Compositional Quality of Milk. I. The Relationship of the Solids-Not-Fat and Fat Percentages

Open Access
Publisher Website
Google Scholar
Abstract
The authors present a critical review of the literature and point out the confusion existing, not only with respect to the relation between the percentages of fat and solids-not-fat, but also in the formulas used to calculate the solids-not-fat from fat percentages. Two schools of thought prevail; one assigning a linear, the other a non-linear relationship. Data are presented from analysis of 717 samples of genuine milk from 5 herds of purebred Holstein, 100 from 2 herds of purebred Ayrshire and 231 samples from 1 herd of purebred Jersey cows. Samples were either 3- or 1-day composites and were segregated with respect to mastitis. The Babcock test for fat and the Mojonnier test for solids were used. Regression equations for calculating solids-not-fat from fat percentages for normal milks are shown for 4 ranges of fat values, 2.75-3.3, 3.3-4.25, 4.25-5.85, and 5.9-6.75%. No single equation was found for the entire range of fat contents. Similar equations are shown for milks from cows 20-25% of which were mastitis positive. The conclusion is drawn that the Jacobson equation (s.n.f. % = 0.4% fat + 7.07) gives low values for milks higher in fat than 3.3%, and that the Overman equation for Jersey milk (s.n.f. % = 0.2457% fat + 8.2340) represents true values for natural normal milk of fat content above 5.8%. Some data are shown to support the belief that the sire may transmit a factor involved in the secretion of high or low solids-not-fat. Suggestions are made for sampling in order to avoid contradictory and confusing results from similar surveys.