Production of Polyhydroxyalkanoates From Cassava Peel Waste: an Integrated Chemical and Biological Approach

Abstract

Polyhydroxyalkanoates (PHA) are biodegradable polymers synthesised by microorganisms, offering a promising sustainable alternative to petroleum-based plastics because they can be synthesised from renewable resources such as biomass. A significant challenge in the industrial production of PHA is the high manufacturing cost, primarily associated with expensive commercial refined carbon sources used as feedstock. Therefore, the use of waste biomass resources to produce PHA has the potential to improve the economic feasibility. Cassava (Manhiot esculenta) is a crop widely cultivated in tropical regions. Cassava processing industries produce large amounts of cassava waste, of which 20% are cassava peels (CP), that are predominantly dumped into landfills, resulting in greenhouse gas emissions. Therefore, this thesis aims to explore the potential of CP as biomass resource for efficient PHA production. Enzymatic and acid hydrolysis of CP were assessed. 97% of conversion from CP to fermentable sugars was achieved through a systematic approach using a design of experiments (DoE) to optimise acid concentration, time, and temperature in acid hydrolysis using H2SO4. Subsequently, cassava peel hydrolysates (CPH) were used as the sole carbon source at plate, shake flask, and benchtop stirred tank bioreactor scales for Cupriavidus necator (C. necator) growth and PHA production. Small-scale (plate and flask cultures) yielded up to 1.5 g/L of PHA, equivalent to approximately 30% (gPHA/gDCW). The optimization of the cultivation strategy in bioreactor, using defined media in the batch and CPH in the feeding phase, resulted in enhanced production, reaching 12.1 g/L of PHA, equivalent to 86% (gPHA/gDCW). An efficient process for producing PHA by integrating a chemical process for the hydrolysis of CP with a biological process for the conversion of CP into PHA using C. necator is presented. This work contributes to the advancement of efficient and sustainable PHA production in biorefinery platforms, and the replacement of petroleum-based plastics, toward achieving Net Zero targets.