Characterisation and quantification of condensed tannins in rapeseed hulls and meals by depolymerization methods
Résumé
Condensed tannins (i.e. polymerized procyanidins) were quantified in crude samples of rapeseed meal and hulls
using two acidolysis methods. When it was applied directly on crude material, the colorimetric butanol-HCl assay
allowed an estimation of two grams of tannins per kilogram of dried hulls. Interestingly, the estimation was more
than double after solvent extraction using sodium dodecyl sulphate (SDS) when the colorimetric method was
applied on both the extract and the insoluble residue. By altering the plant tissues organization, SDS treatment
may allow a better accessibility of the depolymerizing reagents. However, this colorimetric assay was judged
approximate because of browning side-reaction likely involving other components. Phloroglucinolysis combined
with HPLC-UV-MS analysis was more rigorous and informative confirming that rapeseed hulls condensed tannins
were essentially constituted of epicatechin units with an average degree of polymerization of close to 11. Ac-
cording to this method, the concentration of condensed tannins in hulls was estimated to 5.19 g/kg in the
optimized conditions established through a factorial experimental design allowing optimization of hydrochloric
acid concentration, reaction duration and temperature.