Glucoamylases were obtained from Aspergillus niger using amylopectin from tiger nut starch as carbon source on the 5th (GluAgTN5) and 12th day (GluAgTN12) of fermentation. The optimal pH for GluAgTN5 at 55°C were 6.5, 7.0, 6.0 while that for GluAgTN12 were 8.5, 6.0, 7.5 at 50°C using cassava, guinea corn and tiger nut starch as substrates, respectively. The enzyme activity in GluAgTN5 was enhanced by Ca2+ and Fe2+ while Zn2+ and Co2+ had inhibitory effects on the enzyme activity. Mn2+and Pb2+, however completely inactivated the enzyme. Enzyme activity in GluAgTN12 was enhanced by Ca2+ while Co2+and Zn2+ had inhibitory effects, Fe2+, Mn2+ and Pb2+ completely inactivated the enzyme. The Michealis-Menten constant, Km and maximum velocity, Vmax obtained from Line-Weaver-Burk plot of initial velocity data at different substrate concentrations were 222 mg/ml and 500 µmol/min, 291 mg/ml and 1000 µmol/min, 137.5 mg/ml and 500 µmol/min using cassava, guinea corn and tiger nut starch as substrate, respectively for GluAgTN5. While that for GluAgTN12 were 176.6 mg/ml and 100 µmol/min, 491 mg/ml and 1000 µmol/min, 131.5 mg/ml and 500 µmol/min using cassava, guinea corn and tiger nut starch as substrate, respectively.
Key words: Glucoamylase, pH, metal ions, Aspergillus niger, tiger nut starch, amylopectin.
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