Dairy Goat Milk Composition
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.
Goat's milk derives many of its most distinctive properties from its lipid fraction. The average total fat content in the milk is similar to that found in other ruminant species, (Table 1) despite reports that the percentage of fat in goat's milk exceeds that of the cow. Such a misconception is most likely derived from the fact that the average percentage of milkfat, as with cow's milkfat, is a variable component, often ranging between 3.0 and 6.0 percent.
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.
The protein fraction of the milk of the goat shows a remarkable similarity to that found in other ruminant species, both in amount and in composition with respect to the specific amino acids. The relative percentage of protein (Table 1) is similar in both the cow and the goat despite past assertions that the protein content of goat's milk is lower. Such reports are likely the result of the wide range which has existed in reported values. This variation in range is due in part to a lack of standardization of protein testing procedures as well as the wide differences encountered among animals accepted as the same breed and interbreed differences. It is expected that current emphasis on standardization of protein testing procedures and efforts to improve goat breed designation will result in more accurate assessment.
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.
Lactose is the major free carbohydrate that has been identified in the milk of the goat, though small amounts of inositol are also found (Table 1). The lactose concentration is usually found to be lower than that found in cow's milk, but the magnitude of the difference is hard to quantify because of the variation in methods of analysis employed. A consensus has not been developed on whether to analyze for lactose in the non-hydrated form or the mono-hydrated form, and this water of hydration is capable of introducing a five percent variation in the reported concentration of the same actual amount of lactose. Efforts are being taken to reduce this confusion.
The total ash (calcium, phosphorous, etc.) content of goat's milk ranges from 0.78 mg to 0.83 mg per 100 g milk and is considered to be slightly higher than that associated with the cow (Table 1). However, the relative percentages of the ash components appear to be comparable (Table 3). As the nutritional value of milk is often evaluated in terms of the calcium and phosphorous that the milk makes available, it is important to note that the concentrations of these two minerals are similar in the cow and the goat. A significant variation between the milks of the two species should be noted in the chloride concentration, which appears to run higher in the goat. While this elevation may perhaps be physiological in origin, it seems likely that it may also stem from the influence of infectious mastitis which causes the salt (sodium chloride) concentration to increase which is endemic to many small goat herds. Significant amounts of potassium, sodium and magnesium are also reported in caprine milk, their concentrations paralleling those found in bovine samples. While few assays have been completed on the citrate in goat's milk, indications are that the citrate level is little different than that found in cow's milk. (Citrate is an important precursor to flavor components in cultured dairy foods.)
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 enzymes of the milk of the goat are similar to those of the cow, although some specific differences have been described. Of primary interest, it has been shown that the level of alkaline phosphatase is slightly lower than that found in work with dairy cattle, but the enzyme demonstrates the same degree of heat susceptibility and therefore serves equally well as a pasteurization marker. Peroxidase activity in the milk of both species is the same in all respects, while the xanthine oxidase level is lower in the milk of the goat. Higher levels of activity are observed for both ribonuclease and lysozyme.
The vitamin contentof goat's milk has been the subject of considerable study. Goat's milk differs from cow's milk in its much lower content of B12. (Table 4).
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.
TABLE 4. Average vitamin content of goat, cow and human milk.
|Vitamin A(1)(2)||1560.0 (1380)||2074.0 (1850)||1898.0 (2410)|
|Thiamine||0.44 ( .38)||0.40 ( .48)||0.16 (0.14)|
|Riboflavin||1.75 (1.61)||1.84 (1.38)||0.36 ( .36)|
|Nicotinic Acid||0.94 ( .84)||1.87 (2.7)||1.47 (1.77)|
|Vitamin B 6||0.64 ( .42)||0.07 ( .46)||0.10 ( .11)|
|Pantotheine||3.46 (3.13)||3.44 (3.1)||1.84 (2.23)|
|Folic Acid||0.0028 ( 0.005)||0.0024 (0.001)||0.0020 (0.005)|
|Vitamin B 12||0.0043 (.0036)||0.0006 (.00065)||0.0003 (,.00045)|
|Ascorbic Acid||21.1 (14.7)||15.0(13,0)||43.0 (50)|
Seasonal variations characterize the concentrations of several of the constituents of goat's milk in a manner analogous to that observed in dairy cattle. Such variations include fluctuations in the amounts of fat, SNF, and minerals (particularly calcium and phosphorous) and follow a pattern in which concentrations are highest in the colostrum and decline to minima in the third or fourth month of lactation and slowly increase therafter. As freshening among goats in temperature zones typically occurs in the winter or early spring, the minima are associated with summer milk production. While the same phenomenon is observed in bovine milk production, the magnitude of the fluctuation in that species does not appear to be as great. In addition to these seasonal variations, the milk of goats is found to have significant daily variation associated with almost every component. These daily fluctuations become especially pronounced after the fourth month of lactation when minimum concentrations have been passed.
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
Goat's milk lends itself to use in the same types of products that are considered for cow's milk, and perhaps surpasses the cow's milk in some categories. Major amounts of goat's milk are processed annually into dried milk, evaporated milk, cheese and yogurt as well as being sold as bottled whole milk. In 1965, 1,300,000 gallons were used in the production of evaporated and canned products. The use of goat's milk in the production of cheese has become very widespread in France. This use is largely based on the capacity of goat milk curd to be frozen and produce a product not only equal but often considered better in flavor than one in which freezing did not take place. This process is normally accomplished without using salt and the curd can be held for up to six months at 5°F.
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
As can be seen from the foregoing analysis goat dairying is an enterprise that is capable of producing a product that rivals the more customary fluid, cow's milk, in most respects, and yet is far less intense in its energy and land use requirements. These characteristics make it an attractive avenue for many individuals who would like to become involved with dairying, but face any of a number of restrictions. A major difficulty faced by the industry has been the lack of a definite compositional profile for goat's milk, without which it has been difficult to compare the milk with that of the cow and thus demonstrate its nutritive value. What information that has been available has often complicated this problem because of the wide range of reported values. The Dairy Herd Improvement Program which is particularly active in California, is making large studies in eliminating this problem by delineating procedures for participants in the program. It is hoped that the information gathered in this program will not only provide information on the nutritional values typical in goat herds in the Western United States, but will also create more rigid controls on the various breeds of goats and clarify differences among these breeds.