Paper Title: Seasonal Changes in the Chemical Composition of Commingled Goat Milk.

Authors: MING R.GUO*, PETER H. DIXON, YOUNG W. PARK1, JAMES A. GILMORE, and PAUL S. KINDSTEDT, Northeast Dairy Foods Research Center

Department of Nutrition and Food Sciences, The University of Vermont, Burlington VT 05405 and 1Agricultural Research Station College of Agriculture, Home Economic, and Allied Programs, Fort Valley State University, Fort Valley, GA 31030


            Production of goat milk cheese in North America has been growing rapidly during the past several years.  However, information on chemical composition and its seasonal variation of year-round bulk-collected goat milk is limited.  The objective of this study was to analyze the chemical composition of commercial goat milk shipments for an entire year to provide fundamental information for cheese making and milk cheese yielding potential and pricing.  Samples were collected weekly from bulk milk shipments to a commercial cheese company over 12 months, beginning in April, 1996, and analyzed for contents (%) of total solids (TS), fat (F), lactose, crude protein, casein, non-protein nitrogen (NPN), ash, minerals, and specific gravity (G). Chemical composition of the goat milk varied widely during the year.  The contents of fat and TS decreased over the first 20 weeks from 3.6% and 12.7% to 3.0% and 11.3%, respectively, and then increased to peak values of 13.4% and 4.4% in January.  Crude protein and casein contents also decreased over the first 20 weeks, from 3.5% and 2.7% to 3.2% and 2.3%, respectively, before increasing gradually to 3.8% and 2.9% in February.  The level of lactose seemed to decrease below mean levels during August and January.  Ash content declined during the first 20 weeks from 0.82% to 0.78%, and then increased sharply to 0.90% by week 36 before decreasing sharply again toward the end of the study.  Calcium content decreased steadily from about 0.16% to 0.14% by week 20 before increasing to 0.16% by around week 40. It was found that TS content could be estimated using the equation: TS = 0.13 G + 1.41 F + 4.28 (r2 = 0.94, p < 0.01). SDS-PAGE analysis showed that the goat milk contained very little as1-casein.


              The results indicated that summer milk had the highest yield potential per kg of protein due to a higher proportion of casein in crude protein.  The milk with the highest crude protein content and lowest casein number was produced in late lactation by does that freshened in the summer.  Perhaps milk from herds bred in the off-season may not have a high cheese yield potential over an entire lactation as that from herds freshening in the spring. Non-protein nitrogen as a percentage of CP followed the same trend as casein number during the year indicating that the non casein nitrogen as % of total N was much higher in the winter milk (~12%) than in milk produced during the summer months (~9%).