Handbook of Vanilla Science and Technology

The mature-green pod (fruit) of the vanilla species used in commerce, including Vanilla planifolia and Vanilla tahitensis, is subjected to a curing process as a means for developing the prized vanilla flavor. V. planifolia, a climbing orchid, is indigenous to Mexico and neighboring Mesoamerica regions. Recently, V. tahitensis has been shown to be a hybrid between V. planifolia (maternal parent) and V. odorata (paternal parent) (Lubinsky et al. 2008). The plants are cultivated commercially, mostly in various tropical regions, requiring 3 to 4 years to set the flower, and flowers once a year. The pod-like fruit, also termed the “vanilla bean”, is allowed to develop for 8 to 10 months and is then harvested, usually at the mature-green stage, followed by a curing process as discussed in this chapter. Annual worldwide production of cured vanilla beans is around 2,000 tons (US Department of Commerce).

Vanilla, first introduced and cultivated in Europe in 1520 by the Spanish Conquistador Hernan Cortes would not set fruit, because fruit set in its native habitat is dependent on vanilla flower pollination by the Melipona bee, a local insect (Childers etal.1959). The discovery by Edmond Albius that a vanilla flower could be hand-pollinated, around 300 years later, created an opportunity for the commercial cultivation of vanilla in alternative global regions, mostly around equatorial zones, such as Madagascar, Indonesia, Papua New Guinea (PNG), and India. On-the-vine growth and development of the vanilla pod is manifested by a rapid increase in tissue mass, followed by a stage of maturation required for the formation of precursor compounds that give rise to aroma and flavor constituents during on-the-vine senescence or by the off-the-vine curing process.

In commerce, cultivated vanilla beans are harvested when green and flavorless. To bring out the prized vanilla flavor, green beans are subjected to a curing process commonly lasting three to six months, depending on various curing protocols in different localities. The objective of the curing process is two-fold: i) Development of the vanilla flavor and ii) Creation of shelf-life for cured beans by drying. Cured dry beans can be stored, distributed, and used subsequently for an ethanolic-water extraction that renders the familiar vanilla extract, as well as usage in other vanilla products. Vanilla cultivation, biosynthesis of flavor constituents, and economic aspects are discussed extensively in other reviews (Ranadive 1994; Dignum et al. 2001a; Havkin-Frenkel and Dorn 1997; Rao and Ravishankar 2000). In this chapter, we focus on the curing process and outline various aspects that might influence the flavor quality of cured vanilla beans, including botany of the vanilla bean, the nature and purpose of the curing process, as well as effects of curing practices in various production regions (see Addendum).

6.2 BOTANY OF THE VANILLA POD

6.2.1 Two fruit regions

The syncarpous fruit of Vanilla planifolia, that is, the fruit with fused ovarian carpels, develops from an inferior ovary that eventually splits open along three lines at maturity, thus forming a capsule. Apparently, two principal parts in the vanilla pod are important for flavor development in the course of the curing process (Figure 6.1):

I the fruit wall, containing a “green” region including the epidermis, ground and vascular tissues of the fruit wall, surrounding the cortex with lesser chlorophyll content and whitish appearance; and II the “white” inner region composed of the three parietal placentae (not including seeds), and the three bands of glandular hair-like cells between them. The glandular hair cells might play a role in the biosynthesis of glucovanillin. 

Fig. 6.1 Cross-section (× 20) of freshly cut green vanilla bean. The figure shows the outer wall composed of a green wall region and the cortex, with lesser chlorophyll density. Also shown is the inner pod portion composed of placental tissue, haircells,and seeds (dark bodies). Reproduced with permission from Havkin-Frenkel, D., French,J.C., Graft,N.M., Pak, F.E., Frenkel, C.and Joel, D.M. (2004) Interrelation ofcuring and botany in vanilla (Vanilla planifolia) bean. Acta Hort. (ISHS), 629, 93-102.

6.2.2 Fruit components

The fruit wall containing the outer “green” region and the cortical region, with lesser chlorophyll content, comprise about 60 and 20% of the fruit weight, respectively. The inner “white” region, containing the placenta, hair cells, and the seed components comprise the balance. However, this weight ratio of the outer and inner portions appears to change during early and advanced stages of on-the-vine pod development, as cel size changes with pod development (Figure 6.2).