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Overview

Milk is an opaque white liquid produced by the mammary glands of female mammals (including monotremes). Mammary glands are highly specialized sweat glands. The female ability to produce milk is one of the defining characteristics of mammals. It provides the primary source of nutrition for newborns before they are able to digest other types of food. The early lactation milk is known as colostrum, and carries the mother's antibodies to the baby. It can reduce the risk of many diseases in the baby. Males of all mammal species retain the breasts that are part of the fundamental mammalian animal structure, hence their nipples. Lactation occurs in males in certain rare circumstances, both naturally and artificially, however, some pharmaceuticals precipitate lactation in males readily. The exact components of raw milk varies by species, but it contains significant amounts of saturated fat, protein and calcium as well as vitamin C.

Physical and chemical structure

Milk is an emulsion of butterfat globules within a water-based fluid. Each fat globule is surrounded by a membrane consisting of phospholipids and proteins; these emulsifiers keep the individual globules from joining together into noticeable grains of butterfat and also protect the globules from the fat-digesting activity of enzymes found in the fluid portion of the milk. In unhomogenized cow's milk, the fat globules average about four micrometers across. The fat-soluble vitamins A, D, E, and K are found within the milkfat portion of the milk (McGee 18).

The largest structures in the fluid portion of the milk are casein protein micelles: aggregates of several thousand protein molecules, bonded with the help of nanometer-scale particles of calcium phosphate. Each micelle is roughly spherical and about a tenth of a micrometer across. There are four different types of casein proteins, and collectively they make up around 80 percent of the protein in milk, by weight. Most of the casein proteins are bound into the micelles. There are several competing theories regarding the precise structure of the micelles, but they share one important feature: the outermost layer consists of strands of one type of protein, kappa-casein, reaching out from the body of the micelle into the surrounding fluid. These Kappa-casein molecules all have a negative electrical charge and therefore repel each other, keeping the micelles separated under normal conditions and in a stable colloidal suspension in the water-based surrounding fluid[1] (McGee 19–20).

Both the fat globules and the smaller casein micelles, which are just large enough to deflect light, contribute to the opaque white color of milk. The fat globules contain some yellow-orange carotene, enough in some breeds — Guernsey and Jersey cows, for instance — to impart a golden or "creamy" hue to a glass of milk. The riboflavin in the whey portion of milk has a greenish color, which can sometimes be discerned in skim milk or whey products (McGee 17). Fat-free skim milk has only the casein micelles to scatter light, and they tend to scatter shorter-wavelength blue light more than they do red, giving skim milk a bluish tint.[2]

Milk contains dozens of other types of proteins besides the caseins. They are more water-soluble than the caseins and do not form larger structures. Because these proteins remain suspended in the whey left behind when the caseins coagulate into curds, they are collectively known as whey proteins. Whey proteins make up around twenty percent of the protein in milk, by weight. Lactoglobulin is the most common whey protein by a large margin (McGee 20–21).

The carbohydrate lactose gives milk its sweet taste and contributes about 40% of whole cow milk's calories. Lactose is a composite of two simple sugars, glucose and galactose. In nature, lactose is found only in milk and a small number of plants (McGee 17). Other components found in raw cow milk are living white blood cells. Mammary-gland cells, various bacteria, and a large number of active enzymes are some other components in milk (McGee 16).

Nutrition and health

The composition of milk differs widely between species. Factors such as the type of protein; the proportion of protein, fat, and sugar; the levels of various vitamins and minerals; and the size of the butterfat globules and the strength of the curd are among those than can vary.[3] For example:

  • Human milk contains, on average, 1.1% protein, 4.2% fat, 7.0% lactose (a sugar), and supplies 72 kcal of energy per 100 grams.
  • Cow's milk contains, on average, 3.4% protein, 3.6% fat, and 4.6% lactose, and supplies 66 kcal of energy per 100 grams. See also Nutritional benefits further on.

Aquatic mammals, such as seals and whales, produce milk that is very rich in fats and other solid nutrients when compared with land mammals' milk.

Nutritional benefits

</tr>
Cow milk (whole)
Nutritional value per 100 g (3.5 oz)
Energy 60 kcal   250 kJ
Carbohydrates     5.2 g
- Sugars  5.2 g
  - Lactose 5.2 g  
Fat3.25 g
- saturated  1.9 g
- monounsaturated  0.8 g  
- polyunsaturated  0.2 g  
Protein 3.2 g
Water88 g
Vitamin A equiv.  28 μg 3%
Thiamin (Vit. B1)  0.04 mg  3%
Riboflavin (Vit. B2)  0.18 mg  12%
Vitamin B12  0.44 μg  18%
Vitamin D  40 IU20%
Calcium  113 mg11%
Magnesium  10 mg3% 
Potassium  143 mg  3%
100 ml corresponds to 103 g.[4]
Percentages are relative to US
recommendations for adults.
Source: USDA Nutrient database</td></tr></table>

Processed milk began containing differing amounts of fat during the 1950s. A serving (1 cup or 250 ml) of 2%-fat milk contains 285 mg of calcium, which represents 22% to 29% of the daily recommended intake (DRI) of calcium for an adult. Depending on the age, 8 grams of protein, and a number of other nutrients (either naturally or through fortification):

Studies show possible links between low-fat milk consumption and reduced risk of arterial hypertension, coronary heart disease, colorectal cancer and obesity. Overweight individuals who drink milk may benefit from decreased risk of insulin resistance and type 2 diabetes.[5]

Interestingly, a study has shown that for women desiring to have a child, those who consume full fat dairy products may actually slightly increase their fertility, while those consuming low fat dairy products may slightly reduce their fertility due to interference with ovulation. However, studies in this area are still inconsistent.[6]

Nutritional and physiological detriments

  • Milk contains casein, a substance that breaks down in the human stomach to produce casomorphin, an opioid peptide. In the early 1990s it was hypothesized that casomorphin can cause or aggravate autism,[7] [8] and casein-free diets are widely promoted. Studies supporting these claims have had significant flaws, and the data are inadequate to guide autism treatment recommendations.[9]
  • Lactose intolerance, discussed below.
  • Cow milk allergy (CMA) is as an immunologically mediated adverse reaction to one or more cow's milk proteins. Rarely is it severe enough to cause death.[4]
  • There are some groups [10] [11]debating the amount of calcium from milk that is actually absorbed by the human body. However, it is argued by detractors that calcium from dairy products has greater bio-availability than calcium from vegetable products. [12]
  • One study [13] demonstrated that men, and to some degree women, who drink a large amount of milk and consume dairy products were at a slightly increased risk of developing Parkinson's disease. The reason behind this is not fully understood, and it also remains unclear why there is less of a risk for women.[14][15]
  • Several sources suggest a correlation between high calcium intake (2000 mg per day, or twice the US recommended daily allowance, equivalent to six or more glasses of milk per day) and prostate cancer.[16] A large study specifically implicates dairy.[17] A review published by the World Cancer Research Fund and the American Institute for Cancer Research states that at least eleven human population studies have linked excessive dairy product consumption and prostate cancer,[18] however randomized clinical trial data with appropriate controls only exists for calcium, not dairy produce, where there was no correlation.[19]

Controversy

A number of groups e.g. PETA and the Vegetarian & Vegan Foundation (VVF) have provided strong scientific evidence linking cows' milk and dairy products to a wide range of illnesses and diseases [20] White Lies.

Common claims cited by anti-milk advocates:

  • White blood cells -- Milk contains varying levels of white blood cells depending upon the health of the source animals, according to guidelines set up by the Food and Drug Administration and statistics reported by the dairy industry.[21]
  • Bovine Growth Hormone(rbst) -- Since November 1993, with FDA approval, Monsanto has been selling recombinant bovine somatotropin (rbST)--or rBGH--to dairy farmers. Additional bovine growth hormone is administered to cattle in order to increase their milk production, though the hormone also naturally fosters liver production of insulin-like growth factor 1 (IGF1). The deposit thereof in the milk of rBGH-affected cattle has been the source of concern; however, all milk contains IGF1 since all milking cows produce bovine growth hormone naturally. The IGF1 in milk from rBGH-affected cattle does not vary from the range normally found in a non-supplemented cow.[22] Elevated levels of IGF1 in human blood has been linked to increased rates of breast, colon, and prostate cancer by stimulating their growth,[23][24] though this has not been linked to milk consumption. The EU has recommended against Monsanto milk.[25] In addition, the cows receiving rBGH supplements may more frequently contract an udder infection known as mastitis, partly responsible for the aforementioned prevalence of blood cells in dairy products.[26] Milk from rBGH-affected cattle is banned in Canada, Australia, New Zealand, and Japan due to the mastitis problems. On June 9, 2006 the largest milk processor in the world and the two largest supermarkets in the United States--Dean Foods, Wal-Mart, and Kroger--announced that they are "on a nationwide search for rBGH-free milk [5]."

No study has indicated that consumption of rBST-produced milk increases IGF1 levels, nor has any study demonstrated an increased risk of any disease between those consuming rBST and non-rBST produced milk. In 1994, the FDA has concluded that no significant difference has been shown between milk derived from rBST-treated and non-rBST-treated cows, nor does any test exist which can differentiate between milk from rBST-treated and non-rBST treated cows. [27]

Lactose intolerance

Main article: lactose intolerance

Lactose, the disaccharide sugar component of all milk must be cleaved in the small intestine by the enzyme lactase in order for its constituents (galactose and glucose) to be absorbed. The production of this enzyme declines significantly after weaning in all mammals including humans (except for most northern westerners and a few other ethnic groups, lactase decline occurs after weaning, sometime between the ages of two and five). Once lactase levels have decreased sufficiently, consumption of small amounts of lactose can cause diarrhea, intestinal gas, cramps and bloating, as the undigested lactose travels through the gastrointestinal tract and serves as nourishment for intestinal microflora who excrete gas. [28]

Nutrition - comparison by animal source

Milk Composition Analysis, per 100 grams

Constituents unit Cow Goat Sheep Water Buffalo
Water g 87.8 88.9 83.0 81.1
Protein g 3.2 3.1 5.4 4.5
Fat g 3.9 3.5 6.0 8.0
Carbohydrate g 4.8 4.4 5.1 4.9
Energy kcal 66 60 95 110
kJ 275 253 396 463
Sugars (Lactose) g 4.8 4.4 5.1 4.9
Fatty Acids:
Saturated g 2.4 2.3 3.8 4.2
Mono-unsaturated g 1.1 0.8 1.5 1.7
Polyunsaturated g 0.1 0.1 0.3 0.2
Cholesterol mg 14 10 11 8
Calcium IU 120 100 170 195

Source: McCane, Widdowson, Scherz, Kloos.[6]

These compositions vary by breed, animal, and point in the lactation period. Jersey cows produce milk of about 5.2% fat, Zebu cows produce milk of about 4.7% fat, Brown Swiss cows produce milk of about 4.0% fat, and Holstein-Friesian cows produce milk of about 3.6% fat. The protein range for these four breeds is 3.3% to 3.9%, while the lactose range is 4.7% to 4.9%. [29]

Milk fat percentages in all dairy breeds vary according to digestible fibre, starch and oil intakes[30], and can therefore be manipulated by dairy farmers' diet formulation strategies. Mastitis infection can cause fat levels to decline.[31]

Spoilage and Fermented Milk Products

When raw milk is left standing for a while, it turns "sour". This is the result of fermentation, where lactic acid bacteria ferment the lactose inside the milk into lactic acid. Prolonged fermentation may render the milk unpleasant to consume. This fermentation process is exploited by the introduction of bacterial cultures (e.g. Lactobacilli sp., Streptococcus sp., Leuconostoc sp., etc) to produce a variety of fermented milk products. The reduced pH from lactic acid accumulation denatures proteins and caused the milk to undergo a variety of different transformations in appearance and texture, ranging from an aggregate to smooth consistency. Some of these products include sour cream, yogurt, cheese, buttermilk, viili, kefir and kumis. See Dairy product for more information.

Pasteurization of cow's milk initially destroys any potential pathogens and increases the shelf-life [32][33], but eventually results in spoilage that makes it unsuitable for consumption. This causes it to assume an unpleasant odor, and the milk is deemed non-consumable due to unpleasant taste and an increased risk of food poisoning. In raw milk, the presence of lactic acid-producing bacteria, under suitable conditions, ferments the lactose present to lactic acid. The increasing acidity in turn prevents the growth of other organisms, or slows their growth significantly. During pasteurization however, these lactic acid bacteria are mostly destroyed.

In order to prevent spoilage, milk can be kept refrigerated and stored between 1 and 4 degrees Celsius in bulk tanks. Most milk is pasteurized by heating briefly and then refrigerated to allow transport from factory farms to local markets. The spoilage of milk can be forestalled by using ultra-high temperature (UHT) treatment; milk so treated can be stored unrefrigerated for several months until opened. Sterilized milk, which is heated for a much longer period of time, will last even longer, but also loses more nutrients and assume a different taste. Condensed milk, made by removing most of the water, can be stored in cans for many years, unrefrigerated, as can evaporated milk. The most durable form of milk is milk powder, which is produced from milk by removing almost all water. The moisture content is usually less than 5% in both drum and spray dried milk powder.

Notes

  1. Dairy Chemistry and Physics, webpage of University of Guelph
  2. Dairy Chemistry and Physics, webpage of University of Guelph
  3. Introduction to Dairy Science and Technology, webpage of University of Guelph
  4. Jones, Alicia Noelle (2002). Density of Milk. The Physics Factbook.
  5. Dairy's Role in Managing Blood Pressure, web page of the US National Dairy Council
  6. [1]
  7. Reichelt KL, Knivsberg A-M, Lind G, Nødland M (1991). "Probable etiology and possible treatment of childhood autism". Brain Dysfunct 4: 308–19.
  8. Christison GW, Ivany K (2006). "Elimination diets in autism spectrum disorders: any wheat amidst the chaff?". J Dev Behav Pediatr 27 (2 Suppl 2): S162–71. PMID 16685183.
  9. Christison GW, Ivany K (2006). "Elimination diets in autism spectrum disorders: any wheat amidst the chaff?". J Dev Behav Pediatr 27 (2 Suppl 2): S162–71. PMID 16685183.
  10. Calcium Rich Foods: Get All The Calcium You Need Without Milk
  11. Feskanich D, Willett WC, Stampfer MJ, Colditz GA. Milk, dietary calcium, and bone fractures in women: a 12-year prospective study. Am J Public Health 1997; 87:992-7.
  12. Brody T. Calcium and phosphate. In: Nutritional biochemistry. 2nd ed. Boston: Academic Press, 1999:761–94
  13. H. Chen et al., Consumption of Dairy Products and Risk of Parkinson's Disease, American Journal of Epidemiology. 2007 May;165(9):998-1006
  14. H. Chen et al., Consumption of Dairy Products and Risk of Parkinson's Disease, American Journal of Epidemiology. 2007 May;165(9):998-1006
  15. "Milk linked to Parkinson's risk", BBC News. 
  16. Giovannucci, E. et al., Calcium and fructose intake in relation to risk of prostate cancer., Cancer Res. 1998 Feb 1;58(3):442-7.
  17. Chan, J.M., Dairy products, calcium, and prostate cancer risk in the Physicians' Health Study. Am J Clin Nutr. 2001 Oct;74(4):549-54. (disputed publication)
  18. The World Cancer Research Fund and American Institute for Cancer Research (1997). "Food, nutrition and the prevention of cancer: a global perspective". Food, nutrition and the prevention of cancer: a global perspective.
  19. Chan JM et al., (2005) Role of diet in prostate cancer development and progression. J Clin Oncol 23:8152-60.
  20. [2]
  21. Greger, Michael. Paratuberculosis and Crohn's Disease: Got Milk? Pro-vegan online publication, January 2001
  22. http://www.idfa.org/reg/biotech/talking2.cfm
  23. Kahan, Z et al., Elevated levels of circulating insulin-like growth factor-I, IGF-binding globulin-3 and testosterone predict hormone-dependent breast cancer in postmenopausal women: a case-control study. Int J Oncol. 2006 Jul;29(1):193-200.
  24. Pacher, M. et al., Impact of constitutive IGF1/IGF2 stimulation on the transcriptional program of human breast cancer cells. Carcinogenesis. 2006 Jun 14
  25. International Scientific Committee Warns of Serious Risks of Breast and Prostate Cancer from Monsanto's Hormonal Milk. Press release of the Cancer Prevention Coalition.
  26. Milk: Epstein, S., America's Health Problem. Web page of the Cancer Prevention Coalition.
  27. http://www.fda.gov/bbs/topics/ANSWERS/ANS00564.html
  28. http://en.wikipedia.org/wiki/Anaerobic_respiration
  29. McGee, Harold (2004). On Food and Cooking: The Science and Lore of the Kitchen, Completely Revised and Updated. New York, NY: Scribner, 13. ISBN 9780684800011. 
  30. http://www.kt.iger.bbsrc.ac.uk/FACT%20sheet%20PDF%20files/kt21.pdf
  31. http://books.google.com/books?id=qJgdAEhQvnMC&pg=PA226&lpg=PA226&dq=mastitis+and+milk+fat+levels&source=web&ots=PrguNhnHdm&sig=W_MS2A7FWTBksmBYvZZk38dRh4A
  32. http://www.fda.gov/fdac/features/2004/504_milk.html
  33. [3]

References

See also

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