Glucoamylase immobilization on a magnetic microparticle for the continuous hydrolysis of maltodextrin in a fluidized bed reactor

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Abstract
Glucoamylase (GA) has been successfully immobilized through its carbohydrates previously oxidized with periodate onto a low-cost magnetic microparticle made of polyethyleneimine-coated magnetite crosslinked with glutaraldehyde (M-GAD) and derivatized with adipic dihydrazide (ADH). A stabilization posttreatment consisting of cross-linking its carbohydrates with ADH, increased the remaining activity from 54 to 71%, calculated on theV m values and measured at 50°C and pH 4.5 with maltodextrin (DE 11-14) as substrate. This treatment also improved the enzyme stability and lowered the deactivation rate constantk d to a third of its value. A 30% maltodextrin solution has been continuously hydrolyzed at 50°C and pH 4.5 in a recycled, fluidized bed reactor (FBR) containing GA immobilized on these magnetic microparticles. They easily settled in this highly viscous medium because of their high density (5 g/mL), and washout of ultrafines was prevented by surrounding the top of the bed with an electromagnet. The small particle size (20 μm) allowed a high enzyme loading in the reactor and also a high bed voidage, which is recommended to avoid extensive pressure drop and consequent channeling problems. The kinetic of hydrolysis fitted with the plug-flow model; this is explained by the insignificant backmixing effects observed. After 2 wk of hydrolysis under process conditions leading to a conversion of 70%, which corresponds to a high-conversion syrup, the immobilized GA only lost 4% of its initial activity.