Moreover, it has been shown that, after a year, the concentration of these compounds, positive for the aroma of the wine, not only do they not increase but, in some cases, they begin to decrease. This is an important finding given that the current norms lay down a minimum period of twelve months for Reserva wines and 18 months for Gran Reserva ones.
The PhD thesis, defended at the Public University of Navarre under the title, "A Study of the Volatile Composition of Red Wines Aged in Different Kinds of Oak Casks".
Ageing of wine in old casks
This was to study the influence of various factors (age of the barrel, length of ageing period, composition of wine) in the extraction of volatile compounds of oak wood during the ageing process of wine.
The study also undertook a study of the evolution of fermentation esters during this ageing process. To this end, different grape varieties were used as well as monovariety red wines.
Four experiments
For the thesis, four chemical analysis experiments were carried out on different types of wine and casks. In the first, a red wine was analysed which had been casked for 12 months in French and American oak that had been in use for 5 years. Subsequently, the evolution of a red wine aged for 18 months in 2-year-old French oak casks. The other two experiments were carried out with a Merlot and a Cabernet Sauvignon – with different alcohol graduation and pH – which had been in one-year-old American oak casks for 17-18 months. Finally, a study of synthetic wines was undertaken.
In order to investigate the influence that the age of the barrel has on the volatile compounds transferred to the aged wine, French and American oak casks which had been in use for five years were used. From the results it was observed that the old casks hardly ceded any volatile compounds to the aged wine. These compounds are positive for the wine because they are what give the wine its aroma (bouquet). The current practice is to use barrels over many years as they are very costly but we have found that 5-year-old casks hardly provide any aromatic compounds.
From the data analysis it can also be concluded that, although all barrels ceded low amounts of volatile compounds, French oak, in general, provides a greater quantity than American. The wines aged in the latter show high concentrations of ethylphenols, substances undesirable for the aroma of the wine.
As regards the influence of the period of time of the ageing of the wine on the accumulation of volatile compounds, the optimum time for wine in the cask was investigated, drawing the conclusion that the maximum concentrations of compounds transferred to wine from wood is reached after 10 to 12 months of the wine being stored in barrels. Of the esters studied, only ethyl lactate increased its concentration during ageing.
It is significant that currently, the wine bodega business apply the norms of six months minimum for crianzas, one year for reservas and 18 months minimum in the wood for a Gran Reserva. These are the norms laid down by legislation but are not based on scientific study. In our investigation, precisely, we observed that, after 12 months in the barrel, from the perspective of the bouquet, wines do not change or the compounds which afford the aroma to the wine begin to decrease.
Finally, it was observed that alcohol content affects the accumulation of volatile compounds in wine more than pH does. That is, wine with a higher alcohol content extracts a greater quantity of volatile compounds from the wood than wine with a lower alcohol content. Nevertheless, the content is inversely related to the accumulation of ethylphenols. Esters evolve in a similar manner in both wines during the ageing process.
It should be taken into account that the alcoholic content depends on the quality of the grape and so is a variable that the wine itself has been provided with. But what can be done, in order to extract the volatile compounds from the wood, is to have the wine in the barrel for less time.