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.