Optimization of dilute nitric acid pretreatment of brewers’ spent grains for sustainable biorefinery development and poly(3-hydroxybutyrate) production

This study presents an integrated biorefinery concept using brewers’ spent grains (BSG) processed via acidic enzymatic proteolysis and dilute nitric acid treatment for the production of protein-rich hydrolysates and poly(3-hydroxybutyrate) (PHB). After aqueous extraction of dextrins, acidic proteolysis of the remaining BSG solids (10% w/v, pH 3, 50 °C, 10 U/g) achieved a protein extraction efficiency of 66.9 ± 0.6%. The protein-rich hydrolysate contained, per 100 g protein, mainly glutamic acid and glutamine (29.8%), aspartic acid and asparagine (9.7%), leucine (8.1%), proline (7.2%) and valine (6.7%). Central composite design was employed for the optimization of dilute HNO 3 pretreatment of the remaining BSG solids to maximize the glucan and hemicellulose to sugar conversion yield, considering HNO 3 concentration (0-1% w/v) and temperature (70-120 °C) as process variables. The optimal BSG treatment was achieved at 0.29% (w/v) HNO 3 , 120 °C and 30 min reaction duration, resulting in 58.1% overall glucan and hemicellulose to sugar conversion yield. The hydrolysate contained 75.1% glucose, 20.7% xylose, 3.7% galactose and 1.5% arabinose. Fed-batch fermentation of Paraburkholderia sacchari using the optimal BSG hydrolysate resulted in 97.7 g/L PHB, 65.5% (w/w) intracellular PHB content, 0.31 g/g PHB yield and a productivity of 2.87 g/(L∙h). This biorefinery concept resulted in the production of 155.1 g PHB and 160.2 g protein-rich extract produced as hydrolysate from 1 kg BSG. • Optimal pretreatment (0.29% HNO 3 , 120 °C, 30 min) achieved 58.1% sugar yield. • Acidic proteolysis extracted efficiently 67% of BSG protein. • Biorefinery co-produced 155 g PHB and 160 g protein hydrolysate per kg BSG. • P. sacchari fermentation reached 97.7 g/L PHB with 2.87 g/(L∙h) productivity.