SUMMAKY. Five hundred and seven samples of iso-electric caseins were typed by starch-gel electrophoresis and their polymorphic compositions were determined in terms of a sl -, ft-and K-casein variants. Clotting times in minutes were determined on 50 ml of 3 % casein solution in milk dialysate containing 0-02 M-Ca as CaCl 2 and 2 mg commercial rennin powder at pH 6-60-6-80. The clotting times ranged from 4-06 to 1-4:1 min. The data indicated that the polymorphic composition of casein significantly influenced its stability to rennin. The polymorphic combinations a sl B+/?A + KB and a sl B+/?AC + /cAB, were the most resistant to rennin action (clotting times of 7-41, 7-39 min respectively). The combinations a a l B + /?B + /cAB and a sl BC + /?B + ACBwere the least resistant to rennin action (clotting times of 4-25 and 4-06 min respectively). In most cases, caseins with lower stability to rennin contained tcB or y?B or both in their composition.It has long been known that milks differ greatly in their susceptibility to the action of rennin (Chevalier et al. 1950 a;Peltola, 1949;McDowall, Dolby & McDowell, 1937;Parisi, 1933). In earlier investigations by other investigators, attempts were made to determine whether differences in the clotting properties of milk with rennin are asso ciated with differences in the caseins or with other constituents of milk. In some instances the results obtained were contradictory. Chevalier etal. (19506) removed the caseinate micelles from both fast-and slow-clotting milks by centrifugation and resuspended them in homogeneous serum prepared from mixed milk. They found that the resuspended caseinate from slow-clotting milk retained its coagulating characteristics as did the caseinate from milks that coagulated faster. They also reported similar results when iso-electric caseins from fast-and slow-clotting milks were tested in artificial milk of the same composition. They concluded that differences in the nature of the caseins accounted for variations in the coagulating properties of milk. On the other hand, Hostettler & Riiegger (1950) found that caseins from fast-and slow-clotting milks gave the same clotting characteristics when reconstituted in solutions of the same Ca and H 2 ion concentrations.A search of the available literature failed to reveal any research work dealing with the effect of polymorphic composition of casein on its clottability with rennin. This has been the main objective of the present investigation, where the clotting times of
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