Glucose oxidase release from calcium alginate gel capsules Ana Blandino*, Manuel Macı ´as, Domingo Cantero Biological and Enzymatic Reactors Research Group, Department of Chemical Engineering, Food Technology and Environmental Technologies, Faculty of Sciences, University of Ca ´diz (UCA), 11510 Puerto Real (Ca ´diz), Spain Received 2 December 1999; received in revised form 15 February 2000; accepted 15 March 2000 Abstract Diffusion of glucose oxidase within calcium alginate gel capsules has been assayed and the experimental data fitted to a simple semi-empirical power equation, which is used to analyse the solute release from polymeric devices. It was found that an increase in the concentration of sodium alginate and calcium chloride gives rise to a reduction in the enzyme leakage. This was verified when glucose oxidase (GOD) diffusion percentages were compared in capsules with thicknesses of the same order of magnitude but obtained under different experimental conditions. So, the use of sodium alginate and calcium chloride solutions of concentrations 0.5% w/v and 2.6% w/v, respectively, lead to a diffusion percentage of 25  2. This percentage was reduced to 8  3 when sodium alginate and calcium chloride concentrations were fixed at 1% w/v and 4% w/v, respectively, even though the thicknesses of the capsules were of the same order of magnitude. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Calcium alginate gel capsules; Enzyme diffusion; Gelling condition; Glucose oxidase 1. Introduction Industrial development of enzymatic reactors requires the use of immobilized enzymes in cases in which the cost of the biocatalyst is high. To a large extent this procedure prevents enzyme losses due to washout and, at the same time, maintains high concentrations of the biocatalyst [1]. It is well known that the effective immobilization of biocata- lysts can be achieved using several techniques, one of which is encapsulation within a gel matrix. This immobilization technique consists of enclosing the biocatalyst, in an aque- ous solution, inside a semipermeable membrane capsule [2]. There are two main advantages inherent in this immobili- zation method: the particle structure allows contact between the substrate and biocatalyst to be achieved in an appropri- ate way and, in addition, it is possible to immobilize several enzymes at the same time [3]. Encapsulation in calcium alginate gels is characterised by the very mild conditions in which the immobilization procedure is carried out and by its low cost and ease of use [4]. Moreover, by changing the gelation conditions it is possible to control easily some of the capsule characteris- tics, such as the thickness [5–7] and permeability to differ- ent substrates of the gel membrane [8 –12]. However, sev- eral disadvantages are also associated with this method, namely the low stability (affected by substances such as citrate, lactate, and phosphate) and the high porosity of the membrane [13,14]. The latter characteristic limits the appli- cation of calcium alginate gel capsules to high molecular weight compounds and whole cells or organelles [15–18]. As regards this problem, pore size is a critical parameter in selecting a matrix for a particular enzymatic immobilization process. Low molecular weight substrates and products can easily diffuse into or out of a matrix with large pores and large pores can also cause problems by allowing the immo- bilized enzyme to leak out. However, there are very few studies that take into consideration the enzyme leakage and practically none that discuss this phenomenon in terms of diffusion in porous materials [19 –21]. In this context, the main objective of this work was to study the diffusion of an enzyme of high molecular weight out of calcium alginate gel capsules, obtained under different experimental condi- tions (i.e. at various sodium alginate and calcium chloride concentrations). Based on the results obtained, the diffu- sional properties of calcium alginate gel capsules are dis- cussed in relation to the structure of the gel matrix. In * Corresponding author. Tel.: 0034-956-83-0907; fax: 0034-956-83- 7565. www.elsevier.com/locate/enzmictec Enzyme and Microbial Technology 27 (2000) 319 –324 0141-0229/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S0141-0229(00)00204-0