Rheological Properties of Enzymatic Hydrolyses of Cassava Starch in a Rheometer as a Reactor

Abstract

To mimic excellent starch hydrolyses in a reactor, cassava starch at concentrations of 25, 32 and 36.23% (w/v), were liquefied by α-amylase at a concentration of 0.031% v/w at 90 oC for 60 min in a concentric cylinder (with 19 mm diameter outer cylinder and 17 mm spindle) of a rheometer at constant shear rate of 56.76, 84.48 and 113.52 s-1 which were equivalent to calculated shear rates of starch liquefaction in a 10-L stirred tank reactor fitted with two-stage Interming Impeller stirring at 200, 300 and 400 rpm, respectively. Also, sugar dextrin solution samples, from liquefaction of cassava starch at the same concentrations and temperature for 3 h in the stirred tank reactor above stirring with dual Intermig at 200 rpm , were saccharified by glucoamylase at a concentration of 0.06% (volume of conc. Enzyme/initial flour weight) at 65 oC for 60 min in the same concentric cylinder and rheometer with the same shear rates. Results showed that in both cases, liquefaction and saccharification, the viscosity decreased and the reducing sugar concentration and DE value increased as a function of shear rate due to degradation of starch chain polymers and dextrin to obtain sugars. Also liquefaction and saccharification in rheometer by increasing shear rate from 10 to 330 s-1 showed the characteristic of shear thinning with a flow behavior index of < 1. As a small volume of starch sample of 8 mL with large surface areas of both cylinder and spindle promoting ideal good mixing (mass transfer) and heat transfer, essentially fast and perfect enzymatic reaction of liquefaction and saccharification were achieved. Peak viscosities of sugar dextrin sample during liquefaction dropped within 10 min and complete liquefaction was achieved within < 60 min. As well, complete saccharification seen from plateau viscosity was obtained within < 5 min. These essentially indicate that good mixing and heat transfer generate fast and complete starch hydrolyses and lead to the design of a good reactor.