Hydrogen production from sugars and sweet sorghum biomass using Ruminococcus albus

In the present work the production of hydrogen from sorghum biomass by pure cultures of the bacterium Ruminococcus albus was investigated. R. albus, an important fibrolytic bacterium of the rumen, can ferment hexoses and pentoses as well as cellulose and hemicelluloses. Therefore, R. albus seems to be very promising for the production of hydrogen from energy crops such as sweet sorghum, with the potential of utilizing not only the free sugars but also the cellulosic/hemicellulosic biomass as well. Batch and continuous stirred tank reactor (CSTR) experiments were carried out using glucose as carbon source in order to investigate the metabolism and calculate the growth kinetics of R. albus. Besides hydrogen, the main metabolic products detected were acetic and formic acids and ethanol. Hydrogen yield ranged from 0.47 to 2.52 mol of hydrogen per mole of glucose in continuous and batch experiments, respectively. Moreover, sorghum water extract containing soluble sugars and the lignocellulosic sorghum biomass before and after water extraction were also tested as potential substrates for hydrogen production using R. albus. The hydrogen productivity of sorghum extract plus that of sorghum residues equaled the hydrogen productivity obtained from the sorghum stalks suggesting that the process could be designed as a single-step process, thus avoiding the separate fermentation of soluble and insoluble carbohydrates as well as the extraction process. Hydrogen productivity has been estimated to be approximately 60 l of hydrogen per kg of wet sorghum biomass, thus suggesting that R. albus is suitable for efficient hydrogen production from sweet sorghum biomass.