By concerning to the other parameters that were monitored (Fig. 1a), in this assay there was a biogas production of 1.99 L. Therefore, a culture inhibition could be occurring, since a biogas production was interrupted after 40 h, although there was still lactose in the medium. Complementarily, the pH was maintained at 5.5 during all the assay. In addition, in Fig. 1b it is possible to observe increases in acetic and butyric acid concentrations, while slight increment was observed in the formic acid concentration. Besides that, the specific hydrogen productivity varied from 80 mmol H2/gvs d in 23 h to 100 mmol H2/gvs d in 7 h. After that, it was reduced probably due to toxic compounds accumulated into the medium by cells.