Microbial valorization of waste digestate: production of chitosan and lignocellulolytic enzymes
Author(s)
Russo, Cristina
Date Issued
July 14, 2023
Type
Doctoral Thesis
Abstract
This PhD thesis research project aims to perform the microbial upgrading of the digestate, a residue of a biogas plant derived from anaerobic digestion. In particular, this waste has been used as a fungal growth substrate for the production of chitosan and lignocellulolytic enzymes.
In CHAPTER 1, an overall introduction on the microbial valorization of wastes is presented, defining the concept of biorefinery and presenting the characteristics of the digestate and its valorization strategies.
In CHAPTER 2, the lignocellulolytic potential of the recently described species Aspergillus olivimuriae on different solid wastes is investigated. The profiles of the glycosyl hydrolases and lignin-modifying enzyme activities of A. olivimuriae were determined in solid-state fermentation, carried out on waste from different supply chains, including residual thistle biomass, spent coffee grounds, digestate solid fraction and barley straw. Among the tested substrates, residual thistle biomass was the most conducive to endo-β-1,4-glucanase and cellobiohydrolase productions while the digestate solid fraction was poorly conducive to enzymatic production and the activity levels observed are far from those reported for hyper-producing strains belonging to the same genus. Consequently, the obtained results are not of potential commercial relevance.
For this reason, the research activities have been focused on the use of the digestate as the basis of a liquid medium for selected fungal strains capable of accumulating high levels of chitosan, a polymer of relevant commercial interest.
Besides CHAPTER 3, where a current state of knowledge, future opportunities and constraints on fungal chitosan production has been described, in CHAPTER 4 experimental procedures have been reported.
For the production of chitosan, priority was given to strains of some fungal species belonging to the subdivision Zygomycota, the cell walls of which predominantly contain chitosan. As growth substrate, a liquid medium derived from the digestate was used; in particular, the liquid fraction of the digestate was used as an extractant of the solid fraction, to recover an enriched liquid phase to be used as a substrate; the extraction conditions were optimized in order to maximize the extraction of carbohydrates with a simultaneous low extraction of potential microbial inhibitors.
After an initial screening phase, aimed at testing the growth and chitosan production capacity of the selected strains, the production process was transferred to laboratory
bioreactor level with the most promising strains. The chitosan obtained through microbial bioconversions was analyzed for its physicochemical properties (degree of deacetylation, viscosity, molecular weight) and, in the light of an important application of this polymer, for its antimicrobial properties.
This Ph.D. thesis work ended with a brief conclusion, where final considerations and future perspectives were given and outlined.
Additional information
Dottorato di ricerca in Scienze, tecnologie e biotecnologie per la sostenibilità
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