The presence of piperonal (heliotropin) as a characteristic constituent of Tahitian vanilla has been a matter of debate for a long time. Definitively Joulain et al. (2007) showed that the supposed characteristic odorant constituent of Tahitian vanilla was p-anisaldehyde and that piperonal was present in Tahitian vanilla but only as a trace element as well as in V. planifolia, confirming previous works (Tabacchi et al., 1978; Ehlers et al., 1994; Lechat-Vahirua and Bessiere, 1998).
At the laboratory of the “Etablissement Vanille de Tahiti,” the high flavored, original and pervasive flavor of the Tahitian vanilla was highlighted by the high-pressure liquid chromatography (HPLC) analysis of the major aroma phenolic compounds. The work focused on the role of the curing process in the development of a unique flavor, and on the aroma composition of some Polynesian cultivars.
More than 300 samples of Tahitian cured vanilla beans were analyzed from three successive annual harvests (2005, 2006, and 2007). Cured beans of Tahitian vanilla are rich in overall aroma compounds with an average of 4.6% of dry matter compared to 2.1–4.2% for V. planifolia. Moreover, the aroma composition of Tahitian vanilla is more different, with a smaller contribution of vanillin to the overall flavor (30% v. 80% for V. planifolia). Whereas its vanillin level is of less influence (1.2% as it reaches 1.7–3.5% for V. planifolia), Tahitian vanilla contains higher amounts of ani-syl molecules (2.1% and less than 0.05% for V. planifolia) and p-hydroxybenzalde-hyde. The most concentrated anisyl molecules are anisyl alcohol and anisic acid (respectively, 1.35% and 0.75% of dry matter); p-anisaldehyde and methyl anisate represent about 200 ppm each, whereas they are only at trace levels in V. planifolia.
Many factors influence the flavor of Tahitian vanilla, such as genetic, agronomic, climatic, and transformation criteria. In fact, the highly valued flavor of Tahitian vanilla becomes more and more intense during the curing process. Nevertheless, if vanilla pods undergo a water loss when exposed to the sun, some aroma compounds are also evaporated and the batches of pods generate a pleasant vanilla flavor. Research on vanilla aroma in French Polynesia focuses on phenolic compounds that contribute to the overall flavor. Fourteen “aroma compounds” from an ethanolic Soxhlet extract were assessed by HPLC during the curing process (see Table 13.1).
In relation to the drying process, there is a subsequent loss of aromatic molecules. When beans are dried to 38% moisture content, they lose approximately one-third of their aroma potential, from 6.5% to 4.6% of dry weight. However, given that the loss of water is more important than the loss of aroma compounds, their concentration per gram of fresh weight actually increases during the curing period.
Differences in volatility between aroma compounds and oxidation reactions occurring in the beans (from alcohol to aldehyde and then to acid function) involve diverse changes of molecule concentrations (see Figure 13.7). The highly concentrated vanillin and anisyl alcohol undergo a significant decrease, whereas vanillic acid and anisaldehyde concentrations increase because of their production due to the oxidation of previous major compounds (respectively, vanillin and anisyl alcohol).
FIGURE 13.7 Evolution of major aroma compounds of Tahitian vanilla during the curing process. Variation in content of the compounds is indicated as a percentage of initial content.
The moisture content has an influence on the flavor composition: drying favors accumulation of acid molecules (anisic acid and p-hydroxybenzoic acid) and decrease of the ratio of anisyl alcohol and vanillin (Figure 13.8). The odor-active compound p-anisaldehyde is a key component of the original Tahitian vanilla flavor, as its concentration doubles during the curing process. Since moisture content produces some effect on aroma quality, it is very important to provide details of moisture content when describing vanilla beans composition (Collard, 2007).
The curing process of Tahitian vanilla is a subtle compromise between optimal drying and conservation of aroma compounds. This results in an original aroma composition.
FIGURE 13.8 Aroma composition of Tahitian vanilla pods for different moisture contents (80%, 60%, and 40%). See Table 13.1 for complete names of molecules.
Tahitian vanilla develops an exotic flavor compared to other vanillas. There is an intra-tahitensis diversity with various cultivars and nuanced flavors.
In Tahitian vanilla, there is a morphological diversity, which is confirmed by different chemical compositions. A study was carried out based on aroma quality of uncured beans of five cultivars of importance in French Polynesia (the most cultivated Tahiti and Haapape, Rea rea, Parahurahu, and Tahiti long).
Each cultivar develops its own aroma characteristics. Table 13.2 shows that the overall aroma amount differs from one cultivar to another and that some molecules are discriminating as they contribute to structure groups by statistical analysis. Performing a factorial discriminant analysis shows very good correspondence between the observed classification obtained with aroma variables and naturally occurring cultivars (see Figure 13.9) (Collard et al., 2006).
FIGURE 13.9 2D plot of fi ve cultivars of uncured Tahitian vanilla beans in factorial discriminant analysis using aroma compounds (see Table 13.2) as variables.
Among Polynesian cultivars, Parahurahu is very specific with a significantly lower overall amount, a lower amount of vanillyl molecules and a higher level of anisic acid. The cultivar Rea rea demonstrates a remarkably lower amount of p-hydroxybenzaldehyde and a higher amount of vanillyl compounds, and it differs from the group formed by Tahiti, Haapape, and Tahiti long, all of which show quite similar aroma compositions. Beans of Tahiti long are characterized by a high amount of p-hydroxybenzaldehyde. Tahiti and Haapape are the cultivars that have the most similar aroma compositions, although Tahiti is richer in overall aroma and vanillyl compounds, and Haapape contains relatively more p-hydroxybenzyl and anisyl components.
| Molecule* | Parahurahu | Rea rea | Tahiti long | Haapape | Tahiti |
|---|---|---|---|---|---|
| No. of samples | 18 | 13 | 15 | 23 | 110 |
| Van_alc | 243a | 909e | 407b | 514c | 614d |
| Van | 806a | 17,718c | 11,732b | 13,337b | 20,184d |
| Van_ac | 87a | 316e | 177b | 197c | 227d |
| Phb | 2504c | 352a | 4221d | 2621c | 2081b |
| Phb_ac | 7101c | 4932a | 7275c | 7202c | 6007b |
| Anis_alc | 7156a | 13,108b | 16,397c | 20,158d | 21,117d |
| Anis_ald | 46a | 65b | 71b | 82b | 65b |
| Anis_ac | 16,026c | 4896a | 8103b | 8736b | 7907b |
| Pro_ald | 1101a | 1421b | 2585c | 2758c | 2628c |
| Pro_ac | 220b | 216b | 219b | 188ab | 169a |
| Σ Van | 1137a | 18,943c | 12,316b | 14,048b | 21,025c |
| Σ Phb | 9605c | 5284a | 11,496d | 9823c | 8088b |
| Σ Anis | 23,228b | 18,069a | 24,571b | 28,976c | 29,088c |
| TOTAL | 35,290a | 43,933b | 51,186c | 55,793d | 60,998e |