Barrels are traditionally associated with winemaking. Like stainless steel, concrete and polyethylene, it is primarily a container. Although the other materials are cheaper, more hygienic and homogeneous, barrels are still popular for many other reasons.
The influence of the physical and chemical characteristics of oak
Alcohol and water evaporate from barrels due to the wood porosity, which also causes the absorption of wine by the wood. Different extractable compounds like ellagitannins, guaiacol, eugenol, oak lactones and furfural are extracted from the wood. These compounds influence the astringency, mouthfeel and aroma intensity of wines. Due to the moderate permeation of oxygen through the pores, reactions like oxidation, polymerisation, co-pigmentation, and condensation involving anthocyanins and proanthocyanidins take place, which can stabilise the colour and decrease astringency. Barrel ageing also favours the natural sedimentation in wine, influencing the wine’s clarity.
The natural characteristics of oak barrels
Barrels are made from wood, which is a natural product. The most commonly used species are Quercus petraea (sessile oak), Quercus robur (pedunculate oak) or their hybrids and Quercus alba (white American oak). Alternative species may also be used for traditional reasons. The wood composition and barrel production process will cause a variation difference in barrels. The main factors influencing the variation are the oak species, wood origin, seasoning and location, and the toasting process. Barrels influence the phenolic composition and colour during maturation, which will consequently be affected by the variation between barrels. Winemakers know this and blend different barrel lots according to their sensory evaluation.
The effect of cooperages on barrel-to-barrel variation
A 2017 Touriga Franca red wine was matured for 12 months under the same maturation conditions in new medium-toasted Quercus petraea barrels. The 49 barrels originated from four different cooperages. No significant differences in the phenolic and colour analyses were found between three of the cooperages. Regarding certain parameters, the fourth cooperage differed significantly from one or more of the other three cooperages. This may be attributed to its processes and wood selection, leading to lower oxygen uptake. However, the cooperage of an individual barrel could not explain these differences.
The effect of the barrel on barrel-to-barrel variation
Chemical analyses in the experiment showed variations between barrels, ranging from 0.01 to 37.2%. The effect of barrel maturation on general characteristics like density, alcohol concentration and the dry extract is small or similar between barrels. Volatile acidity and residual sugar concentration variations are probably due to microbial activity and not the influence of the different barrels. Differences in polymerisation reactions can be caused by a variation in oxygen permeation, which can also be attributed to cooperage processes and not the individual barrels. In summary, it can be concluded that variation in the chemical characteristics of individual barrels depends on each specific parameter and is not uniform.
Barrel sample requirements
Bearing in mind that barrel-to-barrel variations occur during maturation, it is important to ensure that the samples drawn represent the population of barrels that will be blended. More samples will be more representative, but the number must be practical and not destructive. The variation between different parameters and the analysis tolerances must also be considered. The above experiment was used to determine the sample number if 2, 5, 10, 15 and 20% precision ranges are required. This implies that in the case of the 20% range, 10% of the results will be above and 10% below the mean value of the barrel lot blended. The number of barrels sampled to obtain a required precision percentage will vary, because general chemical characteristics differ less barrel to barrel than analytical parameters like polymeric pigments, polymerisation and related anthocyanins. Cellars will consequently have to decide on the required focus and sample size when samples are drawn.
Pfahl, L., Catarino, S., Fontes, N., Graca, A. & Ricardo-da Silva, J., 2023. Understanding the effect of barrel-to-barrel variation on color and phenolic composition of a red wine. Wine Business Monthly, May: 38 – 44.