Effect of mixing on enzymatic liquefaction of sago starch

Abstract

The effect of mixing as a function of agitation speed and impeller diameter on the rate and degree of enzymatic liquefaction of sago starch was carried out using a stirred tank reactor with a single Rushton turbine impeller. The performance of the reactor as a mixing device was first examined using different concentrations of carboxymethylcellulose, which exhibited pseudoplastic behaviour similar to that of the solution during the sago starch liquefaction process. A correlation between mixing time ( t m ) and Reynolds number (Re) in the form of t m = bRe c is presented; the constants for the correlation depended on viscosity of the fluid. For the two ratios of impeller diameter ( D i ) to tank diameter ( D t ) used, 0.407 and 0.542, agitation speed gave significant influence on both overall rate and degree of liquefaction of sago starch. Mixing time ( t m ) was independent of impeller diameter used, and correlated well with the overall rate of liquefaction ( P) (calculated as the reducing sugar produced divided by time of liquefaction) and expressed as P = 1.95 t m −0.362.