Chemical analysis and thermal stability of African mahogany (Khaya ivorensis A. Chev) condensed tannins
Résumé
The forest policies of new Congo basin countries have increased the wood timber industry output since the year 2000. Thus, a high content of underutilized wood waste from sawmill, furniture and plywood industries is generated. Among them, Khaya ivorensis A. Chev bark, sapwood and heartwood account for the less valorized wood wastes. Therefore, an attempt was made to study the chemical variability and thermal stability of tannins extracted from the sapwood and heartwood by the acetone/water method (7:3, v:v); the potential recovery of these polyphenols for industrial applications was also aimed. Quantitative analysis pointed out that the heartwood was the most abundant in phenolic units, and a significant difference (P < 0.05) was found in condensed tannin content between the bark, sapwood and heartwood. This result indicated an intra-tree variability while no significant difference was found for inter-tree tannin contents (P > 0.05). These tannins were characterized by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) and Fourier transform infrared (FTIR) spectroscopy. Their chemical structures were fisitinidin- and gallocatechin-type units. No evidence of free cathechin moiety was found in these extracts. Moreover, oligomers up to seven tannin monomers free from glycosyl structures were found in the K. ivorensis condensed tannins. Moreover, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) pointed out good thermal stability of these hardwood species' tannin. These findings could be useful for future valorizations of African mahogany wood wastes as source of tannins for chemistry or composite materials.