Effects of thermo-vacuum treatment on secondary metabolite content and antioxidant activity of poplar (Populus nigra L.) wood extracts

A renewed interest in producing thermo-treated wood extracts and the potential applications of these extracts was observed in chemical, pharmaceutical, and food industries. Poplar (Populus spp.), belonging to the Salicaceae family, is one of the most cultivated woody plant for industrial purposes, one of the least expensive hardwoods, rarely used in the production of fine furniture, but extensively used for pulp and panel productions and have therefore an important economic impact worldwide. The aim of this study was to verify the influence of thermo-vacuum treatment (at 180, 200, and 220 °C) on wood extracts obtained via three different extraction techniques: maceration, ultrasound-assisted extraction and accelerated-solvent extraction. Effect of temperature on extraction was verified by measuring the total contents of polyphenols, tannins and flavonoids. Secondary metabolites are often related to antioxidant activity measured by several in vitro tests, including the 2,2-diphenyl-1-picrylhydrazyl radical-scavenging method, the ferric-reducing-ability power test, and the beta-carotene bleaching assay. Our results showed the presence and the effect of heat treatments and extraction techniques on polyphenol and flavonoid contents. Extracts obtained from wood heated at 200 and 220 °C showed the highest flavonoid and polyphenol contents, and, we observed a relationship with the shown antioxidant activity levels. Our study clearly showed the differential effects of temperature and extraction technique on both antioxidant activity and secondary metabolite contents. The detailed knowledge about the extractives from poplar wood can contribute, on the one hand, to better understand the effect of temperature during thermo treatment, and, on the other demonstrates the potential of this species as a source of bioactive compounds for nutraceutical or pharmaceutical applications by identifying appropriate extraction techniques.