a Percentages of fatty acids are in relation to saponifiable fraction (not to total lipid fraction).
Fibers (hemicelluloses, cellulose, and lignin) constitute nearly 8% of the fresh weight, which is high when compared with fruits and vegetables (between 1 and 4%).
Sugars are mainly composed of sucrose and their total content is comparable with that found in many vegetables (<4%) and far less than that found in most fruits (12%).
Proteins are present in relatively small quantities, which are commonly observed in most fruits and vegetables (<1%). Among these proteins, we note the presence of exceptionally high glucosidase activity (on an average, around 1000 nkatal/g of fresh fruit) closely linked to the aromatic development of vanilla; this will be discussed in detail at the end of the chapter.
Lipids represent 2% of the fresh weight, which is high compared to many fruits and vegetables (excluding oleaginous fruits), where the content commonly observed is around 0.2–0.4%. Unsurprisingly, it is known that during the “killing” of beans, a considerable oily phase appears on the surface of water. However, a great deal of research study has been carried out on this lipid fraction (Ramaroson-Raonizafinimanana et al., 1997, 1998a, 1998b, 1999, 2000; Maestro et al., 2007).
Organic acids are mainly represented by malate and citrate (80% of all organic acids), and their total content is similar to that found in low-acid fruits and vegetables.
The main mineral elements are potassium and calcium; their respective contents (470 mg and 170 mg) are high when compared with most fruits and vegetables.
Finally, the glucovanillin content represents about 1.7% of the fresh weight of the green fruit (and sometimes more), which is really exceptional and characteristic of V. planifolia, and the reason for its commercial value. Glucovanillin is dealt with in greater detail in the final part of the chapter.
These different data on the basic composition of the green fruit were confirmed on the whole in a recent publication (Odoux and Brillouet, 2009); they nevertheless need to be supplemented by more systematic studies, which are currently absent.
Anatomy and Morphogenesis of Vanilla Bean
Morphology, Anatomy, and Histology of Mature Vanilla Bean
Part of the information presented below is largely taken from the following references: De Lanessan, 1886; Villiers et al., 1909; Swamy, 1947; Roux, 1954; Odoux et al., 2003; and French, 2005.
The mature green vanilla bean has a roughly triangular transverse section with a central cavity containing numerous black seeds (Figure 10.4). From an anatomical and histological viewpoint, from the outer to the inner part of the fruit we find the epicarp, the mesocarp, and the endocarp (Figure 10.5d).
FIGURE 10.4 Cross section of mature vanilla bean.
FIGURE 10.5 (See color insert following page 136.) From the flower to the mature bean. Cross sections (3 μm) of a vanilla bean embedded in Technovit 7100 resin, at different stages after staining with periodic acid-Schiff (PAS)–Naphthol Blue Black: (a) 9 days after pollination (dap); (b) 14 dap; (c) 60 dap; (d) 8 months after pollination. (Data from Odoux et al., 2003. Annals of Botany 92: 437–444.) The walls and the storage sugars are stained in pink; the proteins in blue. en, endocarp; ep, epicarp; fu, funicle; me, mesocarp; pl, placenta; s, seed; vb, vascular bundle.
The epicarp is made up of a layer of contiguous cells, which vary in length, are polygonal in shape, and run parallel to the long axis of the bean. These thick-walled cells, which are stained in intense pink by Schiff reagent (Figure 10.5d) due to their biochemical composition (cellulose, hemicelluloses, and pectic substances), differentiate into a thick cuticle on the outer part of the fruit. The layer of the cells that forms the epicarp provides a protective layer for the bean.
The mesocarp makes up the majority of the fruit’s volume (Figure 10.5d). It consists of parenchyma cells. It is composed of vacuolated cells that increase in size from the epicarp or endocarp toward the central part of the mesocarp, where their size may reach 300 μm. This considerable increase in size appears to be accompanied by an increase in ploidy through endoreplication (S. Brown, pers. comm.).
The mesocarp is vascularized. There are three groups of three triangularly arranged vascular bundles (Figure 10.5d). They mark the center of each carpel and, from an evolutionary viewpoint, represent the main vascular bundle of the macro-phylla. Among these triangularly arranged groups of bundles, we note the presence of three additional vascular bundles located at the center of the mesocarp, midway between the epicarp and the endocarp. The vascular bundles of the bean are of the closed collateral type, as with most monocotyledons.
Inside the mesocarp, across two of the three groups of three vascular bundles, a radial layer of specialized cells can be observed on cross histological bean sections (Figure 10.5a). The cells that make up this layer are aligned along the ray of the bean and contain carbohydrate reserves in the form of starch granules. These two layers that radiate out from the inner to the outer part of the mesocarp mark the location of the two future dehiscence lines (Figure 10.5a).