. Within 70 and 200 h, the production of H2 and biogas increases more slowly and it was observed that cell density and butyric acid concentration remained practically constant, while lactic acid increased. In the interval of 200–500 h, there was an increment of hydrogen and biogas evolutions and of concentration of butyric acid in a slower rate than observed initially. It suggests that there was a change in the production kinetics of these byproducts. At the same time, concentration of lactic acid was constant, biomass concentration returns to grow after 300 h and the productivity decreased slowly and was maintained constant after 300 h. After 550 h, hydrogen and biogas productions increased more slowly, consequently, productivity
decreased. Concentration of butyric acid also increased until reaching a plateau of 15 g/L after 700 h of process. Lactic acid returns to increase and cell concentration remained constant. This analysis indicates that the profile of butyric acid concentration showed the same trend of the hydrogen evolution, revealing that predominant metabolic route of production butyric acid induced higher hydrogen productivity and longer duration of process. On the other hand, the profile of metabolites also shows that lactic acid impaired the cell growth and hydrogen productivity which could indicate that this acid is toxic to the biomass and that its metabolic route does not lead to H2 formation.