Measurement of Protein Stability in Sterile Concentrated Milk

Abstract

Gold sols were used to determine stability of protein to Ca++ by adding 0.5ml of 0.01 to 0.02% protein solution to 10ml of gold sol, followed by 1.0ml of m CaCl2. Optical densities were determined at 535mμ after 1 hr. Reference protein solutions served as standards.Stable milk proteins had higher optical densities than the unstable ones. α-Casein lost its stability to Ca++ and noncasein and nonprotein N increased during storage. Losses of gold sol stabilities of sterile κ-casein at zero time, and at 35C for 30 days, and at 65C 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. κ-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 35C storage.Changes in protein structure which affect protection of the gold sol may be detected with 5m NaCl instead of m CaCl2. The gold sol and CaCl2-centrifugal methods agreed for determining stability of milk proteins.