Handbook of Vanilla Science and Technology

Naturally occurring pod senescence and consequent browning may also be instigated by applied ethylene, as observed by Arana (1944). Our own work revealed that ethylene-induced browning is intensified when the gas is applied in oxygen. This is in accordance with the view that enhanced oxygen accessibility is stimulatory to the oxygen-dependent browning reaction (Figure 6.5). The treatment also stimulated bean-end splitting. Pod browning, signifying loss of cellular and tissue organizational integrity, instigated by on-the-vine senescence or by applied ethylene, is emulated by the off-the-vine curing process. It is worth noting that the Tahitian curing method (described in Section 6.9) is based on allowing on-the-vine pod browning and completion of the process without artificial killing after the bean has been harvested. 

Fig. 6.5 Mexican vanilla beans were harvested 7 months after pollination and sorted by background color (green and yellowing). Green or yellowing beans were held in 20-liter glass jars and ventilated at a rate of 200 ml/minute with different gas mixtures consisting of air, air + ethylene, oxygen, and oxygen + ethylene. The concentration of applied ethylene was 10pl/liter gas. Reproduced with permission from Perfumer & Flavorist magazine, Allured Business Media, Carol Stream, IL.

6.4 OFF-THE-VINE CURING PROCESS OF VANILLA BEANS

The commercial curing process, that is, off-the-vine induced destruction of cellular and tissue organization, creates conditions allowing the free flow and interfacing of previously compartmentalized cellular constituents, resulting in enzyme-substrate interactions as well as unrestricted access to atmospheric oxygen. These conditions launch the onset of hydrolytic and oxidative reactions that contribute to the formation of the prized vanilla flavor.

Cellular and tissue de-compartmentalization is obviously fundamental for accessing precursor metabolites for enzyme-catalyzed generation of flavor and aroma constituents. In whole green beans, phenolic compounds are restricted to the pod interior, as evidenced by catechin staining. In killed beans, the phenolic compounds have diffused from the inner portion of the pod and have populated the entire tissue, including the outer wall region (results not shown). Catechin staining indicated, moreover, that killing by freezing was more thorough and uniform than killing by dipping beans in hot water at 65°C for 3 minutes (Havkin-Frenkel and Kourteva 2002). One important consequence of killing and attendant de-compartmentalization is free diffusion of glucovanillin, the vanillin precursor, from the bean interior to other regions in the vanilla pod. Unrestricted mobility of the compound creates conditions for contact with and hydrolytic cleavage of the compound by β-glucosi-dase-catalyzed action, observed mostly in the outer pericarp tissue (Arana 1943; Ranadive et al. 1983; Havkin-Frenkel and Kourteva 2002; Dignum et al. 2001a), and perhaps also with β-glucosidase found in the seeds (Jiang et al. 2000). Odoux et al. (2006) observed, in the same vein, that glucovanillin hydrolysis was low in green beans, even though β-glucosidase activity was substantial, whereas in curing beans glucovanillin formation was robust although the enzyme activity was barely measurable. From these results they concluded that a state of de-compartmentalization defined the conditions for onset of glucovanillin hydrolysis. It might be argued by extension, that these very conditions underscore the requirement for contact between other glycosyl hydrolases and their respective precursor substrates that might contribute to the production of flavor and aroma constituents in the curing vanilla pod.

It has been suggested that curing-associated flavor formation might stem also from some synthetic activity and not merely from degradative processes. There is no hard evidence to support or refute this concept. We argue, however, that the probability of synthetic events occurring in cells and tissues experiencing organizational collapse is questionable because:

• Biosynthesis is often dependent on precise structural assembly of enzymes and proteins catalyzing biosynthetic pathways, mitochondrial ATP producing machinery for example (Lenaz and Genova 2009). These conditions are not expected to prevail in killed vanilla pods.

• Biosynthesis is generally dependent on free energy input and may not proceed in the killed pod, where metabolic machinery for energy production (ATP or reduced pyridine nucleotides) is not expected to survive.

• A strong proteolytic activity, unleashed by killing, can readily disrupt the molecular and structural integrity of enzymes required for biosynthetic processes in intact cells.