Abstract

Glucoamylases were obtained from Aspergillus niger using amylopectin from tiger nut starch as carbon source on the 5^th (GluAgTN5) and 12^th 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, K_m  and maximum velocity, V_max 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.