Handbook of Vanilla Science and Technology

It has also been suggested that micro-organisms might contribute to an overall vanilla flavor (Ranadive 1994) and perhaps also to vanillin formation, because various microorganisms colonizing the vanilla pod during the curing process (Roling et al. 2001) manifest glycosidase activity and efficacy to convert ferulic acid to vanillic acid and related compounds (General et al. 2009). Additional studies might reveal whether hydrolytic release of vanillin is catalyzed exclusively by the bean endogenous enzyme or, alternatively, might also originate from colonizing micro-organisms.

6.4.1 Purpose of curing

Following pollination and subsequent fruit set, the developing vanilla pod undergoes rapid growth for 3 months followed by growth cessation. Next, the fully grown vanilla pod enters a period of maturation, lasting several months. During on-the-vine bean development, lasting 8 to 10 months, flavor precursors accumulate, mostly in the placental tissue surrounding the seeds in the inner core of the bean (Figure 6.6). Pre-mature harvesting of the pod, even at full size, results in formation of poor flavor upon subsequent curing.

Fig. 6.6 Time-course of change in the content of various metabolites in the green outer tissue (solid lines) and the innerwhite tissue (dashed lines) of a vanilla beanduring pod development on the vine. Beans were harvested green at various stages of development. The various metabolites, present as glucosides, were hydrolyzed and the resulting aglycons determined, as described previously (Podstolski et al. 2002). Reproduced with permission from Perfumer & Flavorist magazine, Allured Business Media, Carol Stream, IL.

However, when mature-green vanilla beans are harvested, they lack flavor. This phenomenon might stem from spatial separation of flavor precursors and corresponding enzymes that catalyze their breakdown to final flavor components. For example, glucovanillin, a vanillin precursor and β-glucosidase, which catalyzes the hydrolytic release of vanillin from glucovanillin, are apparently sequestered in different tissue regions in the vanilla pod. Thus, while glucovanillin is found mostly in the inner portion of the pod, estimation of β-glucosidase activity indicated that the enzyme reaction rate was roughly 10-fold higher in the outer fruit wall than in the inner pod region, including the placental tissue and the hair cells (see Section 6.5.3). This was also confirmed by histochemical staining of a cross-section of vanilla pod for the enzyme activity (results not shown). These data, indicating that β-glucosidase is localized mostly in the pod outer region, suggest that in intact tissues of green beans, the enzyme is spatially separated from glucovanillin and likewise, other glycosyl hydrolases might also be separated from their flavor precursors. The purpose of the curing process, then, is to create conditions for substrate-enzyme interaction and, thereby, onset of enzyme-catalyzed formation of vanillin or other flavor constituents, as well as onset of enzymatic and non-enzymatic oxidative reactions, by allowing contact with atmospheric oxygen. An additional objective is the drying of cured beans, as a preservation method for retaining the formed flavor compounds. Vanilla flavor contains around 250 identified constituents (Adedeji et al. 1993), chief among them is vanillin. Because the vanillin content in cured beans is a major criterion for bean quality, previous studies on the curing process focused on the production of vanillin from glucovanillin, and this topic will also receive special attention in the present chapter.

6.4.2 Traditional methods of curing

The curing process is comprised of four major stages including killing, sweating, drying, and conditioning.

6.4.2.1 Killing

The fundamental purpose of the killing stage is to bring about the cessation of the vanilla bean vegetative life and, furthermore, to disrupt cellular and tissue organization in the green bean, such that previously segregated enzymes and their corresponding substrates can come in contact and interact (Arana 1943; Theodose 1973). This reasoning is supported by the observation that disruption of green bean tissue by mechanical means, tissue maceration by chopping or grinding for instance, initiates a curing process (Towt 1952), including rapid degradation of glucovanillin to vanillin, apparently by β-glucosidase-glucovanillin interaction. Modern methods of killing, aimed at instigating cell and tissue disorganization, are based on the observation that killing by the ancient Mexican method consisted of wilting beans in the sun until they became brown, a manifestation of tissue death (Balls and Arana 1941a). Contemporary killing methods consist of:

• sun killing;

• oven killing;

• hot water killing;

• killing by scratching;

• killing by freezing (Childers et al. 1959).