Effect of Calcium Chloride and Heat on Solutions of Mixtures of β-Lactoglobulin and Casein

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Abstract
SUMMARY Tlle effect of heat and calcium ehloride concentration on the precipitation in the ultracentrifuge of casein and /3-1aetoglobulin has been studied. With fl-lactoglobulin alone (1% solution) at pH 6.5 with 2 mM calcium chloride per liter, the protein is totally precipitated after heating at 90 ° for 30 rain. and ultracentrifuging (127,000 X G for 45 rain.). With ~-lactoglobulin (1%) and casein (2%) together under the same con- ditions, no precipitate is obtained until the calcimn chloride concentration exceeds 4 ram per liter. Eleetrophoretic examination of the supernatant solution, when 50% of the total protein is precipitated (S mM calcium chloride per liter), together with the in- solubility of the precipitate in versene, suggested that the B-lactoglobulin is in the sedi- ment. This conclusion was confirmed by the determination of the phosphorus to measure the casein distribution. Similar treatment of an unheated mixture showed that all of the B-lactoglobulin was in the supernatant solution. Also determined was the amount of calcium bound to the precipitated proteins, both alone and mixed. The total results sup- port the assumption that the proteins in the mixture studied react independently and not as a complex. In the course of studying the effect of heat and calcium chloride concentration on the precipitation of fl-lactoglobulin (9) and casein (10), it was observed that in a mixture of fl-lactoglobulin and casein, the fl-lactoglobulin was not precipi- tated with a concentration of calcium chloride that would precipitate it alone. Since several reports (3, 6) bare indicated that fi-lactoglobulJn and a-casein form a complex when tile two are heated together, the precipitation of the mixture of fl-lactoglobulin and casein by calcium chloride was studied quantitatively, to see whether there was evidence for complex formation. The solubility studies were also supplemented with eleetrophoretie studies of the soluble and precipitated fractions. The effect of heat on a fl-lactoglobulin -easein mixture containing 11o calcium was also studied electrophoretiea lly.