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Composition of Casein in the Smallest Casein-Containing Particles of Milk

Eastern Utilization Research and Development Division, U. S. Department of Agriculture, Washington, D. C.

ABSTRACT

In a previous publication (1), analyses of centrifugally separated casein colloid fractions were presented which showed that over a broad size-range all of the particles contain a phosphoprotein which has the same phosphorus: nitrogen weight ratio, 0.0563. For the 12 samples analyzed, the standard deviation from this average is 0.00035. The smallest casein colloid particles, representing from 5 to 10% of the total casein, were not removed in the centrifuging times used; threfore, the results do not apply to these sizes. It has recently been reported that the constituents of the centrifugable casein micelles are in equilibrium with similar constituents free in solution (5, 6). Therefore, it seemed worthwhile to make a careful analysis of the casein obtained from these smallest particles.

Portions of a sample of skimmilk were centrifuged in a Servall¹ angle-head centrifuge for 30 min., 60 min., and 2 hr., at an average centrifugal force of 27,000 times gravity, and in a 70-cc. capacity air-driven, air-floated bowl rotor for 90 min. at 490 r.p.s.