The Australian Wine Research Institute’s Analytical Service - Analyses

Oak volatiles

The use of oak for the ageing of premium wine products is commonplace in the wine industry. It is primarily used to increase complexity and to add oak flavours to wines. There is now a method for analysing the effect of oak storage on wine flavours and, from this, an opportunity for winemakers to gather information that will allow them to determine optimal barrel storage.

Volatile compounds derived from oakwood are important contributors to wine aroma and flavour. Such compounds include:

cis- (and trans-) oak lactone

Cis- (and trans-) oak lactone--apparently the most important oak-derived flavour compound in wine--is responsible for a variety of sensory characteristics and is chiefly responsible for the greater intensity of vanilla and coconut-like aromas found in wines matured in different types of oak. These lactones are present in the raw material and vary substantially from one sample to another. The amounts present in oak can be affected by seasoning but not in a predictable way. The presence of these compounds in oak is not greatly affected by coopering variation, but in some samples, increasing toasting levels can slow down the rate of lactone extraction and reduce their impact on wine flavour.

Guaiacol and 4-methylguaiacol

Guaiacol and 4-methylguaiacol have smoky aromas and are indicators of the relative toast levels of barrels. These compounds are formed almost exclusively by the degradation of lignin during the toasting process. The degree of toasting is, therefore, an important factor in imparting a smoky character in wines. The use of wood smoke is another source of these compounds and the burning of oak offcuts during the coopering process may also enhance this character.

Vanillin

Vanillin is the main flavour compound in natural vanilla. It can be extracted in significant quantity from unheated oak wood and in even higher amounts from strongly heated wood. It is generally considered to be an important contributor to the character of barrel-aged wines. Vanillin can be transformed by yeast metabolism during fermentation and vanillin concentration can be reduced considerably if primary fermentation is carried out in the barrel. In general, vanillin is formed in increasing amounts at increased toasting levels and levels are not influenced by origin, or seasoning period. Analysis of this compound can be used to determine future toasting levels that are appropriate for the chosen wine variety and style.

4-EP and 4-EG (Brettanomyces)

4-Ethylphenol and 4-ethylguaiacol are products of Brettanomyces yeast and are found in virtually all Australian barrel-aged red wines. 4-ethylphenol is responsible for the 'sweaty saddle/band-aid' aroma of red wines when present in high concentration, and is generally regarded as detrimental to wine quality. Analysis of wine in barrels will determine whether this compound has been formed in high concentration during maturation and may indicate the need for changes in how the wine is subsequently handled. These compounds can be analysed separately to the oak flavour compounds where the latter is not required.

Furfural and 5-methylfurfural

Furfural and 5-methylfurfural are generated by the breakdown of carbohydrates, in particular cellulose and hemicellulose, during the toasting process. The concentration of these compounds can decrease during heavy toasting levels. These compounds have a sweet, caramel or butterscotch aroma and are often present at levels greater than 1000 µg/L in wine.

How does the analysis work?

A new analytical method employing gas chromatography/ mass spectrometry and using deuterium-labelled standards has been developed for accurately determining the concentration of all of these compounds in wine or in oak shavings in a single analysis. A small volume of wine or sample of chips/shavings is extracted with a mixture of ether and pentane, and the pentane extracts analysed directly. Oak shavings and chips must first be steeped in a model wine for several days, the model wine extract is then extracted in exactly the same manner as an ordinary wine.

How can this analysis assist winemakers?

The analytical data will enable the industry to assess the total flavour potential of oak wood and to evaluate the relative merits of various types of oak treatment in wine production. If a winery observes significant barrel-to-barrel variation in perceived wine quality, the analytical data will enable the winemaker to determine whether the raw material (oak lactone concentration), the toasting level (guaiacol and 4-methylguaiacol), or Brettanomyces activity (4-ethylphenol) has contributed to the flavour differences and, if so, to refine oak handling for future vintages. The analysis may also serve as an objective indicator for the purchase of new barrels or for the re-shaving and toasting of old barrels. Furthermore, analysis of wines during barrel storage may provide information that will enable winemakers to optimise the desired oak derived flavours by manipulating the time spent in barrels of different types.

Wineries and suppliers can also determine the flavour potential of oak by analysing subsamples of oak lots prior to coopering, or can assess the flavour potential of oak chips prior to addition. Analysis of oak chips prior to addition can also be undertaken in conjunction with an analysis for trichloroanisole (TCA) to ensure that the wine does not become accidentally tainted by such an addition.

Unfortunately, the analysis is not suitable for assessing the flavour potential of finished barrels prior to use.

Only 20 mL of wine is needed for analysis and samples should be submitted in glass containers, as the effect of contact with plastic polymers is not known.