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The mechanism of milk clotting

II. Role of hydrogen ion concentration

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The American Journal of Digestive Diseases


Rennet clotting of milk is accompanied by no change in the hydrogen ion concentration but pepsin clotting shows an asymptotic diminution in the hydrogen ions due to their adsorption by paracasein. Milk coagulates spontaneously at various pH zones, insoluble casein salts being formed between pH 2.0 and 3.0, isoelectric casein at pH 4.7 and calcium caseinate at about pH 6.5. Optimum rennet coagulation is at about pH 6.0 coinciding with complete conversion of casein to paracasein. At about pH 5.3 acid precipitation is initiated and rennin activity practically ceases. The rennet zone is broad and the acid zone narrow; the optimum of the former is pH 6.2 and that of the latter is at pH 4.7. The rate of clotting with pepsin diminishes in more acid ranges until 2.7 when it increases again. At alkaline pH no clotting takes place.

The role of the hydrogen ion concentration in milk clotting has been considered more from the standpoint of digestion and absorption of milk from the alimentary tract rather than from its function in the actual mechanism of milk clotting (2). In this respect both hydrogen ion concentration and pH range affect the clotting of milk as they do the clotting of blood (3).

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From the New York City Children’s Hospital.

This work was aided by grants from the Nutrition Research Fund and the Milk Research Council.

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Kugelmass, I.N. The mechanism of milk clotting. American Journal of Digestive Diseases and Nutrition 4, 523–525 (1937).

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