Efficient Enzymatic Hydrolysis and Polyhydroxybutyrate Production from Non-Recyclable Fiber Rejects from Paper Mills by Recombinant Escherichia coli

Non-recyclable fiber rejects from paper mills, particularly those from recycled linerboard mills, contain high levels of structural carbohydrates but are currently landfilled, causing financial and environmental burdens. The aim of this study was to develop efficient and sustainable bioprocess to upcycle these rejects into polyhydroxybutyrate (PHB), a biodegradable alternative to degradation-resistant petroleum-based plastics. To achieve high yields of PHB per unit biomass, the specific objective of the study was to investigate various approaches to enhance the hydrolysis yields of fiber rejects to maximize sugar recovery and evaluate the fermentation performance of these sugars using Escherichia coli LSBJ. The investigated approaches included size reduction, surfactant addition, and a chemical-free hydrothermal pretreatment process. A two-step hydrothermal pretreatment, involving a hot water pretreatment (150 °C and 15% solid loading for 10 min) followed by three cycles of disk refining, was found to be highly effective and resulted in an 83% cellulose conversion during hydrolysis. The hydrolysate obtained from pretreated biomass normally requires a detoxification step to enhance fermentation efficiency. However, the hydrolysate obtained from the pretreated biomass contained minimal to no inhibitory compounds, as indicated by the efficient sugar fermentation and high PHB yields, which were comparable to those from fermenting raw biomass hydrolysate. The structural and thermal properties of the extracted PHB were analyzed using various techniques and consistent with standard PHB.