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jS-Casein was modified by trypsin, chymosin or phosphoprotein phosphatase. Each modified /?casein was fortified to the test-milk which was pretreated at 10°C by immobilized thermolysin to curdle non-enzymatically at 35°C. Fortification with chymosin-modified /?-casein, devoid of the hydrophobic moiety near the C-terminus, increased the curd tension as did native jff-casein. The test-milk fortified with trypsin-modified /^-casein, which has been deleted some part of the hydrophilic region near the N-terminus, had similar curd tension to un fortified test-milk. The testmilk fortified with dephosphorylated /?-casein, formed a softer curd than that with native /2-casein. Trypsin-modified /?-casein or dephosphorylated ^-casein retained its associative ability, but chymosin-modified jS-casein revealed negligible ability to associate. These results led to the conclusion that the phosphoryl group on the hydrophilic moiety of ficasein contributes to the increase in curd tension, while the hydrophobic moiety is responsible for the association of jS-casein.Clotting of casein micelles modified by coagulants and successive milk curdling are known to be extremely temperature dependent,1'2) i.e., milk treated by rennet or pepsin does not coagulate at a temperature lower than 1 5°C, while clotting and curdling proceed within a few seconds or minutes at 20°C or more depending on the extent of hydrolysis of milk protein. In the previous report,3) it was clearly demonstrated that the milk curdling phenomenon proceeds nonenzymatically at as high a temperature as 35°C. In this phenomenon, /?casein was revealed to be more effective in increasing curd tension than as-casein.4) It is well known that the reversible association of /?casein depends on temperature,5>6) i.e., jS-casein exists in the monomer form in solution at low temperatures, but associates to form polymers at higher temperatures. From these facts, it can be postulated that the role of /?casein in milk curdling is related to its association properties. However, full details are not available.

The tension values were determined for milk curds formed by six prote'ases using a curd tension meter. In curds with relatively high tension values, which were prepared by rennet, Mucorrennin and pepsin (acid proteases),

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