The tension values were determined for milk curds formed by six protéases using a curd tension meter. In curds with relatively high tension values, which were prepared by rennet, Mucor rennin and pepsin (acid proteases), α_s- and β-casein were negligibly hydrolyzed during incubation after clotting, whereas both casein fractions in the fragile curds prepared by Alkaline-protease from Bacillus subtilis, trypsin and thermolysin (alkaline proteases) were electrophoretically degraded during further incubation after clotting. This phenomenon was more prominent in the β-casein fraction. However, when alkaline proteases were employed in an immobilized form and removed from the test-milk at the initiation of clotting, the degradation of β-casein was decreased, and curd tension values were increased. Inhibition of the enzymatic degradation of β-casein by addition of sodium chloride (5%) led to an increase in curd tension. When the test-milk treated by each immobilized alkaline protease was fortified with β-casein, curd tension value increased significantly. These results suggest that β-casein might be essential for the hardening of curd.

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