Production of microbial protein from fermented grass

• Industrially relevant lactic acid productivity (9.9 g/L/h) was achieved at 45 °C. • Solid removal is essential for lactic acid extraction via membrane electrolysis. • Lactic acid can be extracted at 13 kWh/kg from solid-free fermentation broth. • The dried MP produced was superior to whole egg, raw chicken, tofu and soybean. • Methane was produced from grass residues at yield up to 0.53 g CODCH4 /g CODsubstrate . Grass is one of the most abundant plant families on Earth, however it has limited applications beyond the use as animal feed due to its low economic and nutritional value. Here we developed a concept for the increased valorization of grass to microbial protein (MP) for human food and biogas. We followed a four-step approach, starting with lactic acid (LA) fermentation of grass juice (GJ) using Bacillus coagulans , followed by LA extraction via membrane electrolysis, aerobic MP production using Cupriavidus necator and digestion of the side-products for biogas production. Temperature (28–55 °C) had a strong effect on the performance of batch GJ fermentations under axenic and non-axenic tests performed with B. coagulans , with the highest LA concentration (16–18 g LA /L) and productivity (0.45–0.56 g LA /L/h) achieved at 45 °C. Continuous non-axenic thermophilic (45 °C) GJ fermentation enabled the highest LA titers (23 g LA /L) at 6 h HRT while 1.5 h HRT enabled the highest productivity (9.9 g LA /L/h). The microbial community was dominated (77–97%, HRT < 6 h) by Lactobacillaceae, regardless of inoculating B. coagulans . LA extraction via membrane electrolysis was possible after removal of solids, achieving maximum LA recovery of 35%, while the lowest energy input was 13 kWh/kg LA . Microbial biomass produced in pH-stat mode using extracted LA contained 58% protein in cell dry weight, while the final product (dried MP) is superior to egg, chicken, tofu and soybean. The side-products of these processes ( i.e. fermentation effluent and unconverted solids) can be used for methane production with 0.085–0.53 g CODCH4 /g CODsubstrate yield. This research delivered a proof-of-concept for the use of grass within a biorefinery concept, allowing for recovery of products with a potentially large market such as MP, LA and biogas.