Finally, other authors have proposed techniques that differ more radically from conventional processes. Since cured vanilla is mainly used in the agrifood industry to manufacture extracts, the pods have to be ground after curing. On the basis of this observation, these authors proposed methods to grind green pods and optimize enzymatic reactions by adding exogenous enzymes to the obtained purée. These mainly concern glucosidase activities, but also polysaccharide hydrolase activities (pectinase, cellulase, etc.), in order to achieve total glucovanillin hydrolysis (Graves et al., 1958; Mane and Zucca, 1992; Brunerie, 1993; Ruiz-Terán et al., 2001).
Some of these techniques have been patented. Although some of these processes will, unlikely, never be implemented, others are highly interesting as they bring defi-nite improvements. The main shortcomings concern the fact that they are not at all tailored to the socioeconomic settings in most vanilla-producing countries, especially in Madagascar, the top vanilla producer, or to the current structure of the world vanilla market. They have therefore practically never been adopted, or only to a marginal extent. However, the vanilla market has been quite volatile in recent years, including the implementation of relatively drastic standards in importing countries (especially in terms of the microbiological quality) which, in the medium term, could represent a challenge to conventional practices.
RELATIONSHIP BETWEEN VANILLA CURING AND AROMA DEVELOPMENT
The initial heat treatments are primarily aimed at stalling pod dehiscence, as already mentioned above, but it is generally acknowledged that these treatments also have a role in initiating the aromatic development phase, which is ongoing throughout the vanilla curing process.
The hydrolysis of different glucosylated precursors is the best-known reaction in the development of the aromatic quality of vanilla. Here, we will not get into a detailed discussion on the aromatic composition of cured vanilla, since this feature will be dealt with in Chapter 12. However, it should still be noted that many aromatic compounds, such as vanillin, vanillic acid, p-hydroxybenzaldehyde, p- hydroxybenzoic acid, and so forth, are present in green vanilla beans in glucoside form, thus without any olfactory properties, and that their hydrolysis is required to enable them to release their aromatic moiety.
Concerning vanillin, the main (quantitatively and qualitatively) aromatic constituent of vanilla, it was shown (Odoux, 2000; Gatfield et al., 2007) that hydrolysis of its precursor was initiated during the scalding and sweating processes, which then continued during the slow drying and even conditioning phases. A generally similar behavior was also observed with other glucoside precursors of vanilla aroma (Dignum et al., 2002; Perez-Silva, 2006).
Since glucovanillin and glucosidase are located in the same tissues but in different cell compartments (cf. previous chapter), studies were carried out to determine in what way the curing process was associated with bringing the enzyme and substrate into contact (Odoux et al., 2006). The findings of this study clearly showed that the initial heat treatments have a definite impact on cell integrity, but the proportion of cells that maintain their compartmentation or not in the tissues could not be clearly determined. The light microscopy findings seemed to indicate that this treatment effect was only partial, which could explain the low level of glucovanillin hydrolysis during the first curing steps, that is, after scalding at 60°C for 3 min and sweating for 24 h in crates. However, the observations during postharvest pod senescence and the freezing/thawing process clearly showed that these treatments led to complete cell structure degradation, accompanied by total hydrolysis (or almost total) of glucovanillin into vanillin, despite the heavy loss of glucosidase activity. In contrast, results obtained in a similar study (Mariezcurrena et al., 2008) seemed to show that the treatment had an imperceptible effect on the tissue cell organization until the fifth or eighth day after heat treatment. In this study, it was noted that the sweating process was conducted with a small number of pods (20) so the thermal inertia was much lower than at the center of a crate, which could explain the observed differences with respect to the cell integrity. Moreover, the authors provided no indications on the glucoside hydrolysis patterns or on the level of glucosidase activity during treatment in this study. All of these results, nevertheless, seemed to indicate that the main consequence of the initial heat treatment with respect to aroma development was that it brought the glucosidase and glucosy-lated aroma precursors into contact via cell decompartmentation.
However, these heat treatments also have a very negative impact on glucosidase activity (Odoux et al., 2003; Marquez and Walizewski, 2008), and the different authors who had measured changes in this activity during vanilla curing noted almost total losses as of the first scalding phase (Ranadive et al., 1983; Dignum et al., 2002; Havkin-Frenkel et al., 2005; Odoux et al., 2006; Perez-Silva, 2006), which led them to question the enzymatic origin of aroma precursor hydrolysis. The possibility of chemical or exogenous enzymatic hydrolysis (e.g., due to microbiological contamination during the drying step) was considered and tested by a few authors (Dignum et al., 2002; Havkin-Frenkel et al., 2005; Odoux et al., 2006; Perez-Silva, 2006). All of the findings seemed to confirm that this hydrolysis was actually the result of endogenous glucosidase activity. Although this activity was considerably stalled by heat treatments and not measurable by enzymatic tests developed by the different authors mentioned, aroma precursors continued to be hydrolyzed, even during very advanced phases of the process. Spectacular glucosidase activity losses (Heckel, 1910; Dignum et al., 2002; Odoux et al., 2003, 2006) were also noted during pod freezing, despite the very high rate of glucovanillin hydrolysis (cf. previous paragraph). These different observations underline that efficient cell decompartmentation of glucosylated precursors and glucosidase(s) is more important than the level of enzymatic activity itself. Heat treatments are thus essential during the conventional vanilla curing process.
Another important feature of the conventional process, which has been the focus of a few research studies over the last decade, is the marked loss of vanillin noted after glucovanillin hydrolysis. Vanillin losses of around 50% are systematically observed during the curing process (Odoux, 2000; Gatfield et al., 2007). Different studies (Gatfield et al., 2006, 2007; Frenkel and Havkin-Frenkel, 2006; Perez-Silva, 2006) have revealed that vanillin released by glucosidase can serve as a basepoint for different chemical or even enzymatic reaction mechanisms. Some of these reactions, which are far from being negative, could give rise to the formation of aroma compounds that are essential in determining the end quality of cured vanilla (see Chapter 12).
It is clear that many chemical and enzymatic reactions take place, as indicated by the color, texture, and odor modifications observed during the different vanilla curing phases. Three nonglucosidase enzymatic systems have been regularly studied during vanilla processing: peroxidases, polyphenoloxidases (PPO), and proteases (Rabak, 1916; Balls and Arana, 1941; Jones and Vicente, 1949a, 1949b; Broderick, 1956; Ranadive et al., 1983; Hanum, 1997; Jiang et al., 2000; Dignum et al., 2001, 2002; Dignum, 2002; Havkin-Frenkel et al., 2005; Marquez et al., 2008; Waliszewski et al., 2009). The findings of these studies indicated that pod browning is due to enzymatic reactions and that peroxidases and PPOs, and to a lesser extent proteases, are highly resistant to heat treatment. Peroxidases and PPO are also involved in oxidation reactions leading to the formation of aromatic molecules via lipid oxidation, and so on. Proteases are involved in the inactivation of enzymes that are essential for the formation of aroma compounds.