Effects of temperature on lactose hydrolysis by immobilized β‐galactosidase in plug‐flow reactor

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Abstract
The hydrolysis of lactose using immobilized β-galactosidase (from Aspergillus niger) on phenol–formaldehyde resin was studied at temperatures between 8 and 60°C and initial lactose concentrations ranging from 2.5 to 20.0%. A model involving enzyme–galactose complex similar to Michaelis–Menten kinetics with competitive product (galactose) inhibition is suitable to describe the lactose hydrolysis reaction. A small degree of lack of fit between the model and the data was found to be due to the formation of oligosaccharides. Thermal deactivation of lactase follows first-order reaction mechanism. The effect of temperature on the reaction and the deactivation rate constants follows the Arrhenius relationship. The Oligosaccharide formation was not significantly affected by the temperature when the initial lactose concentration was 5%. A design equation for the plug-flow immobilized lactase reactor was developed from the reaction and the deactivation kinetics and was used to find the optimal operating temperature. The optimal temperature was found to be dependent on the operating time but not on the lactose concentration or the conversion. The optimal operating temperature is 60°C when operating time is short but is close to 35°C for a long operating time. A preliminary economic analysis indicates that the optimal operating temperature is 43, 38.5, and 33°C when the operating time is 300 days, 1000 days, and infinity, respectively.