Influence of Mastitis on Properties of Milk. VIII. Distribution of Soluble and Micellar Casein

Abstract

The effect of mastitis on the distribu- tion of soluble and micellar casein in skim milk was investigated. Paired quar- ter milk samples from individual cows were selected on Wisconsin Mastiffs Test positive (>20 mm) and negative (< 10 ram) reactions. Approximately 51 to 78% of the total casein was sedi- mented from positive and 71 to 94% from negative skim milks by centrifu- gation at 56,000 to 144,000 N g for 1 h at 4 C. Dialysis of positive against negative skim milk (1:10 v/v ratio) for 36 h at 4 C increased casein sedimenta- tion in positive skim milk, The concen- tration of soluble casein was higher and that of mieellar casein lower in positive than in negative skim milks under vary- ing conditions of calcium concentration, pH, and temperature. Soluble casein content was highest in both skim milks at about pH 5.5 and lowest at pH 6.5 for negative and pH 7.0 for positive skim milks.