Abstract:
Acid proteases represent an important group of enzymes, extensively used in food and beverage
industries. There is an increased demand for acid proteases adapting to the industrial extreme
environment, especially lower pH. Thus, this necessitates the search for a better acid protease
from fungi that best performs in industrial conditions. The fungal isolates were isolated from
grape and dairy farm soil using potato dextrose agar and further screened for protease production
based on the hydrolysis of clear zone on skim milk agar. The potential fungi were then subjected
to secondary screening under solid-state fermentation (SSF). After the secondary screening, the
potential fungus was identified to the genus level by the macroscopic and microscopic methods.
The growth conditions and media composition for the potential fungus were further optimized
under SSF. The crude enzyme produced by the potential isolate was characterized after partial
purification by acetone and ammonium sulfate precipitation. A total of 9 fungal isolates showed
protease production in primary and secondary screening; however, one potential isolate (Z1BL1)
was selected for further study based on its protease activity. The isolate was identified to the
genus Aspergillus based on their morphological features. The maximum acid protease from the
isolate Z1BL1 was obtained using fermentation media containing wheat bran as a solid substrate,
1 mL of 3.2 x 106 inoculum size, 50% moisture content, and pH 4.5 upon 120-h incubation at
30°C. The acetone-precipitated enzyme exhibited the maximum activity at 50°C and pH 5 with
stability at pH 4–6 and temperature 40–60°C. Thus, the acid protease produced from Aspergillus
showed suitable enzyme characteristics required in the industry and could be a candidate for
application in the food industry after further purification.
