Roasted coffee wastes as a substrate for Escherichia coli to grow and produce hydrogen

After brewing roasted coffee, spent coffee grounds (SCGs) are generated being one of the daily wastes emerging in dominant countries with high rate and big quantity. Escherichia coli BW25113 wild-type strain, mutants with defects in hydrogen (H2)-producing/oxidizing four hydrogenases (Hyd) (ΔhyaB ΔhybC, ΔhycE, ΔhyfG) and septuple mutant (ΔhyaB ΔhybC ΔhycA ΔfdoG ΔldhA ΔfrdC ΔaceE) were investigated by measuring change of external pH, bacterial growth and H2 production during the utilization of SCG hydrolysate. In wild type, H2 was produced with rate of 1.28 mL H2 (g sugar)-1 h-1 yielding 30.7 mL H2 (g sugar)-1 or 2.75 L (kg SCG)-1 during 24 h. In septuple mutant, H2 production yield was 72 mL H2 (g sugar)-1 with rate of 3 mL H2 (g sugar)-1 h-1. H2 generation was absent in hycE single mutant showing the main role of Hyd-3 in H2 production. During utilization of SCG wild type, specific growth rate was 0.72 ± 0.01 h-1 with biomass yield of 0.3 g L-1. Genetic modifications and control of external parameters during growth could lead to prolonged and enhanced microbiological H2 production by organic wastes, which will aid more efficiently global sustainable energy needs resulting in diversification of mobile and fixed energy sources.