Oak barrels have an important effect on wine aroma, colour, and stability. In addition, the product undergoes spontaneous clarification. The quality of aged wine depends on many different factors, mainly the wood composition, wine composition and ageing time. First, we shall review the different studies carried out on the above-mentioned factors. In addition, we shall look at the works carried out on the formation of undesirable compounds (ethylphenols), which are mainly found in wine ageing in used barrels. Finally, studies made on the processes of sorption of some wine volatile compounds by wood and by lees and new methods of ageing wine in wood will be presented.

Review of quality factors on wine ageing in oak barrels

Quantitative analysis of 4-ethylphenol and 4-ethylguaiacol in red wine.

The concentrations of 11 phenols and 5 furans were measured in 12 categories of distilled spirits by HPLC methodology, together with the total antioxidant status (TAS) of the same beverages. Ellagic acid was the phenol present in highest concentration in all beverages. Moderate amounts of syringaldehyde, syringic acid, and gallic acid, as well as lesser amounts of vanillin and vanillic acid, were measurable in most samples of whiskey, brandy, and rum but were largely undetectable in gin, vodka, liqueurs, and miscellaneous spirits. 5-(Hydroxymethyl)furfural was the predominant furan in the former three beverages, notably cognac, with 2-furaldehyde the next highest, but these were undetectable in most of the latter beverages. Highest TAS values were given by armagnac, cognac, and bourbon whiskey, all three of which tended toward the highest concentrations of phenols. Negative TAS values were exhibited by rum, vodka, gin, and miscellaneous spirits in line with the low or undetectable phenol concentrations in these beverages. Wood aging is the most likely source of phenols and furans in distilled spirits. Those beverages exposed to this treatment contain significant antioxidant activity, which is between the ranges for white and red wines, with the potential to augment the antiatherosclerotic functions attributable to the ethanol that they contain.

Phenolic constituents, furans, and total antioxidant status of distilled spirits.

Stir bar sorptive extraction (SBSE) coupled with gas chromatography/mass spectrometry (GC/MS) was used to analyse wine samples for three applications: flavour and compositional analysis; 2,4,6-trichloroanisole (TCA), a common off-aroma in wine; and agrochemicals. SBSE was found to be orders of magnitude more sensitive than modern conventional methodology, allowing for lower detection and quantitation levels, and improved confirmation of identity; SBSE often gave better signal to noise in scan mode than other methods in selective ion monitoring (SIM) mode. With the help of their characteristic mass spectra all agrochemicals could be identified unambiguously at concentrations of 10 μg L−1 in wine and a further 100 constituents were detected in a Cabernet Sauvignon sample. Thus it is now possible to analyse complex samples such as wine by scan mode, with better confirmation of identity, and without sacrificing sensitivity, where previously SIM methodology had to be used.

Application of stir bar sorptive extraction for wine analysis.

Red wine was stored in different oak barrels or in stainless steel, and samples were taken for two years to determine 79 aroma compounds. Aging in oak affects 41 compounds. The type of wood affects 11 compounds. At least seven different processes seem to take place concurrently in aroma evolution, and five such processes, affecting 37 compounds, are linked to the oak cask. These are extraction from the wood, oxidation of wine alcohols and amino acids, microbiological formation of ethyl phenols, sorption processes, and condensation of acetaldehyde with polyphenols. The wood can release linear gamma- and delta-lactones, beta-damascenone, and ionones. Some compounds are released very fast from wood, which suggests they lie in the external part of the wood. Some extraction profiles are too complex to be explained by physical processes. Finally, the levels of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and 2-ethyl-5-methyl-4-hydroxy-3(2H)-furanone increase even in the reference wine, which suggests the presence of a precursor.

Concurrent phenomena contributing to the formation of the aroma of wine during aging in oak wood: an analytical study.

Furfural, 5-methylfurfural, and vanillin co-occurred in 64 barrel-aged red, white, and model wines with the reduction products, furfuryl alcohol, 5-methylfurfuryl alcohol, and vanillyl alcohol, and with the corresponding ethyl ethers of these alcohols. Hydrolytic studies in a model wine have shown that 5-methylfurfuryl ethyl ether is formed rapidly from 5-methylfurfuryl alcohol, but both decomposed quickly under the conditions. Vanillyl ethyl ether was also formed relatively rapidly, and both this ether and vanillyl alcohol were stable in the model wine. The formation of furfuryl ethyl ether from furfuryl alcohol and the subsequent decomposition of these two compounds were comparatively slow. The relative concentration of these aromatic alcohols and ethers in the barrel-aged wines was consistent with the observed stability of the furan derivatives, but low concentrations of vanillyl alcohol and vanillyl ethyl ether observed in all samples showed that factors other than solvolytic degradation were responsible for reducing the concentration of these compounds in wine. Furfuryl ethyl ether, which had an aroma threshold of 430 µg/L in a white wine, was found at approximate concentrations of up to 230 µg/L in the wines.

Formation and Degradation of Furfuryl Alcohol, 5-Methylfurfuryl Alcohol, Vanillyl Alcohol, and Their Ethyl Ethers in Barrel-Aged Wines.

A method for the rapid and accurate analysis of vanillin in wine, using stable isotope dilution analysis and gas chromatography/mass spectrometry, has been developed and applied to 64 oak barrel-aged white, red, and model wines. Following barrel fermentation and maturation on yeast lees, the concentration of vanillin in the white wines was only about one-third of that in the model wines stored for the same period. Once the yeast lees were removed, however, the white and model wines accumulated vanillin at a similar rate, which indicated that biological reduction of vanillin occurred only prior to racking. After 93 weeks in barrels, the concentration of vanillin in the red wines was less than one-half that in the model wines, and vanillin was further depleted during subsequent bottle storage of the red wines for 2 years at cellar temperature. For the model and red wines, the mean concentration of vanillin in barrels made from French oak, seasoned and coopered in Australia, was significantly higher than tha...

Philip J. Spillman

Papers

Formation and Degradation of Furfuryl Alcohol, 5-Methylfurfuryl Alcohol, Vanillyl Alcohol, and Their Ethyl Ethers in Barrel-Aged Wines.

Accumulation of vanillin during barrel-aging of white, red, and model wines.