Protein loss by the microencapsulation of an enzyme (lactase) in alginate beads

Alginate gel beads are used in many applications as matrices for release or immobilisation. The problems of enzyme stability and low entrapment efficiency by this microencapsulation procedure are well known, especially with water-soluble substances like peptides. Microencapsulation of the enzyme lactase into alginate beads resulted in a protein loss of about 36%. During the crosslinking step, the bead weight decreased due to the 'syneresis' phenomenon: the carboxylate groups of the guluronate monomers complex the calcium cations. The water leakage was 44% when 1% sodium alginate was used. The contraction phenomenon by the alginate crosslinking and subsequent water leakage is the reason for the loss of water soluble peptides. A reduction of water leakage was achieved decreasing the pH of the alginate solution from 6.5 to 4.3. To hold back the water in the beads, 1% of bentonite was added and the water leakage was reduced to 28%. The protein adsorption onto alginate beads was shown to be affected by pH. Namely, the maximum of protein adsorption was found at a pH slightly below the isoelectric point of lactase (4.61). On charging the protein positively, a binding to anionic alginate occurred. On decreasing the pH of the alginate solution and adding bentonite, the protein loss was reduced from 36 to 3% without lowering of the enzyme activity. (C) 1998 Elsevier Science B.V.