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DAIRY GOAT MILK COMPOSITION
By: John C. Bruhn, FST, UC Davis
Goat husbandry has been part of agriculture since almost the first use of domestic animals and presently its popularity is increasing throughout the world, this increase is reflected to a greater degree by the rise in the number of small herds maintained by individuals either as a source of income or as an avocation. Goats are particularly suited to this role because they have minimal land use and attention requirements yet still allow an individual to become actively involved in dairying. The goat's milk produced by such enterprise is typically sold as whole milk or processed in cheese, evaporated milk or dried milk products. Because of this increased interest, it is valuable to be aware of the factors affecting the composition and nutritional value of caprine milk. Further, it is worthwhile to compare the milk of goats with that of cows and note benefits or limitations which may result from differences found.
There are also district breed differences in fat composition. It should be remembered, however, the quality and quantity of feeds, genetics season, stage of lactation, etc. all influence the average percentage of goat milkfat. In California DHIA goat records indicate that the dairy goats on test produced milk with a 3.9% milkfat.
The fatty acids in the milkfat are arranged in the triglycerides in accordance with a pattern that appears to be universal among ruminants. The percent unsaturated fatty acids (ol.eic and linolenic) do not differ from the average found for cow's milk (Table 2). Because of this, goat's milk does not appear to offer an advantage over cowls milk in use in diets restricting the intake of saturated fats. A major difference between the milkfat of the goat and the cow is the percentage distribution among specific short chain fatty acids. Goats have an appreciably higher proportion of capric, caprylic and caproic acids. The high amounts of these specific fatty acids are responsible for the characteristic flavor and odor associated with goat's milk.
In terms of cholesterol, goat's milk appears to offer a specific distinction in comparison to cow's milk, Cow's milk typically contains about 14 to 17 mg cholesterol per 100 g milk, while goat's milk is more usually recorded at 11 to 25 mg per 100 gram of milk. More work, however, needs to be done to establish if there is a real difference.
Two misconceptions about goat's milk are associated with the fat globule size. First, it is often claimed that goat's milk is naturally homogenized. This statement is derived from the observations that milkfat from the goat does not cream quickly. This slow creaming was attributed to the belief fat globules in goat milkfat are very much smaller than those found milk. However, the size of the fat globules, on an average is only smaller than those found in cow's milkfat. The apparent reason for creaming is the lack of a protein (agglutenating euglobulins) which individual fat globules to cluster and rise. This protein is found in cow's milk. Creaming at higher temperatures, where the rate of clustering is not as dependent on the protein, is probably somewhat related to the fat globule size. Secondly, it is often proposed that the apparent "small" globules in goat's milk render the fats more digestible. No evidence has been presented to substantiate that point of view.
A recent report has suggested that the biological value of products processed from caprine milk may be slightly higher than encountered in those made from bovine milk.
Structurally, the milk protein casein of the goat's milk is sufficiently different from that found in cow's milk to be easily differentiated in the lab. The casein miscelles typically exist either as much larger or much smaller aggregations than are found in bovine milk. Because of this it has been suggested that, although the quantity and distribution of amino acids in the casein fractions of the milks of the two species are similar, the sequency of assembly is almost certainly different. This difference is further substantiated by the fact that goat casein is associated with a lower mobility in an electrophoretic field. A similar difference appears to be found in the lactalbumin portion as well, with perhaps more clinical significance. The lactalbumin of bovine milk elicits an allergic response from many individuals, a serious problem, especially for young children. These individuals are often able to consume the milk of goats without suffering that reaction, an effect attributed to the dissimilarities in structure of the two proteins.
It must be realized that the concentrations of the various elements of the ash fraction demonstrate a wide variation, not only in response to the various points in the lactation cycle, but on a daily basis as well. Accurate evaluation entails the averaging of values obtained for a single animal over an extended time or using an average determined from samples taken from several different animals in the same herd on the same day.
Trace mineral analysis of both goat's milk and cow's milk are very similar in profile, only slight differences existing in the concentrations recorded for cobalt and molybdenum, differences associated with vitamin B, and xanthine oxidase levels respectively. The association of both cow's milk and goat's milk with infantile anemia appears to stem from low levels of iron and copper in these fluids, and the condition is easily reversed by the addition of those trace minerals to the diet.
The meaning of this difference is not entirely clear. Differences in B6 are uncertain when the recent USDA data are examined. Despite the fact that the concentrations of B6 and B12 are equal or exceed those concentrations found in human milk, anemia developed by infants and experimental animals is frequently attributed to deficiencies of these vitamins. However, the fact that the addition of copper and iron to the diet acts to eliminate the anemic condition removes much of the suspicion with which these levels are regarded. It has also been suggested that such an anemia could result from low levels of folic acid; however, the concentration of this vitamin does not differ significantly from that found in cow's milk.
It is remarkable that caprine milk derives its vitamin A potency entirely from the vitamin itself and entirely lacks the precursor carotenoid pigments characteristic of bovine milk, which also causes goat's milk and milkfat to be much whiter in color than the milk of the cow.
(1) Vitamin A expressed in International Units/liter; all others as mg/liter.
A factor of prime importance in milk production is the type and adequacy of the feed that goats use. One of the attractions of goats is that they can be maintained on pasture that would be marginal or inaccessible to other dairy animals, but such pastures are seldom sources of optimum diets for any animal and one might expect the quality of the milk to suffer. Surprisingly, the nutritive value of the goat's milk remains constant over a wide range of feeding conditions and similar concentrations for nutrients should be expected from most feeds. A diet of low quality does not assert itself, however, in causing a decrease in the amount of milk produced. The major controlling element in the diet in this event is the energy content of the feed.
GOAT'S MILK PRODUCTS
A major difficulty that must be faced by many small dairymen, and especially common with goat herds, is meeting government standards of sanitation for a commercial product. Many small operators have been forced out of the business because of the expense and time commitment involved in satisfying health standards, and now raise goats only for an avocation or for noncommercial distribution of their products.
GOATS IN THE FUTURE
|About the author: The assistance of R. Rodden in preparing this literature review is acknowledged. Dairy Goat Composition Source: John C. Bruhn, FST, UC Davis, Davis, CA 95616-8598|
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