Biorefinery Electrification of Brewers’ Spent Grains Using Plasma Bubbles for Sustainable Production of Poly(3-Hydroxybutyrate)

Biorefinery electrification could be implemented for sustainable refining of crude renewable resources eliminating the use of high temperatures and pressures, alkaline and acidic solutions and conventional solvents. A plasma bubble reactor using air as the feed gas was employed to enhance poly(3-hydroxybutyrate) (PHB) production from brewers' spent grains (BSG) facilitating the fractionation of protein and lignin as value-added co-products. Plasma factors (voltage, pretreatment duration, duty cycle, initial solids concentration) were evaluated with the highest overall conversion yield of glucan and hemicellulose (73 %, 0.494 g sugars/gBSG) achieved for initial solids concentration of 150 g/L. The produced BSG hydrolysate led to higher PHB production efficiency (130 g/L total dry weight, 74.4 gPHB/L, 0.32 g/g yield, 2.64 g/L/h productivity) than conventional hydrolysates produced by enzymatic hydrolysis alone and enzymatic hydrolysis combined with hydrothermal treatment. The protein and lignin losses in the remaining BSG solids were 24.1 % and 29.2 % respectively. When renewable electricity is employed, the global warming potential was estimated at 0.59 kg CO2-eq per kgPHB and the abiotic depletion potential fossil at 11.12 MJ per kgPHB for PHB production based on plasma treated BSG. In the proposed biorefinery concept, 1 kg BSG could be converted into 147.6 g PHB with a solid fraction containing 137.4 g protein and 98.4 g lignin that could be subsequently fractionated as value-added co-products. The rationale of this study is to demonstrate that BSG plasma treatment can be used for the exploitation of the full potential of BSG for the production of PHB with improved environmental impact and minimal losses of protein and lignin.