Handbook of Vanilla Science and Technology

At the end of a sweating period, beans have attained a brown color and have developed most of the flavor and aroma characteristic of cured beans. However, at this stage, beans contain about 60 to 70% moisture and are, therefore, subject to spoilage by micro-organisms upon prolonged standing. Subsequently, beans that have completed the sweating period are dried to a moisture content of 25 to 30% of the bean weight (Ranadive 1994), a process that lends shelf-life to cured vanilla beans. Drying might also lead to the expulsion of volatile compounds, such as hexanal or other aldehydes and other compounds that impart “green” unripe notes to vanilla flavor. The most commonly used drying methods are sun and air-drying. These methods are occasionally supplemented by oven drying. Sun drying consists, traditionally, of spreading the beans on racks in the morning sun and transferring the sun dried beans to a shaded area in the afternoon. This protocol may be carried out daily for 3 months. Theodose (1973) divided the process into rapid and slow drying where, in the former, beans are held in the sun for a few hours every day and then wrapped in cloths and placed indoors. This process is repeated for 5 to 6 days until the beans become supple, a sign of sufficient drying. In slow drying, the beans are placed on shelves in a well-aerated room and are moved outside into the sun every 2 to 3 days. This method of drying may last one month. Other workers (Kamaruddin 1997; Ratobison et al. 1998) proposed using drying equipment based on solar energy. Because drying is the longest stage in the curing process, Theodose (1973) proposed combining traditional drying with hot air drying to shorten the drying period. Drying is the most difficult stage in the curing process. Uneven drying may result from varying bean size, differences in bean moisture content, and from variable environmental conditions, when outdoor drying methods are practiced. The latter may include weather conditions during sun drying or from variations in the relative humidity during sun or air-drying. The drying stage is apparently critical to the development of the full rich vanilla flavor, but prolonged drying may lead to loss of flavor and in vanillin content.

Bean appearance and suppleness are used by practitioners the trade as an index for moisture content. When beans are judged to have reached sufficient dryness, they are placed in wooden boxes and held for “conditioning” for an additional few months. This stage may be viewed as a continuation of the drying process where additional moisture and volatiles may be lost. However, this stage might also be accompanied by enzymatic and non-enzymatic oxidative processes that alter the vanilla flavor. Arana (1944) emphasized the probable importance of oxidative enzymes in general, and peroxidative enzymes in particular during conditioning, suggesting that vanillin or other phenolic compounds might be oxidized to quinones or other complex structures that might give rise to additional flavor notes. We show (below) that curing is also associated with lipid oxidation and, apparently, an additional origin for oxidant-induced flavor formation in a curing vanilla pod. The low rate of oxidative reactions might account for the prolonged conditioning period, lasting 5 to 6 months.

6.5 ACTIVITY OF HYDROLYTIC ENZYMES OCCURRING IN A CURING VANILLA POD

Senescence in plants is accompanied by extensive hydrolytic breakdown of cellular macromolecules, catalyzed by various hydrolytic enzymes (Rogers 2005; Hopkins et al. 2007). This process is illustrated, for example, by β-D-glucosidase-catalyzed hydrolytic cleavage of glucovanillin recorded in a senescing vanilla pod (Odoux et al. 2006). Given that the curing process is a mimic of senescence, it is reasonable to expect activity of a host of hydrolytic enzymes in killed vanilla beans, as outlined below.

6.5.1 Protease activity

Killing and subsequent curing is associated with proteolytic activity in the vanilla pod. This conclusion is inferred from changes in the bean protein content, showing precipitous decline within 24 hours of killing, but a persistent level of protein content afterward (Figure 6.7). Wild-Altamirano (1969) showed that on-the-vine pod development is associated with a decline in protease activity, although the enzyme activity remains steady when beans have matured. Following killing the pod protease activity declined within 2 days to about 60 to 70% of the initial pre-killing level and remained steady afterward (Figure 6.8). Apparently proteases resist the severe killing conditions, since proteolytic activity has been shown to survive extreme scalding, for example, 30 minutes at 80°C. The same harsh conditions de-activated other enzymes, various glucosidases or phenylalanine ammonia lyase for instance (Dignum et al. 2002a).