Improved fermentation strategies for enhancing polyhydroxybutyrate production from paper mill fiber rejects

This study explores upcycling non-recyclable fiber rejects from recycled paper mills, rich in structural carbohydrates (∼50 %), into polyhydroxybutyrate (PHB), a biodegradable plastic alternative. The process involved biomass pre-cleaning, pretreatment, and fermentation to enhance PHB yields. Pre-cleaning, including repulping and screening, reduced ash content from 30.4 % to 20 %. Sequential hydrothermal pretreatment (150 °C for 10 min) followed by disk milling (3 cycles) was applied, achieving sugar yields exceeding 60 g/L during enzymatic hydrolysis. The hydrolysate was then used as the sole carbon source for PHB production by recombinant E. coli LSBJ. The study systematically optimized carbon source loading and inoculation size to maximize PHB production. Under optimal conditions, using 50 g/L reducing sugar and a 2.5 % inoculum, the highest PHB titer and yield achieved were 14.7 g/L and 0.29 g/g sugar, with a productivity of 0.2 g/L-h, surpassing previously reported titers from lignocellulosic materials at the shake-flask scale.