Enzyme and Microbial Technology 34 (2004) 603–610 Diffusion of glucose and maltose in polyacrylamide gel D. Yankov ∗ Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. Bontchev Str., Bl. 103, 1113 Sofia, Bulgaria Received 9 August 2003; accepted 20 January 2004 Abstract The diffusion of glucose and maltose in different polyacrylamide gels (PAAG) was investigated. The method applied was non-steady-state diffusion into gel beads from a finite solution. The diffusion coefficients were determined applying Crank’s solution of the diffusion equation. It was found that the diffusion coefficients in PAAG were about 15–85% lower than the corresponding coefficients in water, depending on gel composition. The influence of different factors, such as monomer concentration, cross-linking ratio, temperature, concentration of the diffusing substance, ionic strength, and the amount of the entrapped enzyme were tested. © 2004 Elsevier Inc. All rights reserved. Keywords: Diffusion; Polyacrylamide gel; Glucose; Maltose 1. Introduction Gels of polyacrylamide or other natural and synthetic polymers are widely used as a matrix for enzymes and liv- ing cells immobilization. The great interest in this technique is a result of its undoubted advantages, such as controllable pore size, no need for enzyme reactive groups for the attach- ment to an insoluble support, high residual activity, etc. In view of the above, the enzyme and cell entrapment is widely used by a number of biotechnological processes. However, for the mathematical modelling of these processes, as well as for the design and analysis of bioreactors, data for the diffusion coefficients of the substrate and of the products in different gels, are required. Usually biochemical reactions are slower than chemical ones. As a result, low feed flow rates are needed in the reactors with immobilized biocatalysts. Thus, it can be assumed with great certainty that diffusion plays a major role in the mass transfer mechanism, mainly in the case of biocatalyst immobilized inside of porous support par- ticles. There is an additional mass transfer limitation in the entrapped systems because of an interaction between the gel matrix and the diffusing substances. In general, the values of the diffusion coefficients in gels are con- sidered lower than the corresponding ones in water. This is because the path length for the diffusant, due to ob- structions caused by gel structure, is increased, while the ∗ Tel.: +359-2-719121; fax: +359-2-707523. E-mail address: yanpe@bas.bg (D. Yankov). volume available for diffusion is decreased. The first effect is called obstruction effect and the second one-exclusion effect. Many of the papers published recently deal with diffusion of small molecules (glucose, sucrose, lactose, ethanol, etc.) in a large variety of gels. They, however, discuss mainly diffusion at temperatures up to 40 ◦ C. In recent years, the selection of strains, capable to produce thermostable en- zymes, has been intensified. As a result, the interest in their properties, in particular for the diffusion characteristics of their immobilized preparation, has increased. This has re- flected in increasing the number of papers discussing the influence of concentration of cells entrapped in gels on the value of the diffusion coefficient. Results are summarised in some excellent reviews, for example that by Muhr and Blan- shard [1] which discusses the diffusion in gels at ambient temperatures; and that by Westrin and Axeisson [2]—the influence of cells’ content in the gel on the values of the dif- fusion coefficient. However, the question of the analogous influence of enzyme concentration on the diffusion charac- teristics of the gels remains open. Likewise, the data for the influence of the concentration of monomers and the degree of cross-linking differs greatly and are often contradictory. For example, some of the authors claim that the diffusion coefficient is independent of the degree of cross-linking [3,4], while others—that there is a strong dependence be- tween them, even with a minimum [5]. Similarly, for the dependence of the diffusion coefficient on temperature Brown and Chitumbo [4] report a change in the slope of the line ln D versus 1/T at temperatures below 25 ◦ C, while 0141-0229/$ – see front matter © 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.enzmictec.2004.01.008