Handbook of Vanilla Science and Technology

Accumulation of glucovanillin during on-the-vine development of a vanilla pod ensues during the fourth month after anthesis. It then rises sharply for the next 3 months and levels off during the last stages of pod development (Havkin-Frenkel et al. 1999). Formed glucov-anillin may be sequestered mostly in the inner white placental tissue around the seeds (Figures 6.1 and 6.3). The distribution of glucovanillin along the longitudinal axis of green vanilla pods may also vary and was found to be as follows: 40% in the blossom end, 40% in the central portion, and 20% in the stem end, indicating uneven tissue distribution with respect to the substrate. This is in keeping with the observation by Childers et al. (1959) and by other studies, noting that vanillin crystals formed during curing appear mostly on the blossom end.

6.6 ACTIVITY OF OXIDATIVE ENZYMES OCCURRING IN A CURING VANILLA POD

Killing of vanilla beans is associated with de-greening and onset of browning reactions, appearing to be a mimic of browning occurring during on-the-vine vanilla pod senescence (Figure 6.4). Browning is observed also during advanced senescence in other ripening fruit (Wilkinson 1970), as well as during stress or disease injury in plant tissues (Schwimmer 1972), arising mostly from oxidation of phenolic compounds (Broderick 1956b). Balls and Arana (1941a, 1942) observed that various killing methods, including chemical, mechanical, or heat stress, but not freezing stress, stimulated a temporary respiratory upsurge, suggesting that the killing-induced increase in oxygen consumption might contribute to onset of oxidative processes in the killed pod. This effect was simulated by applied ethylene, leading to a brief upsurge followed by a decline in CO₂ evolution in green beans, as the ethylene-treated pod continued to ripen (Balls and Arana 1941a, 1942). Furthermore, application of ethylene resulted eventually in pod browning (Arana 1944). Other studies, showing ethylene-induced H₂O₂ accumulation accompanying respiratory upsurge in plants (Chin and Frenkel 1976), suggest that ethylene-induced browning may stem from ethylene-induced oxidative stress, that is, oxygen consumption for the production and activity of reactive oxygen species (ROS). In keeping with this view, we observed that co-application of ethylene and oxygen amplified ethylene-induced browning (Figure 6.5). These data infer that bean browning, as occurring naturally during on-the-vine senescence or as induced by ethylene, may reflect onset of oxidative conditions in vanilla beans. Because various stress conditions are associated with the accumulation of H2O2 and other reactive oxygen species (Kocsy et al. 2001), it is a reasonable assumption that stress conditions, employed in vanilla bean killing, may also lead to the onset of oxidative conditions and in keeping with the results showing formation and accumulation of ROS in the curing bean (Figure 6.9).