Transglutaminase is a Ca²⁺ -dependent enzyme that covalently polymerizes proteins through the formation of ε-(γ-glutamyl)lysyl cross-links. We have reported that highly concentrated protein solutions were firmly gelatinized by transglutaminase. The technique of gelation of an α_s1-casein solution with transglutaminase was applied to the preparation of an α_s1-casein film. The α_s1-casein film obtained showed a high tensile strength (105 g/cm²) and strain (72%). It was insoluble in water, 10% 2-mercaptoethanol, 6.6 M urea, 10% SDS and 6M guanidine hydrochloride. Even if it was diluted with water and then heated at 100°C for 10min, it remained insoluble. However, it was gradually hydrolyzed by chymotrypsin. These results suggest the usefulness of the α_s1 -casein film as a supporting material of immobilized enzymes, a medical polymer and an edible film.

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