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April 2017 WINES&VINES 39 PRACTICAL WINERY & VINEYARD BARRELS Much research has focused on ellagitan- nins, a specific class of wood tannins. The sensory properties (bitterness and astrin- gency) of isolated ellagitannins have been investigated, and recent research has de- scribed the determination of their perception threshold, thanks to a half-tongue test. 4-6 Products of interaction between ellagitan- nins and grape flavonoids also were identi- fied in red wine; they could be involved in the color change of wine during élevage. 7 Beyond ellagitannins, other non-volatile compounds are released from oak such as coumarins 8 and lignans. 9-11 However, little research data has referred to the sensory properties of non-volatile compounds and more particularly to sweet components of the taste. Various parameters such as the origin of oak or cooperage techniques can influence the composition of wood and subsequently its effect on wine taste. Some coopers classify the qualities of wood according to ring width (also called grain) or geographic origin (for- ests). However, previous studies have dem- onstrated that species is a better indicator of chemical composition than morphological parameters or provenance. 12,13 In practice, two main species of European oak are used for cooperage and occur together in most French forests: Quercus petraea Liebl (sessile oak) and Quercus robur L. (peduncu- late oak). As reported by various authors, the average levels in oak-lactone and ellagitan- nins are respectively higher and lower in sessile oak than in pedunculate oak. 13-16 But high inter-individual variations are observed within each species and deeply affect the significance of the species effect. 17-19 For example, A. Prida et al. showed that some sessile oaks have low levels of oak- lactone in the range of those assayed in pe- dunculate oaks. 18 A similar situation was described for ellagitannins. 13,20 The odorant and taste-active compounds seem not spe- cific to either sessile or pedunculate oak, and the quantification of oak lactone or ellagitan- nins in wood samples does not allow dis- crimination between the oak species. In this research, the authors present data related to the identification of a new class of sweet wood compounds and distribution of these compounds in Quercus robur and Quer- cus petraea. This data enabled the establish- ment of a method to accurately discriminate between the aforementioned species. Identification of sweet triterpenosides occurrence in wine The novel methodology of taste-active com- pound identification was developed and implemented in this research. The method consists first in off-line centrifugal partition chromatography allowing the researchers to obtain 15 different wood fractions, according to their affinity to different solvents used in the study (n-heptane/ethyl acetate/metha- nol/water). The collected fractions were freeze-dried and tasted in order to find ones character- ized by sweet taste. The most prominent ones were analyzed and then purified using preparative HPLC. Finally, purified com- pounds were analyzed using Fourier trans- form mass spectrometry (FT-MS, Orbitrap analyzer) jointly with two-dimensional nu- clear magnetic resonance (2D 1 H and 13 C NMR). This technique allowed obtaining the structural elucidation of the purified com- pounds. The tandem mass spectrometry (MS/MS) spectra obtained with resonant and non-resonant fragmentation modes were compared, thus providing complementary information about the molecular structure. Two oleanane-type triterpenoids substi- tuted with galloyl and glucosyl moieties were identified for the first time, one of which exhibits sweet properties. These compounds, which have never been reported, were named Quercotriterpenoside (QTT) I and II. 21 145 Jordan Street, San Rafael, CA 94901 415-457-3955 • www.boswellcompany.com he Premier Silicone Rubber Bung T