Résumé:
Tannins are secondary metabolites with high molecular weight which are widely distributed in plants. Their main role is to defend against predatory herbivores and microorganisms. They have various effects on biological systems: they are biological antioxidants, ion chelators and protein precipitation agents. In ruminants nutrition, tannins can reduce the breakdown of food proteins and increase their intestinal absorption via their molecular coating. It is in this context that this thesis work falls, the objective is to highlight this biotechnological path through a set of experimental works: (i) selection of native substrates and by-products rich in tannins, (ii) determination of their composition in macronutrients (minerals, nitrogen and fibers) and in secondary metabolites (total phenols, total tannins, condensed tannins), (iii) evaluation of their biological activity by the in vitro gas production method coupled with polyethylene glycol, (iv) study the effects of purified condensed tannins from the substrate having the highest biological activity on the various ruminal fermentation parameters. An investigation was carried out and allowed to select seven substrates: four shrubs of the genus Acacia (Acacia dealbata, Acacia horrida, Acacia cyanophylla and Acacia pycnantha) and three by-products (leaves of date palm, artichoke stems and pomegranate bark). All substrates present high crude protein content, except by-products (<100 g/kg DM) and moderate concentrations of cell wall components. The CT content is comprised between 120.5 and 680.4 g/kg DM in shrubs and between 23.1 and 170.4 g/kg DM in by-products. The
evaluation of the biological activity of crude tannins via the polyethylene glycol technique shows that its addition increased the volume of gas produced as well as the concentration of the various fermentation parameters (methane (CH4), ammonia (NH3) and volatile fatty acids (VFA)) in all forages. The highest increases were recorded for the species Acacia horrida. Therefore, this species was retained and these condensed tannins were extracted and purified by chromatography on a Sephadex LH-20 column. Purified tannins from A. horrida were incubated in vitro with two different forages, alfalfa hay and barley straw at three different concentrations: 50, 100, 150 mg/g DM. The results show that the purified tannins of A. horrida have no effect on the fermentation parameters (P> 0.05) despite its high CT content. The effects of A. horrid tannins may be strongly related to their structure and molecular weight more than to their concentration. For this reason, it would be interesting to complete this work by determining the structure and molecular weight of these tannins and by studying tannin-protein interactions. Moreover, in vivo studies in habituated animals should be conducted to assess the effects of these tannins.
