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44 WINES&VINES April 2017 discrimination of oak species, according to their individual con- centration in wood. Differentiation according to a triterpenoids index Beyond absolute concentrations in individual triterpenoids, it seemed that samples could be grouped in two categories ac- cording to their relative amounts of QTTs and Glu-BA. To express this relative composition, a trit- erpenoids index (TI) was calcu- lated as base 10 logarithm of the ratio between the sum of con- centrations in QTTs and the con- centration in Glu-BA (all given in µg/g). [QTT I] + [QTT II] + [QTT III] [Glu - BA] TI = log The average values of this index were calculated for sessile and pedunculate oak wood sam- ples (see "Mean Logarithm of the Ratio Between ∑QTT and Glu-BA" and "Mean, Minimal and Maxi- mum Values of Observed in Sessile and Pedunculate Oak"). Mean TI was positive for ses- sile samples and negative for pe- dunculate samples (1.9 and -1.5 respectively). The application of one-way analysis of variance test revealed that these differences were statistically significant (p- value less than 0.1%). More in- teresting, confidence intervals were smaller than for absolute concentrations in triterpenes ex- pressing a narrower range of val- ues. All samples of sessile oak had positive TI values (1.2 - 2.4), whereas all samples of peduncu- late oak exhibited negative TI values (-2.2 to -0.8). Contrary to absolute concen- trations in individual triterpe- noids, there was a huge gap (two log points) between the closest values of both species, i.e., be- tween the lowest value of sessile oak (1.2) and the highest value of pedunculate oak (-0.8). Consequently, the calculation of a TI reflecting the relative composi- tion in triterpenoids of wood ap- peared to avoid any ambiguity in the assignment of the botanical species. In practice, a positive TI value might indicate that the sam- ple was from sessile oak, whereas a negative TI value might corre- spond to a pedunculate oak sam- ple. These results highlighted probable differences between spe- cies in terms of triterpenoids bio- synthesis, with variations of the enzymes involved in the decoration of the non-sugar parts of mole- cules. Such differences already had been observed and related to the great diversity of triterpenoids ob- served in the plant kingdom. 26, 27 Identification of natural sweet compounds This research confirmed empirical k n o w l e d g e a b o u t i m p a r t i n g sweetness with barrel élevage. The application of centrifugal partition chromatography cou- pled with gustotemetry allowed the identification of a class of BARRELS PRACTICAL WINERY & VINEYARD 2.5 2 1.5 1 .5 0 -.5 -1 -1.5 -2 -2.5 MEAN LOGARITHM OF THE RATIO BETWEEN ∑QTT AND GLU-BA. Error bars indicate 95% confidence intervals. *** significant p<0.001. Log (∑[QTT]/[Glu-BA]) *** n Sessile n Pedunculate