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 (Aw) is around 0.85, which is much higher than the 0.65 Aw, 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 Aw 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.