Packed-bed reactor performance with immobilized lactase under thermal inactivation A. Illanes, C. Altamirano, A. Aillapa´n, G. Tomasello, and M. E. Zun˜ iga School of Biochemical Engineering, Universidad Cato´lica de Valparaı ´so, Valparaı ´so, Chile Thermal inactivation of immobilized lactase has been studied in the presence of substrate and products. Modulation factors of thermal inactivation have been determined for lactose and galactose based on a two-stage series-type mechanism of inactivation. Galactose was a positive modulator of enzyme stability while the opposite occurred with lactose; the glucose effect was negligible. A model for packed-bed continuous reactor operation with chitin-immobilized lactase from Kluyveromyces marxianus var. marxianus was developed considering modulated thermal inactivation and appropriate kinetic expression for lactose hydrolysis. Experimental results were in good agreement with the model which was a far better representation of reactor performance than the conventional model not considering the effect of substrate and products on enzyme inactivation. © 1998 Elsevier Science Inc. Keywords: Enzyme inactivation; enzyme reactor performance; immobilized lactase; modulation of thermal inactivation Introduction Enzymes are labile catalysts. Inactivation at the process temperature is usually the limiting factor for reactor perfor- mance. Most information on thermal inactivation of en- zymes has been gathered in the last twelve years and refers to nonreactive conditions. 1 There are several cases in which higher 2,3 and lower 4,5 enzyme stability have been reported in reactor operation, and substrate protection against ther- mal enzyme inactivation is a well-documented fact; 6,7 however, information on the effect of other substances that interact with the enzyme during catalysis is sparse, 8 and few efforts have been done to evaluate individual effects 9,10 and incorporate them to the design and evaluation of enzyme reactor performance. 11–13 In a previous work, thermal inactivation of penicillin acylase was studied in the presence of substrate (penicil- lin) and products, (6-aminopenicillanic acid, phenylace- tic acid) and modulation factors were determined: peni- cillin and 6-aminopenicillanic acid were positive modulators (protectors) while phenylacetic acid was a negative modulator of enzyme stability. 14 A different enzyme system was chosen to develop and test a model for continuous reactor performance, considering thermal inactivation modulated by catalytic effectors. The hy- potheses were: firstly, substances that interact with the enzyme during catalysis are potential modulators of enzyme stability, and secondly, modulation factors are relevant parameters for proper prediction and evaluation of reactor performance. Chitin-immobilized yeast lactase (CIL) was chosen as a case study because it is a thermolabile enzyme of increasing industrial relevance, 15 with a well-known kinetic behavior. A process for pilot plant production of such a catalyst has already been developed by the authors. 16 Yeast lactases are inhibited by the product galactose competitively, but the glucose effect is mild to negligible, 17,18 therefore, according to our hypothesis, lactose and galactose are potential mod- Address reprint requests to Dr. A. Illanes, Universidad Catolica de Valparaı ´so, School of Biochemical Engineering, P.O. Box 4059, Avenida Brasil 2950, Valparaı ´so, Chile. Received 6 March 1997; revised 17 November 1997; accepted 24 Novem- ber 1997 Enzyme and Microbial Technology 23:3–9, 1998 © 1998 Elsevier Science Inc. All rights reserved. 0141-0229/98/$19.00 655 Avenue of the Americas, New York, NY 10010 PII S0141-0229(98)00027-1