Obtención de biopolímeros de interés industrial mediante la transformación de residuos lignocelulósicos y ligninas residuales por Streptomyces

Abstract

The depletion of petrol resources and the great awareness of society about the preservation of the environment, have promoted the development of sustainable methodologies to transform renewable sources into products with high added value. Residual lignocellulosic biomass result of agricultural practices and the residual lignin from the pulp and paper industry generate serious environmental problems generate environmental problems by accumulation in the field or by discharge into surface streams. Thus, in recent years, these wastes have been used in countless research works exploring its great biotechnological potential to to develop products of great industrial interest. In this work, the optimization of the parameters that affect a Solid State Fermentation (SSF) process of two agricultural residues (wheat straw and barley straw) by selected strains of Streptomyces has been carried out applying the statistical method Ortogonal Partial Least Squares Discriminant Analysis (OPLS-DA). Fermented substrates were later used as thickeners in the production of environmentally friendly oleogels. Likewise, the potential use as thickener in of residual lignins (Kraft and alkali lignin) functionalized with the laccase SilA were also evaluated. Thus, the optimal enzyme reaction conditions were determined by the Response Surface Methodology (MSR) in order to achieve the highest polymerization degree of the lignins. On the other hand, the production and characterization of hydrogels made with residual lignins functionalized with SilA was carried out and their potential utility as antibacterial agents and adsorbents of textile pollutants was evaluated. Finally, the biodegradability and ecotoxicity of the products obtained were analyzed. Most remarkable results showed that both agricultural residues were successfully transformed by Streptomyces strains during the SSF process. The efficiency of this process, determined as a function of the enzymatic activity and as the solubilization of lignin, was mainly attributed to the xylanase activity produced by both strains. The analysis of all results through the OPLS-DA method allowed to select the strain S. MDG 301 growing on wheat straw at 45 ºC during 7 days of incubation for further applications. The rheological characterization of the oleogels obtained from this fermented wheat straw under the selected optimal conditions, showed lower values of the viscoelastic functions compared to oleogels formulated using uninoculated wheat straw. However, it was noticeable the polymerizing action of laccase SilA on residual lignins, which allowed us to obtaining oleogels with rheological characteristics comparable and even superior, to that used in commercial lubricants. Furthermore, the polymerizing action of SilA on residual lignins was demonstrated when these functionalized lignin were used to formulate hydrogels together with different proportions of chitosan. The films obtained from these hydrogels presented versatile physicochemical characteristics for use in different industrial sectors. It is remarkable that the compatibility of both polymers (lignin and chitosan) was achieved thanks to the oxidative action of the SilA on the phenolic moiety of lignins and those films formulated with 70% chitosan and 30% lignin, presented a great water retention capacity and a good structural matrix. Moreover, these films showed a high adsorption efficiency of textile dyes, as well as a significant antibacterial capacity against Escherichia coli and Staphylococcus aureus. Finally, the adequate characteristics in terms of biodegradability and eco-safety of oleogels and hydrogels designed in this work could assume an innovative strategy within environmentally friendly technologies and also complies with the requirements of the circular economy.