Solid state fermentation: acid protease production in controlled CO2 and O2 environments.

The effect of the partial pressure of O(2) and CO(2) on the acid protease production in solid state fermentation by Aspergillus niger on wheat bran was studied. A fermentation system was used, which allowed on-line reactor measurements and continuous data acquisition of pH, temperature, gas flow, pressure drop and CO(2) production. Six paired combinations of CO(2) and O(2) concentrations were studied. The results showed a direct relationship between pressure drop, production of CO(2) and temperature increase. The pH evolution patterns were similar in all cases but different if the measurements were made on-line or on a liquid homogenate of the fermented substrate. Acid protease production was increased when the gas had 4% CO(2), (vol/vol), and it reached its highest level, a 43% increase over air, with a mixture of 4% CO(2) and 21% O(2). The protease production was strongly related to the mold metabolic activity as represented by the total CO(2) evolved.