Inoculation effect of thermophilic microorganisms on protease production through solid-state fermentation under non-sterile conditions at lab and bench scale (SSF)

The production of protease enzyme was evaluated through the solid state fermentation (SSF) of soy fibre, a waste product that acted as a sole substrate for the fermentation, at a laboratory and bench scale using a 500-mL (batch size 115 g) and 10-L (batch size 2300 g) bioreactors. The objective was to assess the effect of the inoculation of the thermophilic bacteria Thermus sp. on the production of the enzyme when working at laboratory and bench scale under non-sterile conditions, since scaling-up and the need of sterilization are the main challenges of SSF, preventing its industrial development. Results revealed that the inoculation led to a substantial increase in the protease obtained on both scales when compared to non-inoculated fermentation. The maximum protease activities increased as a result of the inoculation from 500 to 800 and from 350 to 670 U/g dry matter of soy fibre in the lab and bench scale bioreactors, respectively. Finally, a very good correlation was found between the protease activities obtained and the fermentation most relevant parameters: oxygen uptake rate (R (2) = 0.81) and temperature (R (2) = 0.82). In this work, we have demonstrated that inoculation is effective even under non-sterile conditions at the kg scale and that this strain is able to compete with autochthonous microbiota and increase the protease production to levels higher than those previously reported in literature.