Concerning the botanical aspect, only the species V. planifolia, Vanilla tahitensis (see Chapter 13), and to a lesser extent V. pompona (Ehlers and Pfister, 1997), have been the subject of research on their aromatic composition. But even in the case of V. planifolia, much research remains to be done using well-defined genetic material and a standardized curing process.
Finally, it should be remembered that extraction and analysis techniques may also result in variability in the aromatic composition (and also in artifacts).
Quantitatively, the major component in cured vanilla is vanillin (3-methoxy-4-hydroxybenzaldehyde, Figure 12.2), which may reach concentrations of several tens of grams per kilogram of dry weight (in other words, several tens of thousands of ppm) (ISO 5565-1). Three other compounds are also commonly quantified, as they are considered as indicators of quality and authenticity (see Chapter 15); these are p-hydroxybenzaldehyde and vanillic acid (Figure 12.2), whose concentrations are generally measured at around 1 g/kg of dry weight (or around 1000 ppm), in other words, around 10 times lower than that of vanillin, and finally p-hydroxybenzoic acid (Figure 12.2), whose average concentration is around 100 mg/kg of dry weight (or around 100 ppm), in other words, 100 times lower than that of vanillin. These figures may of course vary considerably, depending on the quality of the product (Gassenmeier et al., 2008; Saltron et al., 2002).
Other compounds have also been measured at concentrations of more than 100 ppm, such as acetic acid or 4-hydroxy benzyl methyl ether (Klimes and Lamparsky, 1976) and also hexadecanoic acid and linoleic acid (Perez-Silva et al., 2006) (Figure 12.2). Adedeji et al. (1993) even report a large number of compounds with concentrations of more than 1000 ppm, such as 2-furfural (which is in fact the second most abundant compound after vanillin in a sample of Mexican vanilla) or 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one (Figure 12.2), which exceeds 3000 ppm in most of the samples analyzed, in other words, concentrations that are higher than those of p-hydroxybenzaldehyde and vanillic acid.
FIGURE 12.2 Chemical structures of the major compounds (quantitatively) found in the volatile fraction of cured vanilla beans.
It should, nevertheless, be noted that techniques for analyzing and measuring these compounds vary greatly from one author to another, and are also very different from the standardized techniques (ISO 5565-2), which explains the differences observed.
In fact, it is generally acknowledged that 95% of the volatile compounds in cured vanilla are found at concentrations of less than 10 ppm (Hoffman et al., 2005).
Although vanillin is the major component quantitatively, this molecule alone does not explain the quality of the global aroma of vanilla; it is even difficult to show a correlation between the vanillin content and the sensory profile of vanilla extract (Gassenmeier et al., 2008). In fact, rather surprisingly, very few studies have been published on the contribution of different aroma compounds to the quality of the aroma and flavor of cured vanilla.
Using GC-olfactometry (but without providing details of the methodology employed), Dignum et al. (2004) found that p-cresol, 2-phenylethanol, guaiacol, and 4-creosol (Figure 12.3) had a considerable impact on vanilla aroma. Also using GC-olfactometry, Perez-Silva et al. (2006) detected 26 aroma-active compounds in a Mexican vanilla extract; of these compounds, 13 are derivatives of the phenylpropanoid pathway (see Chapter 10), of which some, such as guaiacol, 4-creosol, acetovanillone, and salicylic acid methyl ester (Figure 12.3), are similar in intensity to vanillin, while their concentrations are 1000 times lower (sometimes even less) in the extract. It should be noted that p-cresol, cinnamic acid methyl ester, and anisyl alcohol (Figure 12.3) have intermediate intensities, even though they have concentrations of just a few ppm. In general, these compounds are responsible for sweet, woody, balsamic, spicy, vanilla-like, and toasted notes. Certain aliphatic aldehydes, alcohols, and acids also have intermediate intensities despite concentrations of less than 10 ppm.
In a study aimed at correlating sensory analysis and chemical analysis in order to classify the quality of vanilla extracts, Hoffman et al. (2005) identified 14 compounds that were detected in almost all the extracts studied (55 samples in total) and contributed positively or negatively to the aroma and flavor.
Acetic acid ethyl ester, hexanoic ethyl ester, octanoic ethyl ester, nonanoic ethyl ester, and hexanal ethyl acetal (Figure 12.3) are highly correlated with the “age-related compounds” criterion; 4-hydroxy benzyl ethyl ether, vanillyl ethyl ether, and acetaldehyde ethyl acetal (Figure 12.3) are highly correlated with the “rummy/resinous” criterion; and p-hydroxybenzaldehyde and vanillin (Figure 12.3) are highly correlated with the “vanillin” criterion. All of these compounds are therefore associated with positive criteria, whereas p-hydroxybenzoic acid, vanillic acid, guaiacol, and nonanoic acid (Figure 12.3) are highly correlated with the “smoky/phenolic” criterion, in other words, a negative descriptor.
FIGURE 12.3 Chemical structures of some aroma-active compounds detected in cured vanilla beans.
Finally, less volatile molecules should be mentioned, which are consequently involved in the quality of the flavor rather than the aroma, and are the subject of recent publications (Gatfield et al., 2006; Schwarz and Hofmann, 2009). Seven molecules (Figure 12.4), including divanillin, were identified in cured vanilla and in extracts, and are involved in the velvety mouth-coating sensation. Divanillin, for example, was found at a concentration of around 170 ppm in Madagascan vanilla (Gatfield et al., 2006). These molecules are mostly the products of condensation between two phenolic compounds.
FIGURE 12.4 Chemical structures of compounds involved in the velvety mouth-coating sensation identified in cured vanilla beans.
COMPOSITION OF GLUCOSYLATED COMPOUNDS IN GREEN VANILLA BEANS
As mentioned in the previous section, the most important compounds in the volatile fraction, both quantitatively and qualitatively, have an aromatic ring and are derived from the phenylpropanoid pathway. Many of them are also found in the green fruit in the form of glycosides—in other words, bound by a glycosidic bond (such as O-glycoside) to one or several sugar groups—and have no olfactory properties.
Historically, the presence of glucosylated aroma precursors in the green vanilla bean was long suspected. After much debate (Lecomte, 1901, 1913; Goris, 1924), it was established that “vanilloside” (glucovanillin) was the direct precursor of vanillin, and that three other precursors also existed, but in smaller quantities. Different publications describe the attempts made to isolate and identify them (Goris, 1947; Janot, 1954). These are glucosides of vanillyl alcohol (“vanilloloside”), of protocatechuic aldehyde (3,4-dihydroxy benzaldehyde), and of an unidentified ester (Figure 12.5).
More recently, glucosides of vanillin, p-hydroxybenzaldehyde, p-hydroxybenzoic acid, and vanillic acid (Figure 12.5) were identified in the green bean (V. planifolia origin Comoros, Réunion, Madagascar, Indonesia), using modern analytical techniques (Leong et al., 1989a,b; Leong, 1991). Moreover, glucose seems to be the major sugar, since the acid hydrolysis of a prepurified extract of vanilla glycosides followed by an acetylation assay makes it possible to obtain around 20% glucose, 1% mannose, and traces of rhamnose (percentage expressed in relation to initial dry matter).