Whey Proteins in Human Milk

The major class of proteins in human milk is the whey proteins. These are defined as those proteins remaining in skimmed milk when casein has quantitatively been removed. The difficulty in preparing whey protein lies in removing casein, including all casein subunits, and doing so without removing whey proteins by co-precipitation. This is conventionally done by acid or rennin-precipitation, which works well for cow’s milk but has been demonstrated to be less efficient for human milk. ¹ The reasons for the problems with precipitating a pure casein preparation quantitatively from human milk are (1) lack of a precise isoelectric point at which human casein micelles as well as subunits are precipitated and (2) the tendency of human whey proteins to aggregate with casein at its isoelectric point and co-precipitate. Thus, most studies determining a ratio for casein:whey proteins in human milk must be regarded with caution. This ratio is often estimated to be 60:40, which is likely an underestimation of the whey proteins due to the co-precipitation with casein. A more correct value would be 70:30. ¹ This difference may not seem important, but it is of great relevance in infant nutrition, since many formula manufacturers use this ratio as a target for their protein mixtures. For further details on the casein composition of human milk, see Chapter 2.