Microbial contaminations (mainly molds and bacteria) of the vanilla beans can occur at the harvest and through the several steps of handling and processing. The curing process, besides enabling the biochemical process of aroma, is essential for the control of these microorganisms, which is beneficial for the preservation of the cured beans.
Curing of vanilla beans is a traditionally well-established process in the main countries of production (Madagascar, Mexico, Comores, Reunion, and Tahiti). It is laborious and takes up to six months, depending on the curing procedures adopted by different producing countries. In general, it is mainly based on the following four steps (Dignum et al., 2001):
• Killing the green beans by a thermal treatment (hot water, oven) to stop the vegetative development of the fresh beans.
• Sweating which takes place during the cycles of sunning and sweating, where the beans are spread out in the sun until they are hot, and then wrapped up in blankets and put into an airtight container overnight to maintain their warmth and moisture.
• Drying the beans slowly to prevent the microbial spoilage and to progressively allow the reactions for the development of aroma.
• Conditioning the dried beans in boxes for 3–4 months to obtain the desired aroma and flavor.
The first stage (killing) reduces the initial microbial load on the surface of the green beans. During curing, the major fungi found on vanilla beans are mainly black Aspergillus and green Penicillium strains (Röling et al., 2001). Several bacterial communities are present on the green beans, but after scalding (immersion in hot water 65–70°C for 2 min), this study has reported only the presence of Bacillus strains (Bacillus subtilis, Bacillus firmus, Bacillus licheniformis, Bacillus pumilis, Bacillus smithii) due to their thermophilic nature as being sporeforming microorganisms. For split beans, which have not been scalded, other genera were found such as Xanthomonas, Cellulomonas, Vibrio, and Staphylococcus. B. subtilis (including B. licheniformis as well) are food-spoilage organisms that have been isolated from other herbs and spices and that have also been implicated in food poisoning (te Giffel et al., 1996). Bourriquet (1954) has identified the presence of Aspergillus niger, Penicillium lividum, Penicillium vanillae, and Penicillium rugulosum on vanilla beans from Madagascar and Comores. Another study has shown that the main mold contaminations of vanilla beans consisted of Penicillium glaucum, Trichoderma sp., Aspergillus oryzae, Aspergillus amstelodami, and Xantochpinirous sp. at a total concentration of 8.4 × 104/g (Bachman et al., 1995).
The second key step is the drying step as the moisture content is a major factor in the preservation of cured vanilla beans because low moisture content is essential to prevent microbial growth. The beans should be well aerated while drying to avoid bacterial fermentation, which leads to undesirable flavor, as creosoted vanilla. When properly cured, the water content of beans must be sufficiently low to prevent the growth and activity of microorganisms. Actually, combination of low water with high phenolic content of which vanillin is the major compound, provide an inhibitory effect of the spoilage in cured beans (Havkin-Frenkel and Frenkel, 2006). According to ISO 5565-1:1999 (ISO, 1999), the maximum moisture specification in cured beans is 38% for classes 1 and 2, 30% for class 3, and 25% for class 4. The water content in cured vanilla beans correspond to water activity (aw) values of, respectively, 0.89, 0.86, and 0.84 (J.M. Méot, pers. comm.). The presence of appropriate water content ranging from 25% to 30% has been reported to result in the desirable texture and appearance of the cured beans (Sreedhar et al., 2007).
Immature beans have been reported to be more susceptible to mold infestation by Penicillium and Aspergillus species (Sasikumar et al., 1992). As Aspergillus and Penicillium are, with Fusarium, the most important genera containing toxigenic isolates, prevention measures based on Good Agricultural Practices and Good Manufacturing Practices should be observed to mitigate preharvest and postharvest contaminations by mycotoxins (FDA, 2009). Mycotoxins are toxic secondary metabolites produced by fungi under specific conditions related to the physiological (microbial growth) and environmental (pH, temperature, aw, preservatives) conditions. The toxigenic fungi may be eliminated during processing, while their corresponding extracellular mycotoxin remains in the food. The most important mycotoxins for human health risk are described in Table 14.1 (FAO, 2001). Among the fungi reported in vanilla beans, A. niger does not have the ability to produce aflatoxins (Schuster et al., 2002). However, this specie has been reported to produce ochratoxin A (Blumenthal, 2004). A. oryzae does not produce aflatoxins despite its very close taxonomic relatedness to Aspergillus flavus group, a major group that produces aflatoxins in food (Blumenthal, 2004). A. oryzae might produce other myc-otoxin such as 3-nitropropionic acid. P. lividum has been reported to produce citrinin and penicillic acid (Frisvad, 1986). However, the risk, if any, of mycotoxin contamination of vanilla beans needs a detailed assessment (Codex Alimentarius Commission (CAC), 1999). A few research papers have reported the contamination of vanilla beans by mycotoxins. Analyzing a total of 681 samples of spices for the natural occurrence of ochratoxin A (OTA) and ochratoxin B (OTB), Scheuer and Gareis (2002) found one sample of vanilla extract that was positive for OTB at 156 ng/g.
| Mold Species | Mycotoxins Produced |
|---|---|
| Aspergillus parasiticus | Aflatoxins B1, B2, G1, G2 |
| Aspergillus flavus | Aflatoxins B1, B2 |
| Fusarium sporotrichioides | T-2 toxin |
| Fusarium graminearum | Deoxynivalenol (or nivalenol), Zearalenone |
| Fusarium moniliforme (Fusarium verticillioides) Fumonisin | B1 |
| Penicillium verrucosum | Ochratoxin A |
| Aspergillus ochraceus | Ochratoxin A |
Source: From FAO. 2001. Manual on the Application of the HACCP System in Mycotoxin Prevention and Control. FAO, Rome. With permission.