The long-term activity of the biocatalyst for the production of sorbitol and gluconic acid from an aqueous solution of fructose and glucose is considerably improved with the aid of permeabilized cells of Zymomonas mobilis immobilized using .kappa.-carrageenan that has been rigidified and then stabilized by K.sup.+ ions. Rigidification of .kappa.-carrageenan is preferably carried out by treatment with glutaraldehyde alone or by treatment with polyethyleneimine or hexamethylenediamine and subsequent exposure to glutaraldehyde. A buffer-free solution is preferred and the pH is maintained by adding Ca.sup.++ ions while simultaneously precipitating gluconic acid.