FIGURE 13.10 2D plot of four cultivars of uncured Tahitian vanilla beans analyzed in factorial discriminant analysis using fatty acids as variables (see Table 13.3).
The other cultivars have similar compositions with slight variations: Tahiti shows a greater amount of stearic acid, Tahiti long is distinguishable by its significantly higher oleic acid content, and Haapape is the oiliest among Polynesian cultivars.
The chemodiversity of Polynesian cultivars is underlined as well by aroma diversity as by fatty acids.
Conclusion
Tahitian vanilla beans have a specific aroma and fatty acid composition. The most original cultivar is Parahurahu, which contains a relatively high level of anisyl molecules and a low amount of vanillyl compounds, as well as a very high level of monounsaturated long-chain fatty acids.
The vanillas most cultivated in French Polynesia, namely Tahiti and Haapape, have homogeneous aroma and lipid characteristics. Their aroma composition and high fatty acid content explain, respectively, their subtle sensory properties and their attractive physical aspect.
Sensory Properties
The sensory properties of Tahitian vanilla are unique. They are often analyzed either by sensory evaluation or by GC-O. These two techniques respectively (i) describe the overall flavor of vanilla and (ii) determine odor-active compounds contributing to the overall vanilla flavor.
Sensory Evaluation
The sensory properties of vanilla beans are primarily due to volatile constituents, but some nonvolatile compounds such as lipids may play a role in modifying the flavor perception (Purseglove et al., 1981). Their odor impact has not yet been well elucidated, but during the curing process, they fix some volatiles and restrict their release out of vanilla pods.
In sensory analysis, Tahitian vanilla is described as showing distinctive flowery, fragrant, perfumed, anise, almond, and cherry notes. The beans are also characterized by a shallow vanilla character as well as weaker phenolic, woody, balsamic, and smoky notes than V. planifolia (Ranadive, 1994, 2006; Petitdidier, 2005).
But there is no clear relationship between these sensory properties and the quantitative analytical parameters (as described in the section “Chemical Composition of Tahitian Vanilla”). For V. planifolia, high vanillin contents, which are indicators of quality, do not necessarily imply good sensory properties. In fact, due to a high odor threshold, some major compounds quantified in vanilla beans are not directly linked to the sensory properties of the extracts since minor compounds are involved in the overall vanilla flavor (Ranadive, 2006; Gassenmeier et al., 2008). That is why a GC-O analysis was performed.
GC-O Analysis
To describe the overall vanilla flavor, recent studies took into account the olfactory impact of compounds by GC-O and not only their concentration in the pods (Scharrer, 2002; Black, 2005; Perez-Silva et al., 2006). Compounds like anisaldehyde and ani-syl alcohol account in great part for the characteristic flavor of Tahitian vanilla and are essential to the anise, sweet notes (www.flavornet.org). Some other compounds, such as aromatic anisyl esters (methyl anisate is present at 200 ppm in Tahitian vanilla), could have an olfactory impact on the overall aroma. Thus, a work on Tahitian vanilla enables the identification of 276 components. Among them, some minor specific anisyl molecules are scented and described as “anise, cherry, sweet” (Da Costa and Pantini, 2006).
Some phenolic compounds identified in Tahitian vanilla were found to be aroma-active by GC-O in V. planifolia beans (vanillin, vanillyl alcohol, p-hydroxybenzaldehyde, p-hydroxybenzyl alcohol, anisyl alcohol, and methyl cinnamate). Sweet, woody, balsamic, spicy, vanilla-like, and toasted notes were attributed to these compounds. Moreover, minor compounds present at less than 2 ppm have been found to contribute to the overall vanilla flavor, such as aldehydes, which were seen with green, oily, and herb-like floral notes, and aliphatic acids, which were described as having sour, buttery, and oily notes (Perez-Silva et al., 2006).
These studies have highlighted the specific anise flavor of Tahitian vanilla known as unique in the global market and considered as a luxury product.
Genetic Specificity
Genetic Relationship between Tahitian Vanilla and Other Cultivated Vanilla Species
In order to determine if Tahitian vanilla was genetically different from the other cultivated vanilla species, a comparative study of the genetic diversity was carried out. This project involved the Bureau des Ressources Génétiques (BRG), the Centre de Cooperation Internationale en Recherche Agronomique pour le Développement (CIRAD), the Etablissement Vanille de Tahiti, the University of Reunion Island, and the Institut de Biotechnologie des Plantes (IBP) (Andrzejewski et al., 2006; Duval et al., 2006). Cultivated species V. planifolia (47 samples), V. pompona (6 samples), and Polynesian cultivars (6 samples) from Reunion Island, Central America, and French Polynesia, and 35 related species from Thailand, Brazil, Guiana, Cameroun, and botanical gardens were analyzed. The amplification fragment length polymorphism (AFLP) pattern of each sample was compared with other samples and the data were used to calculate dissimilarities between the samples according to the Sokal and Michener index. The structure of the genetic tree represented the species delimitation with a high level of accuracy (bootstraps of 100%). This study indicated that Tahitian vanilla is genetically very distinct from other cultivated or wild vanilla species.
Tahitian vanilla was also compared with several wild or cultivated species of Vanilla from Central America in collaboration with the Riverside University (Lubinsky et al., 2008). The putative origin of the Tahitian vanilla was assessed. Patterns of DNA sequences from the nuclear internal transcribed spacer (ITS) and chloroplast genomes were compared between Polynesian cultivars and samples collected in the tropical forest of Central America: V. planifolia, V. odorata, V. insig-nis, V. pompona, and a few other species. These data indicated that the two genetically closest species to Tahitian vanilla were V. planifolia and V. odorata and thus indicated that these two species are closely related to the ancestors of the Tahitian vanilla. This study favors the hypothesis of Porteres (1951) who assumed that Tahitian vanilla resulted from hybridization between V. planifolia and another hybrid involving V. odorata.
Polynesian Genetic Diversity
At the same time, the genetic diversity among Polynesian cultivars was also analyzed. The morphological diversity observed among Tahitian vanilla, which allowed local growers to distinguish 14 cultivars, was related to genetic diversity. This genetic diversity was due to a variation at the level of the DNA sequences highlighted by the comparison of AFLP patterns between the different cultivars. AFLP patterns were defined by the presence or absence of each of the 529 tested AFLP markers. The comparison of these AFLP patterns indicated that in spite of the genetic diversity existing within Tahitian vanilla, all the accessions are gathered and form only one monophyletic group (Andrzejewski et al., 2006; Figure 13.11). Genetic diversity is weak (maximum distance between accession = 0.091) but coherent for a plant that is propagated by stem cutting and not by sexual reproduction. For some cultivars, many AFLP patterns were observed, indicating that diversity was underestimated by the growers and that new cultivars had to be defined.