Handbook of Vanilla Science and Technology

Fig. 6.3 Magnified views (400 ×) of cross sections of green vanilla beans. (A) Cross section of a developing vanilla bean. The hair cells are distinct. (B) Cross section of an older bean showing the senescing inter-placental hairs (left) and white parenchyma cells of the fruit wall (right). The hair-like cells contain enzymes in the vanillin biosynthetic pathway. These cells release abundant lipid seen as globular bodies (arrow). The parenchyma cells, comprising the white cortical portion of the frit wall, contain degradative enzymes. Reproduced with permission from Perfumer & Flavorist magazine, Allured Business Media, Carol Stream, IL.

6.3 ON-THE-VINE CURING PROCESS IN A VANILLA POD

Naturally occurring on-the-vine senescence of a vanilla pod (Figure 6.4) might be a context for viewing cellular and metabolic changes occurring during off-the-vine (commercial) curing of vanilla bean, as discussed in Section 6.4. It is commonly observed that at the end of vanilla pod development and maturation, lasting around 9 to 10 months, the vanilla pod manifests de-greening and onset of yellowing. This change, made visible by chlorophyll degradation and, subsequently, unmasking of yellow carotenoid pigments, is a universal mark for the onset of ripening in fruit, including the vanilla pod. Another pronounced feature is the subsequent onset and progressive pod browning, stemming mostly from oxidative degradation of phenolic compounds. Yellowing and browning represent different and contrasting cellular states: The former marks an end point in pod development, where cellular processes are under genetic and tight metabolic control. Browning, in contrast, marks the loss of cellular organization and metabolic control and denotes the onset of degradative processes that have escaped cellular regulation. The latter may include degradation of vital cellular biopolymers and loss of membrane-driven compartmentalization of cellular constituents (Hopkins et al. 2007; Lim et al. 2007) and, importantly, activity of cell wall degrading enzymes, for example, protein degrading enzymes and enzymes that catalyze the hydrolytic cleavage of various glycosylated compounds, notably, glucovanillin. Further work might also reveal enzyme-catalyzed degradation of lipid and membrane-lipid and probably nucleic acids. Onset of pod yellowing and subsequent browning also represent a contrast from an energy perspective. Whereas yellowing and other ripening-related processes, representing organized cellular reactions, are predicated on free energy input, pod browning, which signifies destruction of cellular organization, is an entropy-driven process, entailing energy dispersion. Destruction of cellular organization, particularly the loss of compartmentalization, is associated with unrestricted mobility and diffusion of matter between intra or inter-cellular compartments. For example, diffusion of vacuole-held constituents, such as organic acids, phenolic compounds, ions, or proteases onto the surrounding cytoplasm, as well as adjoining tissue regions, might lead to deleterious consequences. An example is cytoplasmic acidification and subsequent death, resulting from stress-induced leakage of organic acids from the cellular vacuole (Yoshida 1991,1994). Pod browning is a salient manifestation of collapsed cellular organization, resulting, in part, from unrestricted and uncontrolled diffusion of harmful metabolites, unhindered enzyme-substrate interaction, as well as accessibility to ambient atmospheric oxygen. Whereas in viable plant tissue bio-membranes function as gas diffusion barriers (Grinberg et al. 1998), bio-membrane destruction in a browning pod results in removal of membrane hindrance to oxygen diffusion and, in turn, onset of enzymatic and non-enzymatic oxidative reactions and, moreover, formation of reactive oxygen species (ROS) arising, apparently, from lipid oxidation, as discussed in Section 6.5. Vanilla pod browning, a hallmark of on-the-vine senescence as well as off-the-vine bean curing, is an expression of collapsed biological order. In a senescing vanilla pod, enzyme-catalyzed hydrolytic cleavage and oxidation of cellular constituents might provide products useful in nutrition and protection of developing seeds.

Fig. 6.4 Vanilla bean undergoing on-the-vine senescence for 12,15,18, and 20 months after pollination. Continued water loss results in curling of the senescent vanilla pod. Reproduced with permission from Havkin-Frenkel, D., French, J.C., Graft, N.M., Pak, F.E., Frenkel, C. and Joel, D.M. (2004) Interrelation of curing and botany in vanilla (Vanilla planifolia) bean. Acta Hort. (ISHS), 629, 93-102.