Effect of Changes in Sulfur Compounds on Stability and Gelation of Caseins and of Sterile Concentrated Milk

Abstract

Strong disulfide reducing agents (Na2SO3 and NaBH4) increased nonprotein N (NPN) in the casein complex from sterile concentrated milk. The S-containing reducing agents mer-captoethanol, thioglycolate and cysteine reduced the disulfide linkages and decreased stabilities to Ca++ of the casein complex, of whole and of k-caseins with slight changes in NPN. Increasing pH to 12 with 01 N NaOH also split the linkages and decreased stability of whole and k-caseins. Decreased stabilities were almost completely restored by air oxidation and neutralization. Degrading k- and whole caseins with NaOH and NaBH4 doubled the S compounds by the p-mercuriben-zoate (PMB) method as compared to reducing them with Na2SO3. The sulfhydryl blocking agents PMB, N-ethylmaleimide, iodacetamide and salyrgan retarded gelation and the disulfide-reducing agents mercapto-ethanol, thioglycolate, cysteine and glutathione promoted gelation of sterile concentrated milk and of [alpha]-casein with and without [beta]-lacto-globulin in the presence of Ca++. [beta]-Lactoglobulin acclerated gelation of [alpha]-casein. The effects of cystine and oxidized glutathione became evident only after long storage. [alpha]-Casein containing glutathione gelled quicker, decreased in stability more than the control, and protein bound SH- increased and nonprotein SH- decreased during storage. Masked SH- groups were not found in either k- or whole caseins.