Vanilla

Eric Odoux

Introduction

The vanilla curing process is designed to produce an aromatically attractive and microbiologically stable product from green pods harvested before complete ripeness and which have no special aroma apart from a vague “plant” odor.

The curing techniques are of broad range, but generally involve four separate steps. Specific terms are used in reference to these steps in the different vanilla-producing countries—which are primarily Spanish-speaking in Central America (Mexico), French-speaking in the Indian Ocean region (Réunion, Comoros, Madagascar), and English-speaking (India, Uganda). Descriptions of these steps in different languages are often ambiguous and confused, further complicated by the fact that the separation between the four steps is often subjective and controversial. It is thus essential to clearly define this terminology before launching into an in-depth discussion on curing processes and their biochemical implications.

The first curing step is designed to stall pod dehiscence (especially in Vanilla planifolia G. Jackson, which is the most widely marketed species) by blocking the normal metabolic processes that take place during maturation. The most common techniques involve in suddenly inducing pod senescence, that is, “killing” the fruit (“mortification” in French). The main technique used for this purpose, which originates from Réunion, involves soaking vanilla pods in hot water—this is termed “scalding” and “échaudage” in French. In Mexico, the pods are traditionally placed in an oven or exposed to the sun, and the respective Spanish terms are “horneado” and “secado al sol.” A common English translation of “horneado” is “oven-killing,” but there is no specific term in French, while “sun-killing” is the English term for “secado al sol,” but again there is no equivalent in French.

The second step involves in maintaining the heat stored during the initial step as long as possible by placing the pods in closed crates that are as heat insulated as possible (generally with gunny sacks) so as to create conditions that are similar to those of a sweating chamber. This step is called “sweating,” or “étuvage” (from an etymological standpoint, this term introduces the notion of humid heat) in French, and “sudor de horno” or “sudado” in Spanish. These terms are more ambiguous and the interpretations may therefore differ markedly. Some authors (Bourriquet, 1954) report that during this step the pods can release a blackish fluid, which some authors explain as being due to drying associated with pod sweating. This drying conflicts with the conditions established during this step, and the phenomenon has not been experimentally assessed (Odoux, 2000; Perez-Silva, 2006). The blackish fluid is probably due to the result of excess condensation (mixed with bits of crushed pods), which is in line with the “sweating” and “étuvage” notions. Other authors (Ranadive, 1994; Dignum et al., 2001) consider that the sweating step also includes the first days of sun drying when moisture loss definitely occurs.

In the third step, the vanilla pods are dried to stabilize the product—this is called the “drying” step, or “séchage” in French, and “asolear” or “secado del sol” in Spanish. These are clear-cut terms describing the pod drying phenomenon, despite the fact that this step obviously involves more than just moisture loss—which is why it is hard to clearly separate the end of the sweating step from the beginning of the drying step. Moreover, “asolear” and “secado del sol” introduce the notion of sun exposure as a pod-drying technique.

The fourth step is termed “conditioning,” and “conditionnement” or “affinage” in French, and “afinado” in Spanish. These terms pool two complementary notions, that is, packaging, storage, and preservation of the product in a heat-tight atmosphere, with the aim of promoting “aromatic maturation” overtime.

This chapter highlights how these techniques and their derivatives ultimately involve in maintaining vanilla pods in high temperature and humidity conditions as long as possible in order to set the stage for a number of enzymatic and chemical reactions that promote the development of the aroma typical of vanilla, while curbing microorganism proliferation (especially molds). However, as discussed here, this curing is a rather continuous process, thus hard to separate into clearly defined steps.

CONVENTIONAL CURING TECHNIQUES

STEPS 1 AND 2: INITIAL HEAT TREATMENT

In V. planifolia G. Jackson, heat treatment is mainly carried out to hamper pod dehis-cence (n.b. Vanilla tahitensis is not subjected to heat treatment because it is not a dehiscent species, see also Chapter 13), which would reduce the market quality of vanilla pods. The pods are voluntarily harvested before the ripeness stage at which dehiscence occurs, but this early harvesting does not stop the process. Different techniques have thus been developed to stop it completely. These techniques are compatible with the way vanilla is cured under local conditions in producing countries, that is, where technical resources are often very limited.

In the first step, the most common technique implemented in the Indian Ocean region (Madagascar, Comoros, Réunion, etc.) involves soaking green pods in a water-filled tank heated over an open fire (scalding process) (Figure 11.1). Then the second step involves placing the pods immediately in covered crates to avoid heat loss (sweating). In theory, scalding should be carried out for 3 min in a water bath at 65°C, while the sweating process takes 24 h. In some countries, the scalding time and temperature are adjusted according to the pod grade, so the pods are sometimes presorted.

FIGURE 11.1 Killing step: green beans are soaked in hot water.

In practice, these two steps are carried out in a variety of ways. For instance, in Madagascar, which is the top vanilla-producing country, many operators are involved, ranging from farmers who produce just a few hundred grams of vanilla beans to exporters who deal with hundreds of tons of this product, and the technical resources available to these operators also differ markedly (Odoux, 1998). Most farmers have no way to control the scalding temperature and time, which means there may be substantial variations in the treatment process. The quantity of pods cured, which may vary considerably between operators, can have a great impact on the heat treatment quality (e.g., edge effects in crates). The sweating process can also be extended for up to a few days if the climatic conditions are unsuitable to start the drying step. In Réunion, where almost all of the vanilla crop is processed in two centralized units (a cooperative and a private company), the scalding and sweating processes are carried out twice to achieve a uniform heat treatment (Odoux, 2000). However, little is known about the actual impact of these alternatives on the end quality of the vanilla.

A second technique (conventionally used in Mexico) involves processing the pods in wood-fuelled ovens (“calorifico”) for 24–48 h at around 60°C. The pods are wrapped in burlap and then in mats (the so-called “maletas”). These “maletas” are abundantly sprinkled with water so that the humidity level will remain high during the process, then they are placed in wooden crates or on shelves in the oven (Bourriquet, 1954; Théodose, 1973; Perez-Silva, 2006), and finally they are sprinkled again. The oven design is generally purely practical, so this has a substantial impact on the heat treatment uniformity and the temperature actually reached in the oven—it is therefore up to the operator to adjust the treatment time accordingly. At the end of this step, the “maletas” are taken out of the oven, the mats are removed and the pods, which are still wrapped in burlap, are placed in large crates and covered for 24–48 h (Perez-Silva, 2006). This is the so-called “sudor de horno” step, which is equivalent to the “étuvage” step in Réunion.

Another method called “secado al sol,” which is also conventionally carried out in Mexico, involves spreading out the pods in the sun as an alternative heat treatment. This technique is seldom used at the present time because very large drying areas are required, and the results in terms of stopping dehiscence are much more uncertain than with other techniques (Arana, 1944).

From a more anecdotal standpoint, it should also be noted that dehiscence can be effectively stopped (Arana, 1944) by scratching the vanilla pods with a sharp object. Historically, this technique was developed in the West Indies and is only used to treat pods produced by Vanilla pompona, that is, very marginal production.

These treatments lead to vanilla bean senescence, resulting in pod browning (Figure 11.2) and relatively marked loss of the initial turgescence. As will be discussed in the last section of this chapter, these structural modifications are the starting point for the formation of vanilla aroma compounds. Considering the range of heat treatment techniques used, the pod color and texture may differ considerably between pods at the end of the treatment, but the long drying step that follows helps to homogenize the final pod appearance.

FIGURE 11.2 Vanilla beans before (in the basket) and after the sweating step.

STEP 3: DRYING

The next step of this process involves intentional slow drying of vanilla pods.

Drying is obviously carried out to stabilize the pods in order to avoid infestation by microorganisms, especially molds. The pod moisture content gradually decreases from around 85% to maximum final levels of 25–38%, depending on the category. In practice, vanilla that is targeted for the agrifood industry (80% of the market) is dried to 18–20% moisture. It should be noted that, even at 18% moisture, the water activity (A_w) is around 0.85, which is much higher than the 0.65 A_w, which means that the level should not be surpassed to avoid microorganism development. However, vanilla can normally be stored without any problems of this sort.

Vanilla pods are usually roughly spread out (Figure 11.3) on blankets and set on racks in the sun for part of the day. After a few hours of sun exposure, the vanilla is enveloped in blankets (Figure 11.4), which in turn are piled up in the shade until the next day (Figure 11.5). This procedure is repeated daily for several weeks. Pods that are considered dry enough are removed and the shade drying phase begins; this is continued until the entire batch is sufficiently dry. During this shade drying phase, the pods are carefully laid out side by side, in a single layer (Figure 11.6), on wooden racks sheltered from the sun in a building. They are regularly monitored and turned over until they are considered dry enough to begin the next conditioning step. The level of pod drying (during either the sun drying or shade drying phase) is assessed empirically by touch, so the operator has to be quite experienced in vanilla curing to be able to pass a proper judgment on the extent of pod stability. The drying rates are highly variable between pods, even between unsplit pods of the same grade (split pods obviously dry much faster and have to be specially monitored). The whole drying process lasts 2–3 months. This curing step is managed in almost the same way in all vanilla-producing countries, irrespective of the techniques used.

FIGURE 11.3 Drying step: vanilla beans are roughly spread out on blankets.

FIGURE 11.4 After exposure to the sun, vanilla beans are enveloped in the blankets.

FIGURE 11.5 Blankets with vanilla beans are piled up in the shade.

FIGURE 11.6 Shade drying step: vanilla beans are laid out side by side on trays.

The main problem with this drying procedure is that it is closely dependent on the climatic conditions, especially during the first outdoor phase when the pods are most susceptible to microorganism contamination. This step coincides with heavy rainfall periods in the main vanilla-growing areas of Madagascar (Théodose, 1973), which can upset the drying process and sometimes give rise to serious mold development problems. To avoid such infestations, artificial batch drying techniques have been proposed (Théodose, 1973), which have even been successfully implemented, from a pod quality standpoint, under large-scale industrial conditions. In Madagascar and Réunion, for instance, plum drying tunnels have been used to dry vanilla. However, such dryers are generally not used in vanilla-producing countries due to socioeconomic constraints rather than for qualitative reasons. More recently, studies have been conducted on drying vanilla in solar dryers (Kamaruddin, 1997; Ratobison et al., 1998).

The slow drying technique, described in this section, is more than just a dehydration step because the reactions that give rise to the development of the typical vanilla aroma, which are initiated during the initial heat treatment, continue because of high A_w and the heat build up during the successive sun-exposure phases.

STEP 4: CONDITIONING

The final step involves conditioning of the vanilla pods in boxes for several months. The aim is to promote full development of the vanilla aroma.

Small bunches of dried pods (generally after sorting by size and color) are tied with raffia, wrapped in waxed paper, and placed in metal or wooden boxes (Figure 11.7) to limit moisture loss as much as possible. These containers are regularly checked to make sure that there is no mold development. The vanilla pods are considered ready for marketing after a few months of storage under these conditions.

FIGURE 11.7 Conditioning step: vanilla beans are placed in wooden boxes with waxed paper.

This step could be compared to wine aging, or cheese ripening, because the aromatic quality of the vanilla pods is clearly enhanced during this period.

This curing process is implemented in roughly the same way in all vanilla-producing countries worldwide. It is highly empirical, relatively uncontrolled, and closely dependent on the prevailing climatic conditions—which can lead to major pod quality defects in some circumstances—but it is tailored to the conditions in which farmers live in most producing countries, even in Madagascar—the top vanilla-producing country. Substantial handling is required throughout this vanilla curing process, thus providing a major source of income for many people.

UNCONVENTIONAL TECHNIQUES

The different faults in the conventional curing process, already mentioned, as well as its duration (over six months before a marketable product is obtained), have encouraged many authors to try to develop alternative curing methods. Moreover, it is known that the final vanillin contents are never higher than 3% relative to the dry matter (Bayle et al., 1982; Derbesy et al., 1982; Arnaud et al., 1983; Derbesy, 1989; Falque et al., 1992; Fayet et al., 1999; Derbesy and Charvet, 2000; Saltron et al., 2002; Gassenmeier et al., 2008), whereas the initial potential is usually over 5% (Arana, 1943; Ansaldi et al., 1988; Leong, 1991; Brunerie, 1993; Odoux, 2000; Havkin-Frenkel et al., 2005). These considerations have also prompted research aimed at boosting vanillin levels in the end product.

Some authors have focused on enhancing control of the basic parameters (temperature, relative humidity, processing time, etc.) that supposedly have an impact on the final vanilla quality, while also trying to create conditions that closely match conventional pod curing conditions. Finally, the goal is to imitate the conventional process while “industrializing” the different steps.

Consequently, the techniques proposed by these authors—three of which are patented—are relatively similar, while mainly differing with respect to the initial state of the raw material, that is, depending on whether the pods are whole, in pieces or ground.

Towt (1952) proposed to grind vanilla pods to obtain a uniform purée, based on the idea that the different chemical and biochemical reactions that take place during the vanilla quality development process would be facilitated and accelerated as compared to using whole pods. This purée is subjected for 48 h to a temperature ranging from 50°C to 55°C, with air injection into the mass to promote oxidation reactions, and it is then dried at 60°C until the moisture level has dropped below 20%.

Kaul (1967) used pods that were whole or cut into pieces so as to preserve the commercial “identity” of the end product, while effectively controlling the process to curb mold development, which is a common issue under conventional curing conditions. The process proposed by this author involves incubating pods at 38°C for one week, or at 65°C for 24 h, or any other intermediary time/temperature combination, in a container that is sealed to hamper moisture loss and promote chemical and biochemical reactions. This curing step is followed by a drying process under conditions that are not outlined in the patent description. The author has noted that this drying step can, in all cases, be conducted rapidly—contrary to conventional procedures—at low or high temperature without being detrimental to the product quality. The author considers that the described curing conditions must be closely followed to obtain a top quality end product.

Karas et al. (1972) proposed to work on pods cut into pieces and placed on racks. The racks are put in an oven at 60°C for 72 h. The pods are dried at 60°C until a moisture content of 35–40% is reached, and then at ambient temperature until the pod moisture content levels off at 20–25%.

These three techniques were patented by McCormick & Company, Inc., and to our knowledge one of them (Karas et al., 1972) is used in Uganda and also by an industrial group in Madagascar.

As early as 1949, Jones and Vicente (1949a) published a study in which relatively similar procedures were compared. Whole, cut up, or ground vanilla pods were oven dried at 60°C for 24 h, and then at 45°C (until the whole pods were flexible), and finally dried and conditioned at ambient temperature. The results of aromatic tests on ice creams flavored with the corresponding vanilla extracts did not reveal any significant differences between treatments.

Bourriquet (1954) also pointed out that in 1950, a Mexican company was already producing a very flavorful product called “vanilla fruit,” which was produced from a green vanilla purée that was heated for 48–60 h and then dehydrated in a dryer.

Théodose (1973) reported the findings of studies carried out at the Antalaha research station in Madagascar in the 1960s, which aimed at simplifying the vanilla curing procedure. One technique was developed that involved scalding and sweating the whole pods in the conventional way, cutting them into 2–3 cm pieces, and then drying them in batches for 12 days to obtain an end product with 20–25% moisture content. Vanilla importers considered that the results were interesting, as the vanillin contents were higher than normal and the aroma was stronger.

Other authors focused more on optimizing the reactions to hydrolyze glucovanil-lin into vanillin by endogenous glucosidase (see Chapter 10 and section “Relationship between Vanilla Curing and Aroma Development,” of this chapter), while using a specific treatment or technology to bring these components into contact. This treatment could be the initial step before a more conventional drying step or when manufacturing a flavor extract.

Growth hormones such as ethylene were studied long back by Arana (1944) and Bourriquet (1954). It was not clearly determined whether vanilla beans are climacteric or not, but their sensitivity to ethylene as a maturation-inducing agent was clearly demonstrated by these authors. This ethylene induction process leads to pod browning and glucovanillin hydrolysis. However, ethylene induces high pod dehiscence which, according to Arana (1944), could affect up to 47.5% of all pods. More recently, Havkin-Frenkel et al. (2005) generally confirmed these high dehiscence rates in pods treated with ethylene in the presence of air or oxygen. Treatments with ethylene, naphthalene acetic acid (hormone of the auxin family), or biotic elicitors have also been tested on pods that had undergone heat treatment (63°C for 3 min), combined or not with scarification (Sreedhar et al., 2007, 2009). According to the authors, these treatments had a positive impact on the formation of vanillin and other aromatic phenolic compounds, while also reducing the vanilla curing time. The results of these experiments were somewhat inconsistent, however, so it is hard to draw clear conclusions. In addition, the physiological impact expected from these molecules on pods that had undergone a senescence-inducing heat treatment could be questioned.

Ultrasound has also been studied as a technique for bringing enzymes and substrates into contact (Obolenski, 1957, 1958, 1959) and, according to the author, this technique can boost the vanillin contents (but the results were quite confusing overall).

Cernudat and Loustalot (1948), cited in Bourriquet (1954), tested the use of infrared radiation treatments as an initial vanilla curing step, and the quality of the end product was considered fairly acceptable.

Crushing vanilla pods—without squashing or breaking them—by around 10 runs through rollers at 20 kPa pressure was also found to enhance the hydrolysis of gluco-vanillin into vanillin (Perera and Owen, 2010).

A technique involving freezing/thawing of green pods has been patented (Balls et al., 1942; Ansaldi et al., 1988) and has been the topic of different publications (Jones and Vicente, 1949a; Odoux et al., 2006). According to Ansaldi et al. (1988), this technique enables hydrolysis of around 80% of the glucovanillin present in green pods. In similar conditions, our findings showed that it is even possible to obtain hydrolysis rates of over 90% (Odoux et al., 2006).

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.

Finally, it is noteworthy to mention that microorganism could be involved in the formation of aroma compounds (Röling et al., 2001).

The findings of studies that have been carried out to date have not revealed the exact nature of the reaction processes involved in the highly complex phenomena that take place during vanilla curing.

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