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Annual Report: CIRAD 2006

Vanilla aroma and images

During fruit ripening, the glucovanillin that has accumulated in green vanilla beans is hydrolysed by a glucosidase, which releases vanilla aroma. However, there is some debate about the location of these two compounds within the bean. Using cellular imagery and the latest spectral deconvolution techniques, it will now be possible to identify these two metabolites very precisely in situ, and eventually to improve bean postharvest processing techniques.

Histocytological section of a vanilla bean © J.L. Verdeil/Cirad

Cellular imagery is currently being revolutionized by the development of in situ marking techniques and improvement in microscope performance, which has really gathered speed in the past decade. It is now possible to analyse the spectral composition of the light in each pixel of an image.Spectral deconvolution, based on this analysis, uses powerful algorithms to decompose a complex emission spectrum into elementary spectra that are recorded as controls. The system was initially developed for biologists wanting to trace several proteins labelled with different coloured tags within the same tissue.

The histocytology and plant cellular imagery platform has used this technique for the in situ location of vanilla bean secondary metabolites with different degrees of fluorescence: vanillin and its glucosylated precursor, glucovanillin. During bean ripening, the glucovanillin that has accumulated in green vanilla beans is hydrolysed by a glucosidase, which releases vanilla aroma.

Locating vanillin precisely within the bean

Purified vanillin and glucovanillin powder samples were observed under a confocal microscope, and their respective emission spectra were recorded. These control spectra were used to analyse the fluorescent light from a bean section and find the pixels in the image that contained one or other spectrum or both spectra superimposed on each other. It also proved possible to distinguish the zones of the image and the section that corresponded to glucovanillin from those that corresponded to vanillin.

Glucovanillin accumulates mainly in the internal part of the bean, more precisely in the placentas, the secretory papillae, and around the seeds, within the bean cavity. Vanillin, for its part, is found in the placenta cells and around the seeds. Experiments conducted on numerous controls confirmed this location. They demonstrated the power of spectral deconvolution to locate secondary metabolites in situ, but also its limitations.

These experimental data offer new prospects for adjusting bean postharvest processing methods. In effect, the method used in producing countries, which is the result of a long tradition, is entirely empirical. Under these conditions, it is almost impossible to diagnose the origin of any quality defects found correctly. This work is an essential stage on the road to a better understanding of how vanilla quality evolves during processing.

Contacts

Jean-Luc Verdeil, e-mail , UMR: Developmental Biology of Perennial Crop Plants (BEPC)

Eric Odoux, UPR: Tropical Food Quality

Partners

  • Centre national de la recherche scientifique (CNRS, France)
  • Institut national de la recherche agronomique (INRA, France)
  • Montpellier regional imagery platform (Rio Imaging, http//mri.cnrs.fr)

For further information

  • Odoux E., Escoute J., Verdeil J.L., Brioulet M., 2003. Localization of ?-D-glucosidase activity and glucovanillin in vanilla bean ( Vanillia plantifolia Andrews). Annals of Botany, 92 : 437-444.
  • Odoux E., Escoute J., Verdeil J.L., 2006. The relation between glucovanillin, ?-D-glucosidase activity and cellular compartmentation during the senescence, freezing and traditional curing of vanilla beans. Annals of Applied Biology, 149 : 43.