From the pasty
fibrous material, 2 cm thick Koji beds were prepared in a clean
sterilized glass trough having 30 cm diameter. The glass trough
was used as a fermentation vessel. After inoculation with A. niger
the Koji bed was covered with cheesecloth. The cloth cover allowed
adequate gas exchange. The troughs were placed in a sterile chamber
and fermented at different experimental conditions. Citric acid
was measured by a spectrophotometric method using pyridine and
acetic anhydride reagents (Sakurai et al., 1997).