The members of the subfamily Vanilloideae, however, have certain characteristics that are unusual in the orchid family (Cameron and Chase, 2000), including those relating to dehiscence. In the case of the genus Vanilla, the capsule has only two dehiscence splits, corresponding to the central axis of two of the three fused carpels. At maturity, the capsule opens along the two dehiscence splits (for Vanilla planifolia and Vanilla pompona), thereby showing two valves (Figure 10.1b); this is dorsal dehiscence. Some species may even be indehiscent (for Vanilla tahitensis).
The vanilla capsule is unilocular (a single central cavity in the ovary that contains all the seeds). Later in the chapter we no longer use the botanical word “capsule,” but the more commonly used “bean” or “pod.”
For V. planifolia G. Jackson (which, unless otherwise indicated, is the only species referred to in the rest of the chapter), the flowers of the vanilla plant are grouped in inflorescences (Figure 10.2) that resemble a cluster, as the inferior ovary simulates a floral pedicel that is absent.
FIGURE 10.2 V. planifolia inflorescence. Before pollination, ovaries face upward with their flower, and then curve downward.
After pollination, the ovary develops very rapidly, doubling in length in a few days, and curves downward (Figure 10.2). Fertilization occurs 1.5–2 months later (Childers and Cibes, 1948; Roux, 1954). Each fertilized ovary produces a bean. The bean reaches its full size and weight between 10 and 15 weeks after pollination (Gregory et al., 1967; Brodélius, 1994; Havkin-Frenkel et al., 1999) (Figure 10.3). As the bean matures, the moisture content decreases from around 90–92% to 82–85% toward the harvesting time (Shankaracharya and Natarajan, 1973; Brodélius, 1994; Ranadive, 1994), in other words, around 8–9 months after pollination (Sreekrishna Bhat and Sudharshan, 2002).
FIGURE 10.3 Immature vanilla beans.
Mature beans are long, green, and curved at the end of the peduncle. They are around 15 cm long and may reach a width up to 15 mm, and weigh around 10–15 g. These figures are only orders of magnitude and may vary considerably depending on genetic factors, physiology of the plant, agronomic or environmental conditions, and so on.
After this period of maturation, whether harvested or not, the fruit becomes pale green in color and begins to turn yellow from its floral tip; the dehiscent fruits (the proportion varies according to the species, and also within the same species) split into two from the floral tip. The beans then darken, some very markedly, again from the floral tip. They then lose their initial turgidity and become completely flexible, marking the senescence of the fruit.
It appears that the fruit releases ethylene (Ducamp et al., 2000); however, some authors have not been able to measure this during the maturation of the bean (Havkin-Frenkel et al., 2005). However, the fruit’s sensitivity to ethylene has been clearly observed during different scientific works; in particular, ethylene increases the rate of dehiscence in beans (Balls and Arana, 1941; Arana, 1944; Havkin-Frenkel et al., 2005).
It would be worthwhile conducting some in-depth research to determine whether or not the fruit is climacteric and, if so, at what stage of its development the respiratory climacteric takes place. These data would be helpful in controlling the harvesting stage for beans and their conservation before curing in a better way.
There are very little data on the composition of mature green beans. Table 10.1 provides values obtained by Garros-Patin and Hahn (1954) for beans from Madagascar that were analyzed in 1950.
| Fresh Weight % | Dry Weight % | |||
|---|---|---|---|---|
| A | B | A | B | |
| Water 79.6 75 | ||||
| Ash | 0.75 | 0.97 | 3.68 | 3.88 |
| Cellulose | 0.79 | 2.65 | 3.87 | 10.60 |
| Reducing sugars | 1.42 | 1.01 | 6.96 | 4.04 |
| Nonreducing sugars | 3.03 | 2.45 | 14.85 | 9.72 |
| Nonnitrogenous substances | 10.85 | 14.41 | 53.19 | 57.7 |
| Ether extract | 1.58 | 2.14 | 7.74 | 8.56 |
| Proteins | 1.75 | 1.37 | 8.58 | 5.50 |
| Acidity | 0.23 | 1.13 | ||
Source: Data from Garros-Patin, J. and Hahn, J. 1954. In: G. Bouriquet, ed. Le vanillier et la vanille dans le monde. Paul Lechevalier, Paris, 559–615.
A and B are two different samples of mature green beans from Madagascar.
The results of the analyses conducted by CIRAD on mature green beans (usual harvesting time) from Madagascar are presented in Table 10.2. These results are not intended to be representative of the average composition of mature green beans from the vanilla plant, but should be taken only as indications. They are, moreover, relatively consistent with the results obtained by Garros-Patin and Hahn.
| (g/100 g FW) | (g/100 g DW) | Percentage of Each Compound in Relation to Total | |||
|---|---|---|---|---|---|
| Water | 83.0 | 0 | |||
| Fibers | 7.6 | 45 | Lignin 62 | Cellulose 27 | Hemicelluloses 11 |
| Sugars | 1.7 | 10 | Sucrose 80 | Glucose 15 | Fructose 5 |
| Lipidsa | 2.0 | 12 | C18:2 54 | C18:1ω9 10 | C16 10 |
| Proteins | 0.5 | 3 | |||
| Organic acids | 0.9 | 5 | Citric acid 50 | Malic acid 30 | |
| Mineral elements | 1.7 | 10 | K 28 | Ca 10 | Mg 2 |
| Glucovanillin | 1.7 | 10 | |||