Production of Polyhydroxyalkanoates from Brewer's Spent Grains: Biodegradable Bioplastics from Agro-Industrial Waste

Production of biodegradable plastics as an alternative to petroleum-based plastics has long been stymied by the high production cost preventing feasible scale-up and market penetration. One potential approach of reducing the production cost is use of low-value or waste feedstocks. This work investigates the potential of polyhydroxyalkanoate (PHA) production from brewer’s spent grains, the major by-product of the brewing industry. More than 30% of barley grains end up as BSG which contain up to 50% carbohydrates. This work focuses on maximizing PHA production from the sugars obtained by enzymatic hydrolysis of these carbohydrates.

To increase the hydrolysis efficiency, the BSG were subject to a two-step hydrothermal pretreatment of liquid hot water pretreatment followed by disk refining. The hydrolysate was used as a carbon source by recombinant E. coli LSBJ harboring PHA biosynthesis enzymes (PhaA, PhaB, PhaC1, PhaG and AlkK). Fermentation was conducted first in 500mL shake-flasks for optimization studies using a Box-Behnken surface response method. This was followed by fermentation in a 2.5L bioreactor operating in fed-batch mode to investigate scale-up. Cells were harvested by centrifugation, after which PHA was isolated from the pellet by dissolution in chloroform and precipitation in methanol. BSG hydrolysate was successfully fermented to PHA bioplastic using a method which is feasible for scale-up. Productivity using BSG substrate was found to be higher than that of pure sugar in defined media. This research provides an innovative and sustainable solution to the challenge of BSG disposal and provides an opportunity of cost reduction in bioplastics production.