Effect of size, seasoning and toasting in the volatile compounds in toasted oak wood and in a red wine treated with them
Introduction
The wood process during barrel-making in cooperage includes a series of stages that influence its oenological quality, specially seasoning and toasting. Both processes affect the structure and final chemical composition of the wood that is going to be in contact with the wine during its aging. Green wood cannot be used for cooperage since it contains a great quantity of water (up to 70%) and its extractable compounds are not compatible with the objective of improving the quality of the wines, since it contains many bitter polyphenolic compounds (ellagitannins excess, cumarins, etc.) and very few aromatic compounds.
Wood seasoning in cooperage usually happens under natural conditions in the open air during a variable time period between 18 and 36 months. Seasoning allows reducing the high percentage of humidity in wood until it is in balance with the ambient humidity, and simultaneously the fibber contraction. Natural seasoning also produces the wood maturation, decreasing bitterness and astringency, and increasing aromatic properties by means of changes in its chemical composition. It mainly results in a loss of hydrosoluble polyphenolic substances, such as ellagitannins, that could be due to various physical and chemical mechanisms, including stave leaching by rain [1] and hydrolytic oxidative degradation [2], [3], [4], [5], [6], along with fungal enzymatic activity (phenol heterosidase, etherase, and depsidase) [7], [8]. In addition, natural seasoning has an effect on the aromatic profile of wood. During this process, the wood volatile compounds such as lactones, phenolic aldehydes or volatile phenols, show different behaviours; some of them increase their concentrations while others decrease or show no significant variations. All together, however, natural seasoning has predominantly a positive effect on volatile composition and sensorial characteristics of oak wood [2], [9], [10], [11], [12], [13]. Artificial seasoning in a kiln or a mixed method that combines open air and kiln drying result in a different evolution of chemical composition of wood, and nowadays natural seasoning in open air can be considered a superior method of seasoning in cooperage [14].
The same reflection should be valid for the oak wood dedicated to the production of alternative products (chips, staves, cubes, etc.) of good quality, respecting on one hand a time of enough drying (more than twelve months), and a stacked of the wood that optimizes the exchanges between wood and atmosphere. However, the long immobilization times of the wood in the cooperage seasoning park and the expenses derived from it, have aroused interest in other unconventional seasoning methods, especially if we take into account the different size of the oak wood alternative products, that allow use other drying systems. Moreover, since the toasting in cooperage has the highest influence in the volatile composition of oak wood [12], [15], [16], [17], [18], it could think about a seasoning method whose priority is to produce an appropriate decrease of bitter polyphenolic substances, as ellagitannins and cumarins, together with reducing the percentage of humidity.
The increasing demand for wood for barrel-making in addition to the rapid extension of alternative aging system, have led to looking into the possibility of utilizing Spanish oak. Quercus pyrenaica is the species that predominates in Spain, and the chemical composition of its heartwood (ellagitannins, low molecular weight phenolic and volatile compounds) and its incidence in characteristics of wine are similar to that of other species that are of recognized oenological quality for barrel-making, showing only quantitative differences with respect to French (Quercus petraea) and American (Quercus alba) species [19], [20], [21], [22], [23], [24], [25], [26]. However, at the moment, the quantity of good quality wood that we can obtain from the Q. pyrenaica Spanish forest is limited. Hence, in the short term, and considering the high chemical oenological quality of Q. pyrenaica wood, we propose the utilizing of chips, segments, staves, and other oak alternatives for wine aging, which would be obtained from wooden remnants from barrel-making as well as from trees with small diameters or physical defects which would normally be inappropriate for cooperage. With regards to the latter idea, studies on special chip-making processes, and other oak wood pieces are being carried out. We present in this work the effect of seasoning way (traditional or unconventional) on volatile composition of Q. pyrenaica chips and staves, at three toasting levels (light, medium and heavy), and the evolution of the wood-released aromatic composition of a Spanish artificially aged wine, using these alternative oak products.
Section snippets
Wood samples
Staves (100 cm × 8 cm × 1 cm) and chips (1 cm × 0.5 cm, approximately) used in this work comes from Q. pyrenaica trees grown in Salamanca (Spain), felled in 2006 and 2007. Staves and chips were subjected to two different seasoning conditions: (i) natural seasoning in cooperage climatic conditions for a two years period in Tonelería Intona, SA, located in Navarra (Spain) (average annual temperature, 14.4 °C; total precipitation, 504 mm3/year; from 2006 to 2007), watering daily during the first 2 months and
Volatile compounds in staves and chips of Q. pyrenaica
A great variety of volatile compounds able to give a higher aromatic complexity to the wine has been identified in toasted oak wood in recent years (notes of vanilla, clove, cocoa, toasty, leather, spicy, toasted bread, etc.). We analyzed some of the most representative of this contribution of wood to wine during the aging process. They belong to very different chemical families such as the volatile phenols, lactones, furanic compounds, pyranones, phenolic aldehydes, phenolic ketones and
Conclusions
Taking into account the joint overall results we can deduce that the process of unconventional seasoning applied in this study produces alternative oak products similar to those obtained in the traditional seasoning. The influence of the wood piece size and the toasting intensity on the volatile composition of AOP was higher than the influence of the different seasoning way.
In general, the differences found among AOP were reflected in the wines treated with them, and they confirm us the
Acknowledgements
This study was financed by Spanish Government (Project RTA-FEDER06-00078, Project JCyL VA030A06). The authors wish to thank Mr. Antonio Sánchez for his help throughout the chemical analysis; and Tonelería Intona S.A. and CESEFOR for its collaboration.
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