Application of ligninolytic enzyme producing bacteria for enhanced bioconversion of ryegrass biomass into bioethanol

Background: Appropriate bioprocessing of lignocellulosic materials into ethanol could address the world’s insatiable appetite for energy while mitigating greenhouse gases. Bioethanol is an ideal gasoline extender and is widely used in many countries in blended form with gasoline at specific ratios to improve fuel characteristics and engine performance. Finding a suitable microbial agent for the efficient conversion of lignocellulose is still an active field of study. 

Objective: To enhance the bioethanol production with effective lignin degradation and utilization of pentose and hexose sugars in an economical way.  

Methods: Ryegrass (Lolium perenne L.) biomass was the substrate. Microbial strains Bacillus mobilis, Bacillus velezensis, and Bacillus cereus, were isolated, identified, determined for their lignin degradation capability, and used as pretreatment agents for the lignin degradation. Various modern spectroscopic analyses, ligninolytic activity, sugar estimation, enzymatic hydrolysis, and liquefaction and fermentation process were conducted. The final data was statistically validated with post-hoc Tukey test, R software and SPSS Statistics 26.

Results: The proximate and ultimate analyses of raw biomass showed that it comprised of total solids 96.54%, volatile solids 92.82%, carbon 48.22%, and sulphur 0.28%. After the application of bacterial pretreatments, the lignin content was considerably reduced to 6.78%, and the cellulose share increased to 57.31%. The LiP and MnP like activity was highest in alkaline lignin culture source with an amount of 0.67 ± 0.1 U/mL and 1.03 ± 0.08 U/mL, respectively. The optimum sugar utilization efficiency was reached at 93.46 %, with the highest bioethanol production of 0.51 g/g and 85.78 % bioethanol yield after the anaerobic fermentation.

Conclusion: In this study, successful delignification of the ryegrass biomass was achieved by bacterial pretreatments and maximum bioethanol was produced. The integration of bacterial pretreatments and C5 and C6 sugar utilizing microbial strains could enhance the commercial bioethanol production. The ryegrass biomass was selected since it is a common agricultural waste in China. The transformation of this biomass into industrial products like ethanol, would not only utilize waste but also accord to environmental safety. However, to meet global energy demand further studies to develop sustainable and cost-effective approaches are still required.