Abstract
Introduction. Mature green vanilla pods accumulate 4-O-(3-methoxy-benzaldehyde)-beta-D-glucoside (glucovanillin), which, upon hydrolysis by an endogenous beta-glucosidase, liberates vanillin, the major aroma component of vanilla. Little is known on the spatial distribution of aroma-generating phenolics, and the enzymes responsible for their liberation (beta-glucosidase) and oxidation (peroxidase). We report here quantitative data with respect to these three components in relation to the anatomy of the pod. Furthermore, the spatial progression of oxidation is shown. Materials and methods. Mature green vanilla pods were analyzed for their contents of phenolics (HPLC), and beta-glucosidase and peroxidase activities by spectrophotometric techniques using p-nitrophenyl glucoside and vanillin as substrates, respectively. Lipids were examined under fluorescence microscopy after Nile red staining. Oxidation development was observed on transverse slices of pods. Results and discussion. Phenolics, and beta-glucosidase and peroxidase activities showed gradients of increasing-decreasing concentrations from the stem to the blossom end of pods. The beta-glucosidase activity is distributed in between the placentae, mesocarp, and trichomes in a [7 / 2 / 1] proportion while that of peroxidase shows a [38 / 1] ratio in the mesocarp and placentae, and was absent from trichomes. Oxidation begins from the blossom end in the placentae, progressively invading the mesocarp and moving towards the stem end. Conclusion. The green mature vanilla pod is spatially heterogeneous for its phenolics, and beta-glucosidase and peroxidase activities, its placentae playing an important role in the liberation of vanillin and its subsequent oxidation.