Valorisation of brewers’ spent grain via alkaline pretreatment for protein concentrate and poly(3-hydroxybutyrate) production

Abstract This work outlines an integrated biorefinery approach for the valorisation of brewer’s spent grain (BSG), targeting protein recovery and poly(3-hydroxybutyrate) (PHB) production. Initially, dextrins and proteins were extracted from BSG, and the residual solids obtained after protein extraction were subsequently subjected to alkaline pretreatment to promote lignin removal and enhance enzymatic hydrolysis. A Central Composite Design was employed to optimise lignin removal while minimising hemicellulose and glucan losses, with optimal conditions determined as 0.71% (w/v) NaOH at 71°C. After 30 min of pretreatment under the optimal conditions, lignin, hemicellulose, and glucan removal reached 31.6%, 26.9%, and 12.5%, respectively, while the enzymatic hydrolysis efficiency of glucan and hemicellulose reached 86.5% and 30.0%, respectively. The resulting hydrolysate was efficiently used as a carbon source for PHB production by Paraburkholderia sacchari DSM 17165 under fed-batch conditions, yielding a dry cell weight of 86.6 g/L with 49.1% PHB (42.5 g/L), corresponding to a PHB yield of 0.28 g/g and a productivity of 1.23 g/(L·h). Based on the proposed biorefinery, 1 kg of BSG could be used for the production of 76.3 g crude protein extract with 45.2% protein purity and 89.7 g PHB. The proposed process demonstrates a scalable and resource-efficient strategy for converting brewery residues into biodegradable polymers and protein-rich food extracts, thereby contributing to circular bioeconomy practices.