Due to their simple polymerization process and low toxicity, natural hydrogels are favorable matrices for the encapsulation of biocatalysts in organic synthesis.
In organic solvents, they form stable compartments which protect entrapped enzymes from detrimental effects of their environment. Material and encapsulation method, however, affect stability and activity of the biocatalyst itself as well as mass transfer in and mechanical stability of the immobilization
matrix. When an alcohol dehydrogenase from Lactobacillus kefir and its cofactor, NADPH+H+, were entrapped in alginate, agar, gellan, and -carrageenan by dropping into aqueous hardening baths, enzyme as well as cofactor loss from the hydrogels was observed. However, immobilization yields of 100 % were easily achieved by adapting four different methods. Best results were obtained with two-phase dispersions of sunflower oil and solutions of gellan or kappa-carrageenan. With regard to the low thermostability of the entrapped alcohol dehydrogenase, the gelling temperature of both materials was decreased below 40 °C by changing polymer and ion concentrations. Influences on the mechanical strength of the hydrogels were investigated by comparing the critical compression forces of gel cylinders with standard sizes. Reasonable gelling temperatures as well as mechanical strengths was observed for 1.5 % (w/v) gellan including 0.05 % (w/v) CaCl2. Encapsulating the alcohol dehydrogenase and its cofactor with this hydrogel composition, stereoselective production of R-phenylethanol from acetophenone was performed with a yield of 80-85 % and an enantiomeric excess of 98 %.