In the recent years, mathematical models have been developed as tools to help developing SSF bioreactor design and establish proper operating strategies to overcome such
difficulties. More sophisticated models for packed-bed bioreactors, which the bed remains static for long periods without the water addition through the fermentation, taking into account the simultaneous heat and mass transfer during the SSF, where the evaporative cooling is the most efficient method to remove the metabolic heat on the medium. However, as high is the water evaporation through convection more accentuated is the decreasing of water activity of the solids [3].The mathematical models should also take into account how the growth rate of the microorganism is affected by the amount of water in the solids [4]. In this case, the isotherm and/or the drying kinetic of the solids must be known since the water balance equation calculates the water content of the solids, while the driving force for evaporation and the growth rate of the process microorganism are related to the water activity.