's milk.]]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.

Types of consumption

There are two distinct types of milk consumption: a natural source of nutrition for all infant mammals; and a food product for humans of all ages derived from other animals.

Nutrition for infant mammals

In almost all mammals, milk is fed to infants through breastfeeding, either directly or, for humans, by expressing the milk to be stored and consumed later. Some cultures, historically or presently, continue to use breast milk to feed their children until as old as seven years. . . . or just go with the flow?. The Times, May 5, 2005.

Food product for humans

In many cultures of the world, especially the Western world, humans continue to consume milk beyond infancy, using the milk of other animals (in particular, cows) as a food product. For millennia, cow's milk has been processed into dairy products such as cream, butter, yogurt, ice cream, and especially the more durable and easily transportable product, cheese. Industrial science has brought us casein, whey protein, lactose, condensed milk, powdered milk, and many other food-additive and industrial products.Humans are an exception in the natural world for consuming milk past infancy. Even those humans who drink milk after eating solid foods are uncommon within the whole of humanity. Most humans lose the ability to fully digest milk after childhood (that is, they become lactose intolerant). The sugar lactose is found only in milk, forsythia flowers, and a few tropical shrubs. The enzyme needed to digest lactose, lactase, reaches its highest levels in the small intestines after birth and then begins a slow decline unless milk is consumed regularly. On the other hand, those groups that do continue to tolerate milk often have exercised great creativity in using the milk of domesticated ruminants, not only of cows, but also sheep, goats, yaks, water buffalo, horses, and camels. The term milk is also used for whitish non-animal substitutes such as soy milk, rice milk, almond milk, and coconut milk. Even the regurgitated substance pigeons feed their young is called crop milk though it bears little resemblance to mammalian milk.

History

cattle, the dominant breed in industrialized dairying today.]]Milking has its advent in the very evolution of placental mammals. While the exact time of its appearance is not known, the immediate ancestors of modern mammals were much like monotremes, including the platypus. Such animals today produce a milk-like substance from glands on the surface of their skin, but without the nipple, for their offspring to drink after hatching from their eggs. Likewise, marsupials, the closest cousin to placental mammals, produce a milk-like substance from a teat-like organ in their pouches. The earliest immediate ancestor of placental mammals known seems to be eomaia, a small creature superficially resembling rodents, that is thought to have lived 125 million years ago, during the Cretaceous era. It almost certainly produced what would be considered milk, in the same way as modern placental mammals.Animal milk is first known to have been used as human food at the beginning of animal domestication. Cow's milk was first used as human food in the Middle East. Goats and sheep were domesticated in the Middle East between 9000 and 8000 BC. Goats and sheep are ruminants: mammals adapted to survive on a diet of dry grass, a food source otherwise useless to humans, and one that is easily stockpiled. The animals were probably first kept for meat and hides, but dairying proved to be a more efficient way of turning uncultivated grasslands into sustenance: the food value of an animal killed for meat can be matched by perhaps one year's worth of milk from the same animal, which will keep producing milk ? in convenient daily portions ? for years.McGee, 8-10Around 7000 BC, cattle were being herded in parts of Turkey. There is evidence of milk consumption in the British Isles during the Neolithic period. The use of cheese and butter spread in Europe, parts of Asia and parts of Africa. Domestic cows, which previously existed throughout much of Eurasia, were then introduced to the colonies of Europe during the Age of exploration.

Other animals sources

In addition to cows, the following animals provide milk used by humans for dairy products:

• Sheep (the ewe)
• Goats (the nanny)
• Buffalo
• Horses (the mare)
• Donkeys
• Camels (including the South American camelids)
• Yaks
• Water buffalo
• Reindeer

In Russia and Sweden, small moose dairies also exist. Donkey and horse milk have the lowest fat content, while the milk of seal contains more than 50% fat. Milk From Cows and Other Animals, web page by Washington Dairy Products CommissionWhale's milk, not used for human consumption, is one of the highest-fat milks, containing up to 50% fat. MSN encarta livescience.com The high fat content of whale's milk is not a product of cetacean's great size, as guinea pig milk has an average fat content of 46%.Human milk is not produced or distributed industrially or commercially; however, milk banks exist that allow for the collection of donated human milk and its redistribution to infants who may benefit from human milk for various reasons (premature neonates, babies with allergies or metabolic diseases, etc.).All other female mammals do produce milk, but are rarely or never used to produce dairy products for human consumption.

Modern production

In the Western world today, cow's milk is produced on an industrial scale. It is by far the most commonly consumed form of milk in the western world. Commercial dairy farming using automated milking equipment produces the vast majority of milk in developed countries. Types of cattle such as the Holstein have been specially bred for increased milk production. According to McGee, 90% of the dairy cows in the United States and 85% in Great Britain are Holsteins (McGee 12). Other milk cows in the United States include Ayrshire, Brown Swiss, Guernsey, Jersey, and Milking Shorthorn. The largest producers of dairy products and milk today are India followed by the United States FAO Food outlook: International dairy product prices are turning down: how far, how fast? FAO online publication, 1 June 2006 and New Zealand. In India, Amul, a cooperative owned jointly by 2.6 million small farmers was the engine behind the success of Operation Flood.

Price

It was reported in 2007 that with increased world-wide prosperity and the competition of biofuel production for feedstocks, both the demand for and the price of milk had substantially increased world wide. Particularly notable was the rapid increase of consumption of milk in China and the rise of the price of milk in the United States above the government subsidized price."A Thirst for Milk Bred by New Wealth Sends Prices Soaring" article by Wayne Arnold in the New York Times September 4, 2007

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 micrometer 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 Dairy Chemistry and Physics, webpage of University of Guelph (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. Dairy Chemistry and Physics, webpage of University of Guelph molecule being broken down into glucose and galactose.]]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).

Processing

In most Western countries, a centralised dairy facility processes milk and products obtained from milk (dairy products), such as cream, butter, and cheese. In the United States, these dairies are usually local companies, while in the southern hemisphere facilities may be run by very large nationwide or trans-national corporations (such as Fonterra).

Pasteurization

Pasteurization is used to kill harmful microorganisms by heating the milk for a short time and then cooling it for storage and transportation. Pasteurized milk is still perishable and must be stored cold by both suppliers and consumers. Dairies print expiration date on each container, after which stores will remove any unsold milk from their shelves. In many countries it is illegal to sell milk that is not pasteurized.Milk may also be further heated to extend its shelf life through ultra-high temperature treatment (UHT), which allows it to be stored unrefrigerated, or an even longer lasting sterilization process.

Creaming and homogenization

Upon standing for 12 to 24 hours, fresh milk has a tendency to separate into a high-fat cream layer on top of a larger, low-fat milk layer. The cream is often sold as a separate product with its own uses; today the separation of the cream from the milk is usually accomplished rapidly in centrifugal cream separators. The fat globules rise to the top of a container of milk because fat is less dense than water. The smaller the globules, the more other molecular-level forces prevent this from happening. In fact, the cream rises in cow milk much more quickly than a simple model would predict: rather than isolated globules, the fat in the milk tends to form into clusters containing about a million globules, held together by a number of minor whey proteins (McGee 19). These clusters rise faster than individual globules can. The fat globules in milk from goats, sheep, and water buffalo do not form clusters so readily and are smaller to begin with; cream is very slow to separate from these milks (McGee 19)..]]Milk is often homogenized, a treatment which prevents a cream layer from separating out of the milk. The milk is pumped at high pressures through very narrow tubes, breaking up the fat globules through turbulence and cavitation. Homogenization of Milk and Milk Products, webpage of University of Guelph A greater number of smaller particles possess more total surface area than a smaller number of larger ones, and the original fat globule membranes cannot completely cover them. Casein micelles are attracted to the newly-exposed fat surfaces; nearly one-third of the micelles in the milk end up participating in this new membrane structure. The casein weighs down the globules and interferes with the clustering that accelerated separation. The exposed fat globules are briefly vulnerable to certain enzymes present in milk, which could break down the fats and produce rancid flavors. To prevent this, the enzymes are inactivated by pasteurizing the milk immediately before or during homogenization. Homogenized milk tastes blander but feels creamier in the mouth than unhomogenized; it is whiter and more resistant to developing off flavors (McGee 23). Creamline, or cream-top, milk is unhomogenized; it may or may not have been pasteurized. Unlike pasteurization, homogenization confers no health or safety benefits to the milk, only the convenience of not needing to shake the bottle oneself. Unhomogenized milk has made a small comeback in a few areas, such as the west coast of the United States where Straus Family Creameries, based originally out of Sonoma, sells one line of organic milk with the cream still on top in old-fashioned glass bottles. They still however pasteurize it to prevent harmful microorganisms.

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. Introduction to Dairy Science and Technology, webpage of University of Guelph 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 === }}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):

• Vitamins D and K are essential for bone health.
• Iodine is a mineral essential for thyroid function.
• Vitamin B12 and riboflavin are necessary for cardiovascular health and energy production, and B12 is difficult to get outside of animal products or else as supplemental pills.
• Biotin and pantothenic acid are B vitamins important for energy production.
• Vitamin A is critical for immune function.
• Potassium and magnesium are for cardiovascular health.
• Selenium is a cancer-preventive trace mineral.
• Thiamine is a B-vitamin important for cognitive function, especially memory
• Conjugated linoleic acid is a beneficial fatty acid that inhibits several types of cancer in mice, it has been shown to kill human skin cancer, colorectal cancer and breast cancer cells in vitro studies, and may help lower cholesterol and prevent atherosclerosis; only available in milk from grass-fed cows.

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. Dairy's Role in Managing Blood Pressure, web page of the US National Dairy CouncilInterestingly, 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.[2]

Nutritional and physiological detriments

• Milk contains casein, a substance that breaks down in the human stomach to produce the peptide casomorphin, an opioid that appears to act as a histamine releaser, and is suspected in some cases to aggravate the symptoms of autism.
• 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.
• There are some fringe groups Calcium Rich Foods: Get All The Calcium You Need Without Milkhttp://www.quackwatch.org/04ConsumerEducation/Nonrecorg/aqa.html debating the amount of calcium from milk that is actually absorbed by the human body. However, calcium from dairy products has greater bio-availability than calcium from vegetable products. Brody T. Calcium and phosphate. In: Nutritional biochemistry. 2nd ed. Boston: Academic Press, 1999:761?94
• One study H. Chen et al., Consumption of Dairy Products and Risk of Parkinson's Disease, American Journal of Epidemiology. 2007 May;165(9):998-1006 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. H. Chen et al., Consumption of Dairy Products and Risk of Parkinson's Disease, American Journal of Epidemiology. 2007 May;165(9):998-1006
• 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. Giovannucci, E. et al., Calcium and fructose intake in relation to risk of prostate cancer., Cancer Res. 1998 Feb 1;58(3):442-7. A large study specifically implicates dairy. 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) 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, however randomized clinical trial data with appropriate controls only exists for calcium, not dairy produce, where there was no correlation.Chan JM et al., (2005) Role of diet in prostate cancer development and progression. J Clin Oncol 23:8152-60.

Controversy A number of advocate groups (e.g. peta) have sprung up protesting that milk presents a health threat. However no study has concluded any causal health risk to normal individuals consuming moderate quantities milk.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. Greger, Michael. Paratuberculosis and Crohn's Disease: Got Milk? Pro-vegan online publication, January 2001

No study has ever conclusively demonstrated that the levels of white blood cells found in normal milk actually pose any health risk to normal individuals.

• Bovine Growth Hormone(rbst) -- Since November 1993, with FDA approval, Monsanto has been selling recombinant bovine somatotropin --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 . 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.http://www.idfa.org/reg/biotech/talking2.cfm Elevated levels of IGF1 in human blood has been linked to increased rates of breast, colon, and prostate cancer by stimulating their growth, 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. Pacher, M. et al., Impact of constitutive IGF1/IGF2 stimulation on the transcriptional program of human breast cancer cells. Carcinogenesis. 2006 Jun 14 though this has not been linked to milk consumption. The EU has recommended against Monsanto milk. International Scientific Committee Warns of Serious Risks of Breast and Prostate Cancer from Monsanto's Hormonal Milk. Press release of the Cancer Prevention Coalition. 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. Milk: Epstein, S., America's Health Problem. Web page of the Cancer Prevention Coalition. 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 [3]."

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. http://www.fda.gov/bbs/topics/ANSWERS/ANS00564.html ==== 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. http://en.wikipedia.org/wiki/Anaerobic_respiration

Nutrition - comparison by animal source

Milk Composition Analysis, per 100 gramsSource: McCane, Widdowson, Scherz, Kloos.[4]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%. Milk fat percentages in all dairy breeds vary according to digestible fibre, starch and oil intakeshttp://www.kt.iger.bbsrc.ac.uk/FACT%20sheet%20PDF%20files/kt21.pdf, and can therefore be manipulated by dairy farmers' diet formulation strategies. Mastitis infection can cause fat levels to decline.http://books.google.com/books?id=qJgdAEhQvnMC&pg=PA226&lpg=PA226&dq=mastitis+and+milk+fat+levels&source=web&ots=PrguNhnHdm&sig=W_MS2A7FWTBksmBYvZZk38dRh4A==Varieties and brands== Cow's milk is generally available in several varieties. In some countries these are:

• Full cream (or "whole" in US and UK, "homo milk" in Canada & some US dairies, about 3.25% fat)
• Semi-skimmed ("reduced fat" or "low fat", about 1.5-1.8% fat)
• Skimmed (about 0.1% fat)

Milk in the U.S. and Canada is sold as:

• Whole varieties
• 2% (reduced fat)
• 1% (low fat)