The Effects of Sucrose and Lactose on the Sizes of Casein Micelles Reconstituted from Bovine Caseins

¹ Agricultural Research Service, USDA, Eastern Regional Research Center, 600 East Mermaid Lane, Philadelphia, PA 19118

The mass distributions of reconstituted bovine casein micelles formed, in various media, with four preparations of whole casein were determined by sedimentation field flow fractionation. Two of the casein preparations were made from the milk of a single Jersey cow, the other two from the milks of Holstein cows, one being from a single cow, and the other from a herd. All media contained CaCl₂, KCl, and piperazine-N,N'-bis(2-ethanesulfonic acid) (PIPES) buffer, pH 6.75. Most of the experiments were conducted in a medium containing either no sugar, .3 M sucrose, or .3 M lactose. Micelle formation was initiated by adding an equal volume of buffered CaCl₂ to a solution of bovine sodium caseinate in a medium whose composition was identical except that it was free of Ca⁺². After 24 h at room temperature, the distribution of apparent micellar protein mass (M_Prot, grams of protein per mole micelles) was determined by sedimentation field flow fractionation. The values of apparent micellar protein mass at the maximum of the micelle peak were in the range 10⁹ to 10¹⁰ for the Jersey casein preparations and 10¹⁰ and 10¹¹ for the Holstein casein preparations. Lactose (.3 M) reduced the apparent micellar protein mass at the peak maximum by 45 to 90%, depending on the preparation. Sucrose reduced the apparent micellar protein mass at the peak maximum for one of the Jersey preparations but increased it somewhat for the Holstein casein from the herd. The results suggest that lactose may affect the micelle size in milk and in products containing casein isolates of milk. We know of no other evidence suggestive of such an effect of lactose. In view of the known positive correlation between increased micelle size and decreased heat stability, the effects of sucrose and lactose on micelle size should be considered when making milk products such as concentrated sterilized milks and sweetened condensed milk.

Key Words: micelles • casein • lactose • field flow fractionation

Submitted on June 29, 1990
Accepted on February 25, 1991