Sequential extraction of antioxidants from paulownia petioles with sc-CO2 and with subcritical water and formulation of hydrogels with the residual solids
Introduction
Paulownia is a deciduous tree of the Paulowniaceae family that grows naturally in China and comprises nine species and a few natural hybrids which are known for their fast growth, wood quality and biological properties (Hawryl et al., 2020). The exploitation of wood generates wastes from other parts of this plant (i.e. leaves, petioles, bark, flowers, fruits), which are not currently used in the wood industry.
Around 30% of the residues in Europe are agricultural and forestry wastes. These wastes can be proposed to obtain several products of industrial interest (as biofuels, biomaterials, and bioactive compounds) using environmentally friendly technologies (Romaní et al., 2018). Agricultural and forestry wastes are considered lignocellulosic materials mainly composed of cellulose, hemicellulose, and lignin. As a general trend, these residues are destined for biofuel production. For instance, a recent study confirms that paulownia wood could be an attractive alternative for bioethanol production (Domínguez et al., 2020). In this context, biomass from fast-growing trees (paulownia, poplar, eucalyptus) can contribute to the energy basis for agro power industry (Valchev et al., 2016).
In terms of its biological properties, the Paulownia genus finds considerable use in traditional Chinese medicine for the treatment of infectious diseases (Cheng et al., 2019; Móricz et al., 2019). The antibacterial, antioxidant and anti-inflammatory activities of paulownia are due to the presence of bioactive compounds (Hawryl et al., 2020). Approximately 80% of the currently known antimicrobial, cardiovascular, immunosuppressive, and anticancer drugs are of plant origin (Pan et al., 2013). Schneiderová and Šmejkal (2015) reported a source of different secondary metabolites (flavonoids, lignans, phenolic glycosides, quinones, terpenoids, glycerides, phenolic acids, and other miscellaneous compounds) isolated from various extracts of P. tomentosa.
The increasing demand for new sources of bioactive compounds corresponds to the growth of public interest in natural products and herbal medicines. Safe and high-quality extracts can be obtained sustainably by employing green technologies, such as pressurized solvent extraction using green solvents (ethanol, water, CO2) (Rebelatto et al., 2020).
Autohydrolysis heating is an environmentally-friendly biorefining process that favors the water auto-ionization what entails hydrolysis of biomass without addition of potentially harmful acid catalysts. Using microwave energy can accelerate hydrothermal treatment of biomass. Unlike conventional heating, microwave can cause internal heating. Microwave can, therefore, reduce total energy consumption by selectively affecting high-microwave-absorption materials (Tsubaki et al., 2018, 2016).
Supercritical fluid extraction technology stands out for its sustainability and safety. A supercritical fluid shows properties intermediate between those of gas and liquids, with higher density and solubilization power towards different solutes (liquid characteristic) and also higher solute diffusivity and lower viscosity (gas characteristics), which facilitates the mass transport (Krivonos and Belskaya, 2020; Náthia-Neves et al., 2020; Rebelatto et al., 2020). Much research activity has been devoted to the potential of supercritical carbon dioxide as a green solvent for vegetable biomass valorization. Among the supercritical fluids employed to recovery volatile compounds, carbon dioxide CO2 is the most widely used because it is a safe solvent for industrial applications and has a low cost and high availability. Carbon dioxide has a critical temperature of 304.25 K and a critical pressure of 7.39 MPa. (De Melo et al., 2014; Osorio-Tobón, 2020)
For the quantitative valorization of petioles, the formulation of hydrogels from the residues left after supercritical or subcritical extraction is potentially viable. Such solid residues contain value added components including antioxidants, proteins, minerals, polysaccharides; which can be used in new applications.
This paper proposes a sequential extraction of Paulownia elongata x fortunei petioles using supercritical fluid extraction (SFE) followed by microwave heated autohydrolysis (MHA) of the SFE solid residue to recover extracts with antioxidant properties. A parallel objective is the formulation and rheological characterization of hydrogels incorporated with the remaining SFE solid wastes.
Section snippets
Material
Raw material was kindly provided by Maderas Alvarez Oroza S.L. (Lugo, Galicia, Spain). Paulownia elongata x fortunei (P.COT 2®) is a non-invasive hybrid clone for native flora with amazing growth, excellent wood quality and strong climatic resistance. In this work, petioles of this tree considered wastes in biomass exploitation were used as raw material. Leaves and petioles were collected together and separated manually in the laboratory. Petioles were dried at room temperature. Subsequently,
Supercritical CO2 extraction
Supercritical CO2 (sc-CO2) extraction was performed at 45 °C for 90 min at two pressures (20 and 30 MPa) to evaluate its influence on the extraction yield and antiradical properties of extracts. Temperature was fixed at a moderate value to limit thermal degradation of phenolic compounds. Moreover, a comparison between the use of pure CO2 as solvent and the addition of ethanol as modifier at different concentrations (5 and 10%) was carried out. These results are shown in Fig. 1. Attending to the
Conclusion
Petioles of Paulownia elongata x fortunei were extracted sequentially by supercritical CO2 extraction and microwave assisted aqueous extraction evaluating extraction yield and to maximize the antiradical properties extracts. In supercritical CO2 extraction, optimal conditions were established at 20 MPa and 10% ethanol as modifier. Microwave heated autohydrolysis led to significantly higher extraction yields, 40% was attained at 220 °C. This behavior was reproduced in total phenol content of 10
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was funded by Spanish Ministry of Economy and Competitiveness, grants number BES-2016-076840 and RYC2018-024454-I and project CTM2015-68503-R. The authors thank Maderas Álvarez Oroza S.L. for providing the raw material used in this work and Carlos Vila for his technical assistance.
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