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Measurement of Protein Stability in Sterile Concentrated Milk¹

Department of Food Science, University of Illinois, Urbana

ABSTRACT

Gold sols were used to determine stability of protein to Ca⁺⁺ by adding 0.5 ml of 0.01 to 0.02% protein solution to 10 ml of gold sol, followed by 1.0 ml of M CaCl₂. Optical densities were determined at 535 mµ after 1 hr. Reference protein solutions served as standards.

Stable milk proteins had higher optical densities than the unstable ones. -Casein lost its stability to Ca⁺⁺ and noncasein and nonprotein N increased during storage. Losses of gold sol stabilities of sterile -casein at zero time, and at 35 C for 30 days, and at 65 C for 4 and 8 days were 37, 62, 77, and 84%, respectively; whereas, by the CaCl₂-centrifugal method they were 31, 55, 78, and 83%, respectively. -Casein restored to concentrated milk stability that was decreased by heating.

Concentrated milk coagulated with rennet lost about 60% of its stability to Ca⁺⁺, whereas about 30% was lost during gelation of concentrated milk at 35 C storage.

Changes in protein structure which affect protection of the gold sol may be detected with 5 M NaCl instead of M CaCl₂. The gold sol and CaCl₂-centrifugal methods agreed for determining stability of milk proteins.

¹ This research was supported by U. S. Public Health Grant EF-141.

² Research Associate.