Enhancing Lignin Dispersion and Bioconversion by Eliminating Thermal Sterilization

Thermal sterilization is widely applied in fermentation to ensure a pure culture. In this study, a facile and energy-efficient strategy by eliminating thermal sterilization (ETS) was unveiled for upgrading lignin bioconversion. Through alkaline solubilization and neutralization, lignin dispersion in aqueous fermentation media was significantly enhanced by ETS. Small-angle X-ray scattering and dynamic light scattering analyses indicated that the lignin colloid size was dramatically reduced. Compared to 20.5 wt % lignin precipitation during the conventional thermal sterilization, precipitated lignin was not observed within the ETS medium. 31P NMR characterization demonstrated an 11.7% increase of phenolic OH in ETS lignin. Ionization of phenolic OH presented more negatively charged groups, strengthening electrostatic repulsion, resulting in smaller colloidal particles. Interestingly, the pure culture of Rhodococcus opacus PD630 was achieved within the ETS medium due to the lack of lignin degradation ability with most natural microbes. R. opacus PD630 cell amount, lignin degradation, and lipid production by ETS increased by 330, 16.6, and 20.7%, respectively. Overall, an energy-efficient ETS strategy that promoted lignin dispersion and bioconversion significantly is reported in this study.