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Newer Knowledge of Dairy Foods
Milk
Milk's Physical Characteristics and Constituents
Milk is a complex colloidal dispersion of fat globules and protein (casein, whey) in an aqueous solution of lactose, minerals, and other minor constituents. Milk's physical characteristics are affected by several factors including the composition and processing of milk. Measurement of milk's physical properties is used in processing, to determine the concentration of milk's components, and to evaluate the quality of milk products. The general physical characteristics of milk are presented in Table 14 (PDF 8k) . The following references provide additional information on the physical characteristics of cow's milk.
References
Chandan, R. Dairy-Based Ingredients . St. Paul, Minn.: Eagan Press, 1997.
Singh, H., O.J. McCarthy, and J.A. Lucey. Physico-chemical properties of milk. In: Advanced Dairy Chemistry Volume 3. Lactose, Water, Salts And Vitamins . 2nd ed. P.F. Fox (Ed.). New York: Chapman & Hall, 1997, p. 469.
Milk's Constituents
The major constituents of milk are illustrated in Figure 2 (PDF 212k) . Milk is made up of 87.4% water and 12.6% milk solids (3.7% fat, 8.9% milk solids-not-fat). The milk solids-not-fat contain protein (3.4%), lactose (4.8%), and minerals (0.7%).
ENERGY
- Milk's energy (calorie) content varies widely and depends mostly on its fat content. For example, whole milk (3.2% milk fat) provides 150 kcal/cup (8 oz.); 2% reduced fat milk provides 121 kcal/cup; 1% lowfat milk provides 104 kcal/cup; and nonfat (fat free, skim) milk provides 90 kcal/cup. The addition of nonfat milk solids, sugars, and other energy-yielding components also influence milk's calorie content. For the calorie content of specific milks refer to Table 13 (PDF 48k) and USDA's Nutrient Database.
- Milk is a nutrient-dense food providing a high concentration of nutrients in relation to its energy content.
Reference
U.S. Department of Agriculture, Agricultural Research Service. USDA Nutrient Database for Standard Reference, Release 12. Nutrient Data Laboratory. http://www.ars.usda.gov/main/main.htm.
PROTEIN
- Composition. Cow's milk is a heterogeneous mixture of proteins ( Table 15: PDF 8k ). About 80% of the total protein in milk is casein and 20% is whey protein. Milk also contains small amounts of various enzymes (e.g., lipoprotein lipase, alkaline phosphatase, lactoperoxidase) and traces of nonprotein nitrogenous compounds (e.g., ammonia, urea, creatinine, creatine, uric acid). For information on these minor constituents, refer to Jensen.
- Casein. Casein, the dominant protein in cow's milk, can be fractionated electrophorecally into four major components: alpha-, beta-, gamma-, and kappa- casein ( Table 15: PDF 8k ). Casein is generally defined as the protein precipitated at pH 4.6, a property used in the manufacturing of cheese.
- Whey. Whey protein, which is more heterogeneous than casein, consists predominantly of beta-lactoglobulin and alpha-lactalbumin. Alpha-lactalbumin has a high content of the amino acid tryptophan, a precursor of niacin. Because of milk's tryptophan content, this food is an excellent source of niacin equivalents. One niacin equivalent is defined as 1 mg of niacin or 60 mg of tryptophan. Other whey proteins present in smaller amounts are serum albumin, immunoglobulins (e.g., IgA, IgG, IgM), protease peptones, lactoferrin, and transferrin. Each of these proteins has unique characteristics.
- Beneficial Effects. Individual milk proteins have a wide range of beneficial health and functional effects. For example, antibacterial properties of peptides derived from bovine lactoferrin have been demonstrated. Also, limited evidence from in vitro and experimental animal studies indicates that milk proteins may protect against cancer. As reviewed by Parodi, whey proteins in particular appear to be anticarcinogenic, possibly as a result of their ability to enhance immunity. In addition, whey proteins have been demonstrated to increase bone strength in experimental animals. Whey protein concentrates and isolates are used as ingredients in a number of formulated food products.
- High-Quality Protein. Cow's milk is an excellent source of high-quality protein, providing varying amounts of all of the essential amino acids that humans cannot synthesize and in proportions resembling amino acid requirements ( Table 16: PDF 7k ). Only the sulfur amino acids (methionine plus cystine) in milk proteins are slightly limiting as compared with the adult's estimated requirement of essential amino acids. Cow's milk protein, which is rich in lysine, complements many plant proteins, which normally are limited in this amino acid. Also, because of its high quality, cow's milk protein is used as a standard reference protein to evaluate the nutritive value of food proteins. The quality of cow's milk protein is determined by any one of the several parameters indicated in Table 17 (PDF 8k).
References
Jensen, R.G. (Ed). Handbook of Milk Composition. New York: Academic Press, 1995.
Fox, P.F. (Ed.). Advanced Dairy Chemistry, Volume 1. Proteins . New York: Chapman and Hall, 1992.
European Dairy Association. Nutritional Quality of Proteins . Brussels, Belgium: European Dairy Association, 1997.
International Dairy Federation. Milk Protein Definition and Standardization . Brussels, Belgium: International Dairy Federation, 1994.
Chandan, R. Dairy-Based Ingredients . St. Paul, Minn.: Eagan Press, 1997.
Kurmann, J.A., J.L. Rasic, and M. Kroger. Encyclopedia of Fermented Fresh Milk Products. An Inventory of Fermented Milk, Cream, Buttermilk, Whey, and Related Products. New York: Van Nostrand Reinhold, 1992.
Dionysius, D.A., and J.M. Milne. Antibacterial peptides of bovine lactoferrin: purification and characterization. J. Dairy Sci. 80 : 667, 1997.
Takada, Y., H. Matsuyama, K. Kato, N. Kobayashi, J.-I. Yamomura, M. Yahiro, and S. Aoe. Milk whey protein enhances the bone breaking force in ovariectomized rats. Nutr. Res. 17 : 1709, 1997.
CARBOHYDRATES
- Lactose. The principal carbohydrate in milk is lactose, a natural disaccharide consisting of one galactose and one glucose unit. Lactose accounts for about 54% of the total solids-not-fat content of whole milk and about 30% of its calories (about 9% of the calories of 2% reduced fat milk). The lactose content of milk varies by species. Cow's milk contains about 4.8% lactose (12 to 12.5% lactose/cup), whereas human milk has 7% lactose (15 to 18 g lactose/cup). The higher concentration of lactose in human milk explains why lactose is used to enrich breast milk substitutes or infant formula. Refer to Table 10 (PDF 10k) for the lactose content of cow's milk and other dairy foods. The lower lactose content of cheeses is due to the removal of lactose-rich whey and the conversion of lactose to lactic acid by select microorganisms in cheese-making.
- Lactose in Infant Feeding and Calcium Absorption. In infants, some lactose enters the colon, where it promotes the growth of certain beneficial lactic acid-producing bacteria that may help reduce gastrointestinal disturbances resulting from undesirable putrefactive bacteria. In addition, lactose favors calcium and possibly phosphorus absorption in infants. However, there is no scientific evidence that lactose improves calcium absorption in adults.
- Other Carbohydrates. Minor quantities of glucose, galactose, and oligosaccharides are also present in milk. Glucose and galactose are the products of lactose hydrolysis by the enzyme lactase. Galactose may have a unique role in the rapidly developing infant brain.
- Lactose-reduced and Lactose-free Milks. These milk products are sweeter tasting than regular milk because most of the lactose in these products is hydrolyzed to glucose and galactose. For information on these products, refer to Dairy Products in Human Nutrition.
- Commercial Applications of Lactose. Lactose obtained from milk or whey is used in infant formula, dry mixes, confections, drinks, candy, and dairy beverages. For more information on the use of lactose, refer to Chandan.
References
Jensen, R.G. (Ed). Handbook of Milk Composition. New York: Academic Press, 1995.
Filer, L.J., and W.A. Reynolds. Lessons in comparative physiology: lactose intolerance. Nutr. Today 32 : 79, 1997.
Miller, G.D., J.K. Jarvis, and L.D. McBean. Handbook of Dairy Foods and Nutrition. 2nd ed. Boca Raton, Fla.: CRC Press, 1999.
American Academy of Pediatrics, Committee on Nutrition. Pediatric Nutrition Handbook. 4th ed. Elk Grove Village, Ill.: American Academy of Pediatrics, 1998.
Ziegler, E.E., and S.J. Fomon. Lactose enhances mineral absorption in infancy. J. Pediatr. Gastroenterol. Nutr. 2 : 288, 1983.
Nickel, K.P., B.R. Martin, D.L. Smith, J.B. Smith, G.D. Miller, and C.M. Weaver. Calcium bioavailability from bovine milk and dairy products in premenopausal women using intrinsic and extrinsic labeling techniques. J. Nutr. 126 : 406, 1996.
Chandan, R. Dairy-Based Ingredients. St. Paul, Minn.: Eagan Press, 1997.
FATS OF MILK (INCLUDING FAT-SOLUBLE VITAMINS)
Milk fat is a natural fat with unique physical, chemical, and biological properties. This fat contributes to the appearance, texture, flavor, and satiability of dairy foods. Milk fat is a source of energy, essential fatty acids, fat-soluble vitamins, and several other potential health-promoting components.
- Composition. Milk fat, the most complex of lipids, exists in microscopic globules in an oil-in-water emulsion in milk. Milk's lipids are mainly triacylglycerols (triglycerides) or esters of fatty acids with glycerol (97-98%), 0.2 to 1.0% phospholipids, 0.2 to 0.4% free sterols (cholesterol, waxes, and squalene, an intermediate of cholesterol), traces of free fatty acids, and varying amounts of the fat-soluble vitamins A, D, E, and K ( Table 18: PDF 7k ).
- Fatty Acids. More than 400 different fatty acids and fatty acid derivatives are found in milk, ranging from butyric acid with 4 carbon atoms to fatty acids with 26 carbon atoms. Milk fat is characterized not only by numerous different fatty acids, but also by their chain lengths. Among animal fats, milk fat is unique because it contains a relatively high proportion of short-chain and medium-chain saturated fatty acids, many of which are not found in other fats. Milk fat contains about 7% short-chain fatty acids (C4 to C8), 15 to 20% medium-chain fatty acids (C10 to C14), and 73-78% long-chain fatty acids (C16 and higher). Although the composition of milk fat varies according to such factors as the breed of the cow and composition of the feed, the fatty acids in milk fat are approximately 65% saturated, 32% monounsaturated, 3% polyunsaturated, with minor amounts of other types of fatty acids. Refer to USDA's Nutrient Database for the fatty acid composition of specific milks and milk products.
- Saturated Fatty Acids. The saturated fatty acids present in the largest amount in milk fat are myristic (14:0), palmitic (16:0), and stearic (18:0). Although, in general, saturated fatty acids raise blood cholesterol levels, individual saturated fatty acids differ in their blood cholesterol-raising effects. Long-chain saturated fatty acids such as lauric (12:0), myristic (14:0), and palmitic (16:0) acids may raise blood cholesterol levels. In contrast, stearic acid (18:0) and short- and medium-chain saturated fatty acids such as butyric (4:0), caproic (6:0), caprylic (8:0), and capric (10:0) acids either have a neutral effect or may lower blood cholesterol levels.
- Unsaturated Fatty Acids. The major unsaturated fatty acids are oleic (18:1), linoleic (18:2), and linolenic (18:3) acids. Oleic acid is the main monounsaturated fatty acid in milk fat. Polyunsaturated fatty acids such as arachidonic acid (20:4) are present in trace amounts in milk fat. Arachidonic acid and its precursor, linoleic acid, are essential fatty acids. Linoleic acid is present in milk fat in a form that favors conversion to arachidonic acid. Omega-3-linoleic acid and its products, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are found in small, but significant, amounts in milk fat. Trans fatty acids (e.g., vaccenic acid) make up about 3% of milk's total fat. Dairy foods contribute less than 0.5 g of trans fatty acids per day (i.e., 0.2 g/day from milk and 0.1 g/day from butter). Average intake of total trans fatty acids per day is 5.3 g. Conjugated linoleic acid (CLA) is a collective term for a mixture of isomers of linoleic acid. The major CLA isomer in milk fat is cis-9, trans-11-octadecadienoic acid (i.e., rumenic acid).
- Cholesterol. Cholesterol is a normal constituent in cow's milk, although this food contains relatively little cholesterol (less than 0.5% of milk fat). The concentration of cholesterol in milks and milk products is related to the fat content of these foods ( Table 13: PDF 48k ). A one-cup serving (8 fluid ounces) of whole, 2% reduced fat, or nonfat (skim) milk contains 33 mg, 18 mg, or 4 mg cholesterol, respectively. Refer to USDA's Nutrient Database for the cholesterol content of milks and other dairy foods.
- Other Milk Fat Components. Milk fat contains several components such as conjugated linoleic acid (CLA), sphingomyelin, butyric acid, and myristic acid that are being investigated for their potential health benefits. Milk fat is the richest natural dietary source of CLA, averaging about 5.5 mg CLA/g. Table 19 (PDF 6k) presents the CLA content of select dairy foods. For more information, refer to Chin, et al. (1992) and Lin, et al. (1995). The CLA content of dairy foods is influenced by a variety of factors, such as the CLA content of the starting milk and the final fat content, the protein content, and processing procedures. Increasing the polyunsaturated fat content of cows' feed increases the CLA content of their milk fat. Sphingomyelin, the most common sphingolipid, is present in cow's milk at a concentration of 0.1 mg/ml, or about 0.2 to 1.0% of milk's total lipids, or about one-third of milk's total phospholipids. Various factors such as the season and cow's stage of lactation can influence the sphingomyelin content of milk fat. The sphingolipid content of milk and other dairy foods is found in Table 20 (PDF 6k) . The presence of butyric acid (4:0), at a level of more than 3% of the major fatty acids in milk fat, is a unique feature of milk fat. No other common food contains this short-chain fatty acid. Another fatty acid of potential health benefit in milk fat is myristic acid (14:0), a saturated fatty acid accounting for about 10% of milk fat.
- Fat-soluble Vitamins. Vitamins A, D, E, and K are associated with the fat component of milk. Refer to Table 21 (PDF 13k) for information on their content in cow's milk.
References
German, J.B., and C.J. Dillard. Fractionated milk fat: composition, structure, and functional properties. Food Technol. 52 : 33, 1998.
Jensen, R.G. (Ed.). Handbook of Milk Composition. New York: Academic Press, 1995.
Fox, P.F. (Ed.). Advanced Dairy Chemistry . Volume 2. Lipids. 2nd ed. New York: Chapman and Hall, 1995.
Wong, N.P., R. Jenness, M. Keeney, and E.H. Marth. Fundamentals of Dairy Chemistry. 3rd ed. New York: Van Nostrand Reinhold Co., 1988.
U.S. Department of Agriculture, Agricultural Research Service. USDA Nutrient Database for Standard Reference, Release 12 . Nutrient Data Laboratory, 1998. http://www.ars.usda.gov/main/main.htm
Katan, M.B., P.L. Zock, and R.P. Mensink. Dietary oils, serum lipoproteins, and coronary heart disease. Am. J. Clin. Nutr. 61(suppl) : 1368s, 1995.
ASCN/AIN Task Force on Trans Fatty Acids. Position paper on trans fatty acids. Am. J. Clin. Nutr. 63 : 663, 1996.
Allison, D.B., K. Eagan, L.M. Barraj, C. Caughman, M. Infante, and J.T. Heimbach. Estimated intakes of trans fatty and other fatty acids in the US population. J. Am. Diet. Assoc. 99 : 166, 1999.
Parodi, P.W. Distribution of isomeric octadecenoic fatty acids in milkfat. J. Dairy Sci. 59 : 1870, 1976.
Parodi, P.W. Cow's milk fat components as potential anticarcinogenic agents. J. Nutr. 127 : 1055, 1997.
Chin, S.F., W. Liu, J.M. Storkson, Y.L. Ha, and M.W. Pariza. Dietary sources of conjugated dienoic isomers of linoleic acid, a newly recognized class of anticarcinogens. J. Food Comp. Anal. 5 : 185, 1992.
Lin, H., T.D. Boylston, M.J. Chang, L.O. Luedecke, and T.D. Shultz. Survey of the conjugated linoleic acid contents of dairy products. J. Dairy Sci. 78 : 2358, 1995.
Shantha, N.C., L.N. Ram, J. O'Leary, C.L. Hicks, and E.A. Decker. Conjugated linoleic acid concentrations in dairy products as affected by processing and storage. J. Food Sci. 60(4) : 695, 720, 1995.
Kelly, M.L., J.R. Berry, D.A. Dwyer, J.M. Griinari, P.Y. Chouinard, M.E. Van Amburgh, and D.E. Bauman. Dietary fatty acid sources affect conjugated linoleic acid concentrations in milk from lactating dairy cows. J Nutr. 128 : 881, 1998.
Vesper, H., E.-M. Schmelz, M.N. Nikolova-Karakashian, D.L. Dillehay, D.V. Lynch, and A.H. Merrill, Jr. Sphingolipids in food and the emerging importance of sphingolipids to nutrition. J. Nutr. 129 : 1239, 1999.
Fox, P.F. (Ed.). Advanced Dairy Chemistry, Volume 3, Lactose, Water, Salts, and Vitamins . 2nd ed. New York: Chapman and Hall, 1997.
Food and Nutrition Board, Institute of Medicine, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, D.C.: National Academy Press, 1997.
Food and Nutrition Board, Commission on Life Sciences, National Research Council. Recommended Dietary Allowances, 10th Edition. Washington, D.C.: National Academy Press, 1989.
U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration. Grade "A" Pasteurized Milk Ordinance, 1997 Revision. PHS/FDA Pub. No. 229. Washington, D.C.: USDHHS, PHS, FDA, 1997.
Berdanier, C.D. Advanced Nutrition. Micronutrients. Boca Raton, Fla.: CRC Press, 1998.
WATER-SOLUBLE VITAMINS OF MILK
Milk and other dairy foods contain all of the water-soluble vitamins in varying amounts required by humans. Refer to Table 22 (PDF 13k) for the average amounts of the water-soluble vitamins in milk. For more information, including a discussion of factors affecting the content of water-soluble vitamins in milk, refer to Fox (1997) and Jensen (1995).
References
Fox, P.F. (Ed.). Advanced Dairy Chemistry, Volume 3, Lactose, Water, Salts, and Vitamins. 2nd ed. New York: Chapman and Hall, 1997.
Jensen, R.G. Handbook of Milk Composition. New York: Academic Press, 1995.
Berdanier, C.D. Advanced Nutrition. Micronutrients. Boca Raton, Fla.: CRC Press, 1998.
U.S. Department of Agriculture, Agricultural Research Service. USDA Nutrient Database for Standard Reference, Release 12 . Nutrient Data Laboratory, 1998. http://www.ars.usda.gov/main/main.htm
Food and Nutrition Board, Institute of Medicine, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, D.C.: National Academy Press, 1998.
Food and Nutrition Board, Commission on Life Sciences, National Research Council. Recommended Dietary Allowances, 10th Edition. Washington, D.C.: National Academy Press, 1989.
MINERALS, TRACE ELEMENTS, AND SALTS IN MILK
Minerals are generally classified into two groups according to the amounts needed in the daily diet. The first group is composed of major minerals such as calcium, phosphorus, and magnesium, which are needed at levels of 100 mg or more daily. The second group consists of trace elements such as iron, zinc, iodine, copper, manganese, selenium, and fluoride, which are needed at levels of a few milligrams daily. Cow's milk also contains salts or electrolytes such as chloride, potassium, and sodium. Table 23 (PDF 12k) presents the content of major minerals, trace elements, and salts in milk.
In addition to the above, cow's milk contains a number of elements in miniscule amounts, such as aluminum, barium, boron, bromine, chromium, cobalt, fluoride, lithium, molybdenum, nickel, rubidium, silicon, strontium, tin, titanium, and vanadium. In general, few studies have analyzed these elements in milk. For more information on minerals, trace elements, and salts in cow's milk, refer to Jensen (1995).
References
Berdanier, C.D. Advanced Nutrition. Micronutrients. Boca Raton, Fla.: CRC Press, 1998.
Jensen, R.G. Handbook of Milk Composition. New York: Academic Press, 1995.
Fox, P.F. (Ed.). Advanced Dairy Chemistry, Volume 3, Lactose, Water, Salts, and Vitamins. 2nd ed. New York: Chapman and Hall, 1997.
U.S. Department of Agriculture, Agricultural Research Service. USDA Nutrient Database for Standard Reference, Release 12. Nutrient Data Laboratory, 1998. http://www.ars.usda.gov/main/main.htm
Food and Nutrition Board, Institute of Medicine, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride . Washington, D.C.: National Academy Press, 1997.
Food and Nutrition Board, Commission on Life Sciences, National Research Council. Recommended Dietary Allowances, 10th Edition. Washington, D.C.: National Academy Press, 1989.
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