Lipids And Proteins In Milk, Particularly Goat Milk
Among all foods, milk is the most complete and most balanced in nutritional contents. The presence of similar content levels of protein, fat and carbohydrates gives milk unique indestructibility over any period of time. Therefore, milk does not spoil nor become inedible or harmful; it only changes from sweet to sour, from liquid to increasingly solid, but retains very acceptable food qualities in the form of sour milk, yogurt and cheeses.
These principles of milk hold regardless of which mammalian species produced it, but there are unique differences in qualities and quantities of individual milk nutrients between animal species, especially comparing monogastric with ruminant animals. Milk from three animal species are of particular interest in human nutrition: milk from cows, goats and humans. It should not surprise that milk from the first two are both more different qualitatively and quantitatively from human milk than between them, because of the difference between monogastric and ruminant physiology and digestion. Research on goat milk has been much less than on milk of the other two species, but distinct differences are becoming more and more known, which mean different consequences in processing for yogurt and cheeses, and in human digestion and health.
Among the nutrients in milk, fats and proteins show the greatest variability in what components they consist of, while carbohydrates are represented mainly by the one component--lactose. Other differences in milk between species are also known for minerals, vitamins, enzymes and minor components, but this discussion shall focus on the fats and proteins.
Scientifically, fats are called lipids, which include mostly the large group of different triglycerides besides cholesterol and other minor lipids. Triglycerides are very interesting nutritionally, because of their wide range of different physical and chemical properties. They can be solid or soft or liquid at room temperature depending on what their component fatty acids are. They can also be influenced greatly by feeds, which make up the daily ration of the milk-producing cow, goat or human.
Fat in milk occurs as small fat droplets of different diameters. Between different goat or cow species as well as between goats and cows, considerable differences exist in the frequency distribution of small and large droplets of milkfat, which provides different degrees of a kind of naturally homogenized milkfat in some milks, mostly for genetic reasons. The greater amount of small fat droplets in milk of many goat breeds compared to many cow breeds has been long a point of emphasis, when goat and cow milk have been discussed for human nutrition and digestibility. Actually, even more important is the chemical composition within the fat droplets.
Fat is predominantly composed of triglycerides, which consist of one molecule of glycerol, to which three molecules of fatty acids are attached. There are more than 200 different fatty acids found in triglycerides of milk fats (Renner, 1982). Fatty acids are of different length, identified by their number of carbon atoms, which are connected by single bonds, called saturated, or by double bonds, called unsaturated, and they can be straight chains or branched and more complex in shape and configuration. Among the multitude of fatty acids and their combinations making up triglycerides, 15 predominate, but the presence of each may vary quantitatively over a wide range depending on influence of feeding, season or body fats being mobilized (Table 1). These major fatty acids have mostly even-numbered carbon chains and only a few are unsaturated, indicated by ":" for the number of double bonds. Some are even essential in human nutrition.
The different lengths fatty acids undergo different routes in digestion and metabolism, when milk and its fat is eaten, with the short and medium chain triglycerides (MCT), up to C14, not being incorporated into body lipids mostly, in contrast to the longer chain fatty acids. Thus, the short chain fatty acids do not contribute to obesity like the long chain fatty acids do, nor to related problems of heart disease (Greenberger and Skillman, 1969).
Capric, caprylic and other MCT have become established treatment (Babayan, 1981) in a variety of malabsorption syndrome cases of patients suffering from chyluria, steatorrhea, hyperlipoproteinemia, also in cases of intestinal resection, coronary bypass, premature-infant feeding, childhood epilepsy, cystic fibrosis and gallstones; and they have been reported to be lowering serum cholesterol, inhibiting and limiting cholesterol deposition in tissues, dissolving cholesterol gallstones, and correcting unthriftiness in growing children (Kalser, 1971; Nutting et al. 1991; White et al. 1991). Goat milkfat with its normally higher contents of short chain fatty acids and MCT appears to have distinct advantages for human nutrition and health, which have gone unexploited so far (Haenlein, 1992). Contrary to current fear of fat in human diet, this kind of fat from goat milk may even have special health and diet advantages.
While fats in human diet have had close scrutiny in recent years, milk proteins only were favored for more emphasis in marketing milk. Yet a not uncommon allergy affliction in some people incriminates a certain protein, more in cow milk, sometimes in soybean milk formula or even in goat milk. A lack of research in this area is responsible for a widespread ignorance as to which protein is responsible and how it acts (Breneman, 1978; Podleski, 1992).
A more detailed knowledge of the different proteins in milk is therefore very desirable, especially since there is also widespread confusion of milk protein allergy with lactose intolerance, which is a separate problem from allergies to milk proteins, has different symptoms and can be alleviated today very easily with the supplementation of lactase enzymes.
The five principal protein groups in milk fall into two major categories by their chemical behavior towards acid or rennet enzyme. Caseins are precipitated from fluid milk at pH 4.6 and 20 degree Celsius or by the addition of rennet without pH change. Caseins average 78 percent of the total milk proteins with a range from 55-86 percent (Renner, 1982). Mastitis will lower the proportion of casein in total proteins of milk drastically. The other category of milk proteins, called whey proteins, remains dissolved in the whey after casein precipitation, consisting mainly of beta-lactoglobulins, alpha-lactalbumins, serum albumin, immunoglobulins and proteose-peptides, besides minor components with also nitrogen content.
Total protein contents average for goat milk 3.56 percent, cow milk 3.29 percent, but human milk 1.03 percent (USDA, 1990). Proteins are composed of a chain of amino acids with very different physical and chemical properties, which explains the different effects of the different proteins in human digestion and metabolism. Already the difference in only one amino acid can make a great difference, how this protein acts and reacts, even in processing for cheeses (Imafidon et al. 1991). This explains why apparently only small changes in amino acid sequence due to genetic mutation or selection can alter the protein's behavior (Gravert, 1990). The more important 18 different amino acids make up an enormous multitude of different protein variants as the amino acids are chained together (Table 3). The first 8 amino acids are essential in human nutrition. The sequence of many cow and human milk proteins has been identified, while goat milk proteins need more research in order to aid better in the problems with allergy, other health concerns and nutrition (Karjalainen et al. 1992).
Many milk proteins have genetic variants which have been identified by letters of the alphabet. Caseins consist of alpha,
beta and kappa types with about 27 genetic variants from A to E for each. The whey proteins also have about 11 different genetic variants (Ehrmann et al. 1993). This gives milk proteins considerable variation between animal species, which can be exploited for processing and human nutrition (Table 4).
UNIQUENESS OF GOAT MILK
Quantitatively, goats producing milk are in a distinct disadvantage compared to cows, which has serious consequences economically. Unless there are other reasons for milking goats, a justification for their keeping may be questioned. However, goats and cows do not only differ anatomically considerably, they also have distinct differences in their physiology and biochemical metabolism (Haenlein, 1980a; 1992a), which gives their presence and husbandry a unique justification. They differ in water requirements, basic metabolism, reproduction, feed preferences, endocrinology, digestion, milk secretion process, milk composition and even muscle (meat) composition.
Jenness' (Parkash and Jenness, 1968; Jenness, 1980) comprehensive work has probably been more quoted than any other references, such as the total absence in goat milk compared to cow milk of alpha-s-1-casein, beta-carotene, agglutinin; lower contents of citric acid, sodium, iron, sulphur, zinc, molybdenum, ribonuclease, alkaline phosphatase, lipase, xanthine oxidase, N-acetylneuraminic acid, orotic acid, pyridoxine, folate, vitamin B12, vitamin C, lower freezing point and pH; while on the other hand higher contents of calcium, potassium, magnesium, phosphorus, chlorine, manganese, vitamin A, vitamin D, nicotinic acid, choline, inositol, medium-chain length fatty acids, small diameter fat globules, and somatic cell counts (Droke et al. 1993). Substantial basic differences between goats and cows have been reported for mineral metabolism (Lengemann, 1970; Haenlein, 1980b; 1991a; 1992b; Ademosun et al. 1992). Furthermore, the lower content of orotic acid in goat milk can be important in the prevention of fatty liver syndromes (Robinson, 1980); the more fragile fat globule membrane in goat milk is important for the control of off-flavors (Patton et al. 1980; Bakke et al. 1977); and the higher glycerol ethers in goat milk are valuable for the nursing newborn (Ahrne et al. 1980).
For milk lipids, it is well established that there is a species difference between cows and goats in the way that acetate furnished by rumen bacteria is used in the mammary gland synthesis of milk fats having significantly more medium-chain length fatty acids (Parkash and Jenness, 1968). There also appears to be a unique difference between goats and cows in the way fatty acids are selected metabolically for attachment to the glycerol molecule in fat synthesis (Jenness, 1980).
For milk proteins, the beta-caseins seem to be more dominant than alpha-caseins compared to cow milk (Jenness, 1980). However, more recent evidence from France and Italy has proven that the previously assumed general absence of alpha-s-1 casein in goat milk is not true (Boulanger et al. 1984; Ambrosoli et al. 1988; Mora-Gutierrez et al. 1991; Haenlein, 1991b). It is now recognized that certain goat breeds and strains within breeds may have either no alpha-s-1 casein or low or high amounts, depending on genetic types. Low amounts also have shorter cheese coagulation time, less curd firmness, less cheese yield and weaker resistance to heat treatments, which can also be related to digestibility in human nutrition.
Ademosun, A.A., Bosman, H.G., Haenlein, G.F.W. and Adebowale, E.A. 1992. Recent advances in nutrient requirements of goats. Proc. Fifth Intern. Conf. Goats, New Delhi, India, ICAR,
Ahrne, L., Bjoerck, L., Raznikiewicz, T. and Claesson, O. 1980. Glycerol ether in colostrum and milk from cow, goat pig, and sheep. J. Dairy Sci. 63;741-745.
Ambrosoli, R., Di Stasio, L. and Mazzocco, P. 1988. Content of alpha-s-1 casein and coagulation properties in goat milk. J. Dairy Sci. 71:24-28.
Babayan, V.K. 1981. Medium chain length fatty acid esters and their medical and nutritional applications. J. Amer. Oil Chem. Soc. 59:49-50A.
Bakke, H., Steine, S.T. and Eggum, A. 1977. Flavor score and content of free fatty acids in goat milk. Acta Agric. Scand. 27:245-249.
Boulanger, A., Grosclaude, F. and Mahe, M.F. 1984. Polymorphisme des caseines alpha-s-1 et alpha-s-2 de la chevre (Capra hircus). Genet. Sel. Evol. 16:157-175.
Breneman, J.C. 1978. Basics of Food Allergy. Ch. C. Thomas Publ., Springfield, Il.
Droke, E.A., Paape, M.J. and Di Carlo, A.L. 1993. Prevalence of high somatic cell counts in bulk tank goat milk. J. Dairy Sci. 76:1035-1039.
Ehrmann, S., Wagner, V. and Geldermann, H. 1993. Milk proteins: Can variants improve performance? Der Tierzuechter, 3/93:44-47.
Gravert, H.O. 1990. Genomanalysis and milk quality. University Kiel, Agric. Coll., Kiel, Germany, In: Bulletin 72:147-154.
Greenberger, N.J. and Skillman, T.G. 1969. Medium chain triglycerides. Physiologic considerations and clinical implications. New England J. Med. 280:1045-1058.
Haenlein, G.F.W. 1980a. Goats: Are they physiologically different from other domestic food animals? Intern. Goat and Sheep Res. 1:173-175.
Haenlein, G.F.W. 1980b. Mineral nutrition of goats. J. Dairy Sci. 63:1729-1748.
Haenlein, G.F.W. 1991a. Advances in the nutrition of macro- and microelements in goats. Proc. VII Reunion Nac. sobre Caprinocultura, Univ. Monterey, Mexico, 290-320.
Haenlein, G.F.W. 1991b. Recent research on milk protein polymorphism and related items. Proc. 8th Ann. Mtg. Amer. Cheese Soc., San Francisco, 6 pp.
Haenlein, G.F.W. 1992a. Role of goat meat and milk in human nutrition. Proc. Vth Intern. Conf. Goats, New Delhi, India, ICAR, II(2):575-580.
Haenlein, G.F.W. 1992b. Recent advances in mineral nutrition of small ruminants. Proc. Nat. Seminar Small Ruminant Production in India by the Year 2000, Andhra Pradesh Agr. Univ., Tirupati (A.P.), India, 60-72.
Imafidon, G.I., Ng-Kwai-Hang, K.F., Harwalkar, V.R. and Ma, C.-Y. 1991. Effect of genetic polymorphism on the thermal stability of beta-lactoglobulin and kappa-casein mixture. J. Dairy Sci. 74:1791-1802.
Jenness, R. 1980. Composition and characteristics of goat milk: Review 1968-1979. J. Dairy Sci. 63:1605-1630.
Kalser, M.H. 1971. Medium chain triglycerides. Adv. Intern. Med. 17:301-322.
Karjalainen, J., Martin, J.M., Knip, M., Ilonen, J., Robinson, B.H., Savilahti, E., Akerblom, H.K. and Dosch, H.M. 1992. A bovine albumin peptide as a possible trigger of insulin- dependent diabetes mellitus. New England J. Med. 327:302- 307.
Lengemann, F.W. 1970. Metabolism of radioiodine by lactating goats given iodine-131 for extended periods. J. Dairy Sci. 53:165-170.
Mora-Gutierrez, A., Kumosinski, T.F. and Farrell, H.M.Jr. 1991. Quantification of alpha-s-1 casein in goat milk from French- Alpine and Anglo-Nubian breeds using reversed-phase high performance liquid chromatography. J. Dairy Sci. 74:3303- 3307.
Nutting, C.W., Islam, S. and Daugirdas, J.T. 1991. Vasorelaxant effects of short chain fatty acid salts in rat caudal artery. Amer. J. Physiol. 261:H561-567.
Parkash, S. and Jenness, R. 1968. The composition and characteristics of goats' milk: A review. Dairy Sci, Abstr. 30:67-87.
Patton, S., Long, C. and Sokka, T. 1980. Effect of storing milk on cholesterol and phospholipid of skim milk. J. Dairy Sci. 63:697-700.
Podleski, W.K. 1992. Milk protein sensitivity and lactose intolerance with special reference to goat milk. Proc. Vth Intern. Conf. Goats, New Delhi, India, ICAR, II(2):610-613.
Renner, E. 1982. Milk and Milk Products in Human Nutrition. Volkswirtsch. Verlag, Munich, 467 pp.
Robinson, J.L. 1980. Bovine milk orotic acid: Variability and significance for human nutrition. J. Dairy Sci. 63:865-871.
USDA. 1990. Composition of Foods; Dairy and Egg Products. USDA- ARS, Washington, D.C., Agr. Handbook No. 8-1.
White, R.P., Ricca, G.F., El-Bauomy, A.M. and Robertson, J.T. 1991. Identification of capric acid as a potent vasorelaxant of human basilar arteries. Stroke 22:469-476.
MCT = medium chain length triglycerides; SAT.= saturated;
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