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Effect of Alkalization, Oxidation, Reduction, and Storage on Elution Patterns of -Casein Obtained by Gel Filtration¹

Department of Food Science, University of Illinois, Urbana

ABSTRACT

-Casein was investigated to determine its molecular changes and degradation during storage, in comparison with chemically induced changes.

Sephadex G-200 did not yield fractionated peaks of -casein other than the void volume by eluting with Tris-HCl buffer with and without mercaptoethanol, M acetic acid, or 6 M urea. Six molar urea containing 0.01 M mercaptoethanol revealed peaks at K_av of 0.45 and 0.55 for - and para--caseins, respectively.

By eluting with 0.2 N NaOH at 4 C, a K_av 0.29 peak was obtained where the stabilizing ability of -casein to _s1 casein was restored after neutralization. Para--casein shifted to the low molecular weight side of K_av 0.44. After sterilizing and storing at 35 C, the zero peak was increased at the expense of the 0.29 peak, indicating aggregation of molecules, and the stabilizing ability of the 0.29 peak decreased.

Sephadex G-50 and G-75 yielded an excellent glycomacropeptide peak after rennin action. This peak increased slightly after sterilizing and storing -casein and a peak of low molecular weight amino compounds, probably amino acids, increased rapidly. Oxidation with periodate revealed almost the same change as in stored -casein. -Casein oxidized with M H₂O₂, reduced with 0.03 M NaBH₄, or alkalized with 0.2 N NaOH at 25 C, yielded a cleaved peak like that produced by rennin, indicating stripping of amino acids without severe degradation of sterilized -casein during storage.

¹ Supported by a grant from the American Dairy Association, 20 Wacker Drive, Chicago, Illinois.