Measurement of Protein Stability in Sterile Concentrated Milk

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Abstract
Gold sols were used to determine stability of protein to Ca++ by adding 0.5 ml of 0.01-0.02% protein solution to 10 ml of gold sol, followed by 1.0 ml of [image] CaCl2. Optical densities were determined at 535 m[mu] after 1 hr. Stable milk proteins had higher optical densities than the unstable ones. [alpha]-Casein lost its stability to Ca++, and noncasein and nonprotein N increased during storage. Losses of gold sol stabilities of sterile kappa-casein at 0 time, 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. K-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 [image] NaCl instead of [image] CaCl2. The gold sol and CaCl2-centrifugal methods agreed for determining stability of milk proteins.