Applying kinetic models to the study of the influence of wood contact surface area/volume ratio on the ageing of Brandy de Jerez

The present study investigates the influence of the casks’ wood surface/volume ratio on the extraction processes that take place during the first stages of the production of Sherry Brandy. For this purpose, the ageing of a spirit at 60% alcoholic strength has been examined in casks of two different volumes: 500 L and 250 L, made from three different types of wood: Quercus alba (American oak), Quercus robur (French limousin oak) and Quercus petraea (Spanish oak), either previously seasoned or non-seasoned with Sherry wine.

TPI, the sum of all the phenolic compounds analyzed (phenol extraction coefficient) and the variation in color compared to the initial distillate (CIEDE 2000) were evaluated in all the aged distillates. The casks with a higher surface/volume ratio produced brandies that generally presented higher values for all these parameters, with different increments depending on the type of oak wood. The seasoning of the casks reduced the contribution of phenolic compounds from the wood to the ageing spirit and resulted in brandies with a lighter color when aged in French and Spanish oak casks, but not when the American oak casks were used.

Four kinetic models have been applied to better understand the evolution of these parameters during the ageing stage: Lagergren, Peleg, Intraparticle and parabolic diffusion. Peleg's model was the best fit in all the cases, which suggests that 2 types of mechanisms are involved in ageing: a rapid and a slow one. Regarding the diffusion models, the results were better fitted to the parabolic model, which suggests that during the ageing of the spirit a number of parallel reactions take place that affect the values actually reached by the parameters considered in our study. It was also observed that higher wood surface/volume ratios resulted in higher values of phenolic compounds in the distillates, but did not represent a significant factor in relation to the color of the final distillates.

The sensory analysis of the samples aged for 16 months demonstrated that only the wine spirits aged in French oak wood presented significant differences depending on the size of the cask; however, all the brandies were registered significant differences associated to the seasoning of the casks.