Economic Research Institute Study paper
Utah State University
Research has demonstrated that the average composition of producer milk in the United States is approximately 3.7 percent butterfat, 8.7 percent nonfat solids, and 87.6 percent serum (water). The combination of these three components varies by cow, age of cow, month of lactation, season of the year, herd, breed, and feeding program. Nonfat solids consist of l actose, protein, and minerals. Minerals and lactose are quite constant in milk. Most of the variability in nonfat solids is caused by protein fluctuation. It goes up and down with butterfat but not by the same amount. On the average, for each 1 percent change in fat, protein changes about 0.4 percent. For individual cows there are variations to this rule.
Christensen, Rondo A., "Should Nonfat Solids Be Used in Pricing Grade A Milk?" (1978). Economic Research Institute Study Papers. Paper 367.