Study On Starch Saccharification Conditions And Enzymatic Characteristics Of Thermostable Glucoamylase

Two mutants of 9-1-D and 18-3-H from Aspergillus niger 3.4309, which produced thermostable glucoamylase were obtained through determination of their enzyme activity. And their optimal reaction temperatures were 60℃, which was 10℃higher than that of the original strain. At their optimal reaction temperature, the enzyme activity of original glucoamylase was 131.23 U/mg, thermostable glucoamylase G9-1-D was 171.10 U/mg, which was 30% higher than the former and thermostable glucoamylase G18-3-H was 142.83 U/mg, which was 9% higher than the former.It took 10 h for the saccharification DE value of thermostable glucoamylases G9-1-D and G18-3-H to reach 70% at 60℃, which was 10 h less than that of the original glucoamylase.The optimal saccharification temperature of G9-1-D and G18-3-H was 70℃, and at this temperature, it took 5 h for the saccharification DE value of thermostable glucoamylases G9-1-D and G18-3-H to reach 70%, which was 15 h less than that of the original glucoamylase.The optimal composite additives of thermostable glucoamylase G9-1-D were 50 mmol/L Na+,0.05 mol/L glycerin,3 g/L xanthan gum,2 g/L sodium alginate. DE value could increase rapidly during the saccharification when the composite additives were added into, the final DE value were 85.50%,7.86 higher than that of the control. The optimal composite additives of thermostable glucoamylase G18-3-H were 50 mmol/L Na+,0.05 mol/L glycerin,2 g/L xanthan gum,3 g/L sodium alginate, the final DE value were 81.40%,5.77 higher than that of the control. The optimal composite additives of original glucoamylase were 50 mmol/L Na+ 0.03 mol/L glycerin,3 g/L xanthan gum,3 g/L sodium alginate. and the final DE value were 76.38%,7.81 higher than that of the control. The results showed that adding composite additives could protect the spatial conformation of the enzymes during the saccharification.Both glucoamylase G9-1-D and G18-3-H were composed of three isoenzyme components, all the components were glycoproteins identified by PAGE with the staining of Schiff's regeant. The component G9-1-D-3' and G18-3-H-3'were identified as thermostable components by the method of SDS-Starch-PAGE, their optimal temperature and optimal pH were both 70℃and pH4.5 respectively, and the molecular weights were both 50.0 kD.