence of chip size:

The evolution of oak-based flavour compounds in barrel matured wines may extend over several years. It may be partially ascribed to the period of extraction from the oak barrel. As a result of the bigger exposure surface, one may expect the extraction period to be reduced relative to the decrease in size of the oak product, so as to obtain the same concentration of flavourants in the wine. The following was proven in a comparative study with chips of two different sizes, 15x15x5 mm and 10x2x2 mm: Although the evolution of volatile components in a model wine was practically completed in a few days, all extractable material was not removed. The various flavour components differ with regard to the percentage thereof that is removed by successive wine model solutions if the same product is used repeatedly. The chip size does not influence the rate of extraction or the extent to which the components are extracted by the wine.

The size of the wood particle has a significant influence on the result of the heating process. This can probably be ascribed to the importance of air during the heating process. With oven treated samples, smaller particles contained more volatile phenols after heating. However, smaller particles may be more exposed to evaporation of the volatile components during storage.

The influence of the sulphur dioxide treatment of oak:

It is common practice to treat barrels before and after being used with sulphur dioxide. This may be done with an acqeous sulphur dioxide solution (usually 4 grams/hectolitre sulphur dioxide) or with sulphur dioxide gas. There has been little research up to now on the influence of such treatments on the volatile oak components of wine. Used barrels that have been treated in this manner, had increased levels of oak lactones.

Unheated and medium heated (one hour at 200C) French and American oak samples that were exposed to a sample wine mixture after having been exposed to sulphur dioxide gas or air in the laboratory under anaerobic conditions, showed the following results:

In all the oak samples 15 to 65% more vanillin was extracted in instances where there had been exposure to sulphur dioxide.
The heated American oak sample had a small, but significantly higher cis-oak lactone concentration following the sulphur dioxide treatment.
In the other wood samples the cis-oak lactone concentrations were higher, but not significantly so in the absence of sulphur dioxide.
The influence of the shelf life of oak chips:

Alternative wood products are often stored in plastic bags or other containers for varying periods of time before being used. In order to determine the possible influence thereof on the wood extraction, sample wine solutions were used for the extraction of French oak chips with light, medium and heavy open fire toasting and American oak chips, oven heated for one hour at 175C and three hours at 200C. Wood extracts were analysed at purchase, after 6 months, following a 28 day sample wine extraction. Between the two different stages of sampling no differences could be found in the concentrations of guaiacol, 4-methyl guaiacol, cis- and trans-oak lactone or vanillin. Consequently the storage of the wood products in question did not have an influence on the extractable measured volatile components (Campbell et al, 2006).

Reference:

Campbell, J.J., Polnitz, A.P., Sefton, M.A., Herderich, M.J., Pretorius, I.P. 2006. Factors affecting the influence of oak chips on wine flavour. Wine Industry Journal 21 (4): 38 – 42.

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