Milk

A glass of cow's milk
A glass of cow's milk

Milk is the nutrient fluid secreted by the mammary glands of female mammals (including monotremes). 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 more diverse foods. 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 both the mother and baby.

The exact components of milk vary by species, but it contains significant amounts of unsaturated fat, protein and calcium. These amounts are not large in comparison to other foods rich in them, including coconuts, fish, and kale respectively, because milk is predominantly composed of water. 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.

Humans, like other mammals, can consume mother's milk during their infancy. In many ethnic groups, people lose the ability to digest milk after childhood (that is, they become lactose intolerant), so many traditional cuisines around the world do not feature dairy products. On the other hand, those cultures that do tolerate milk have often exercised great creativity in using the milk of domesticated ruminants, especially cows, but also sheep, goats, yaks, water buffalo, horses and camels. 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.

Human milk is fed to infants through breastfeeding, either directly or by expressing the milk to be stored and consumed later.

The term milk is also used for non-animal substitutes such as soy milk, rice milk, almond milk, and coconut milk, and even the regurgitated substance pigeons feed their young, called crop milk, which bears little resemblance to mammalian milk.

Contents

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History

Holstein cattle, the dominant breed in industrialized dairying today.
Holstein cattle, the dominant breed in industrialized dairying today.

Animal milk was first 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 8–10).

Around 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. [citation needed]

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. 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 USA[1] and New Zealand.

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Other milk animals

Goat's milk can be used for other applications such as cheese and other dairy products.
Goat's milk can be used for other applications such as cheese and other dairy products.

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

In Russia and Sweden, small moose dairies also exist. Donkey and horse milk have the lowest fat content, while the milk of seals contains more than 50% fat.[2]

Whale's milk, not used for human consumption, is one of the highest-fat milks. It contains, on average, 10.9% protein, 42.3% fat, and 2.0% lactose, and supplies 443 kcal of energy per 100 grams[citation needed].

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.).

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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).

Schematic of a micelle.
Schematic of a micelle.

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 roughtly 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[3] (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.[4]

A simplified representation of a lactose molecule being broken down into glucose and galactose.
A simplified representation of a lactose 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 (McGee 16).

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Processing

A milking machine in action.
A milking machine in action.

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).

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Pasteurization and raw milk

Pasteurization kills many harmful micro organisms 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 dates 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 even longer lasting sterilization.

Those preferring raw milk argue that the pasteurization process also kills beneficial microorganisms and important nutritional constituents. The resulting pasteurized product is said to contribute to its own indigestability, be less nutritious, and turn rancid (as opposed to sour) with age. However, unpasteurized milk can harbor harmful disease-causing bacteria such as tuberculosis, brucellosis, salmonella, diphtheria, polio, and escherichia coli.[5] The cows must be maintained in very sanitary conditions and a watchful eye kept as to disease testing and vaccinations for this to be completely safe. Cheeses made with raw milk are regarded as safer as the milk typically had to be heated some anyways to make the cheese, and this would have killed off many of the dangerous organisms possibly present.

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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 quicker 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 as 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.[6] 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. Some have suggested that homogenized milk is harder to digest or not as suited to some people as is unhomogenized, it was just for convenience so one didn't have to shake the bottle, and isn't as important as is pasteurization which is done for safety purposes.

Unhomogenized has made a small comeback in a few areas, such as the west coast of the United States where Srauss 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.

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Additives & Flavored Milk Drinks

Milk, sold commercially in countries where the cattle (and often the people) live indoors, commonly has vitamin D added to it to make up for lack of exposure to UVB radiation. Milk often has flavoring added to it for better taste or as a means of improving sales. Chocolate flavored milk has been sold for many years and has been followed more recently by such other flavors as strawberry and banana.

South Australia has the highest consumption of flavoured milk per person in the world, where Farmers Union Iced Coffee outsells Coca-Cola, a success shared only by Inca Kola in Peru and Irn-Bru in Scotland.

Switzerland has a soft drink based on milk that tastes and looks much like SevenUp. This popular "milk-cola", named Rivella, is in fact the national soft drink and comes complete in low calorie & low sugar varieties even. To taste it one would not think at all that it is based from milk.

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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:

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Nutritional benefits

Cow milk (whole)
Nutritional value per 100 g
Energy 60 kcal   250 kJ
Carbohydrates     5.2 g
- Sugars  5.2 g
  - Lactose 5.2 g  
Fat 3.25 g
- saturated  1.9 g
- monounsaturated  0.8 g  
- polyunsaturated  0.2 g  
Protein 3.2 g
Water 88 g
Vitamin A  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 IU 20%
Calcium  113 mg 11%
Magnesium  10 mg 3% 
Potassium  143 mg   3%
100 ml corresponds to 103 g.[7]
Percentages are relative to US RDI
values for adults.
Source: USDA Nutrient database

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, and obesity. Overweight individuals who drink milk may benefit from decreased risk of insulin resistance and type 2 diabetes.[8]

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Nutritional/physiological detriments

The following additional issues are often cited as warranting consideration:

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Lactose intolerance

Lactose is a simple sugar that is present in all milk of the species of origin, and is digested with the help of the enzyme lactase. The production of this enzyme declines significantly after weaning in all mammals, including human beings. Lactose intolerance is the condition in which lactase is not produced in adulthood. With lactose intolerance, the result of consuming too much lactose is excess gas production and often diarrhea. Lactose-intolerant adults can drink about a cup (250 ml or 8 oz) of milk per day without severe symptoms.

Most adults in the world are lactose-intolerant (McGee 14: "lactose tolerant adults are a distinct minority on the planet"). Lactose-intolerant adults stop producing significant amounts of lactase sometime between the ages of two and five. A relatively recent genetic change caused some populations (McGee 14: "several thousand years ago"), including many Northern Europeans, to continue producing lactase into adulthood.

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Common Misconceptions

Contrary to popular belief, milk has not been found to be in anyway related to an increase mucus production or asthma.[17]

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Nutrition - comparison by animal source

Milk Composition Analysis, per 100 grams

Constituents unit Cow Goat Sheep Water Buffalo
Water grm 87.8 88.9 83.0 81.1
Protein grm 3.2 3.1 5.4 4.5
Fat grm 3.9 3.5 6.0 8.0
Carbohydrate grm 4.8 4.4 5.1 4.9
Energy K cal 66 60 95 110
K J 275 253 396 463
Sugars (Lactose) grm 4.8 4.4 5.1 4.9
Fatty Acids:
Saturated grm 2.4 2.3 3.8 4.2
Mono-unsaturated grm 1.1 0.8 1.5 1.7
Polyunsaturated grm 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.[2]

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Distribution

Glass milk bottles used for home delivery service
Glass milk bottles used for home delivery service
A brick of French UHT milk
A brick of French UHT milk

Because milk spoils so easily, it should, ideally, be distributed as quickly as possible. In many countries milk used to be delivered to households daily, but economic pressure has made milk delivery much less popular, and in many areas daily delivery is no longer available. People buy it chilled at grocery or convenience stores or similar retail outlets. Prior to the widespread use of plastics, milk was often distributed to consumers in glass bottles, and before that in bulk that was ladled into the customer's container. In the UK, milk can be delivered daily by a milkman who travels his local milk round (route) using a battery-powered milk float, although this is becoming less popular as a result of supermarkets selling milk at lower prices. In New Zealand, milk is no longer distributed in glass bottles. In India, milk is sold in plastic sachet.

In the United States bottles were replaced with milk cartons, which are tall boxes with a square cross-section and a peaked top that can folded outward upon opening to form a spout. Now milk is increasingly sold in plastic bottles. First the gallon and half-gallon sizes were sold in plastic jugs while the smaller sizes were sold in milk cartons. Recently milk has been sold in smaller resealable bottles made to fit in automobile cup holders.

The half-pint milk carton is the traditional unit as a component of school lunches. In the US, pictures of missing children were printed on the larger milk cartons as a public service until it was determined that this was disturbing to children.

Milk preserved by the UHT process is sold in cartons often called a "brick" that lack the peak of the traditional milk carton. Milk preserved in this fashion does not need to be refrigerated before opening and has a longer shelf life than milk in ordinary packaging. It is more typically sold unrefrigerated on the shelves in Europe than in America.

Glass milk containers are rare these days. Most people purchase milk in plastic jugs or bags or in waxed-paper cartons, such as those made by Tetra Pak. Ultraviolet light from fluorescent lighting can destroy some of the proteins in milk so many companies that once distributed milk in transparent or highly translucent containers are now using thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better.

Milk comes in a variety of containers with local variants:

Brazilian Yakult, an example of the use of milk.
Brazilian Yakult, an example of the use of milk.
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Varieties and brands

Cow's milk is generally available in several varieties. In some countries these are:

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

In Canada "whole" milk refers to creamline (unhomogenized) milk. "Homogenized" milk refers to milk which is 3.25% butterfat. Generally all store-bought milk in Canada has been homogenized. Yet, the term is also used as a name to describe butterfat content for a specific variety of milk. Modern commercial dairy processing techniques involve first removing all of the butterfat, and then adding back the appropriate amount depending on which product is being produced on that particular line.

In Britain, it is possible to get Channel Island milk, which is 5.5% fat.

In the United States, skim milk is also known as "fat free" milk, due to USDA regulations stating that any food with less than ½ gram of fat per serving can be labeled "fat free".

Full cream, or whole milk, has the full milk fat content (about 3-4% if Freisian- or Holstein-breed are the source). For skimmed or semi-skimmed milk, all of the fat content is removed and then some (in the case of semi-skimmed milk) is returned. The best-selling variety of milk is semi-skimmed; in some countries full-cream (whole) milk is generally seen as less healthy and skimmed milk is often thought to lack taste. Whole milk is recommended to provide sufficient fat for developing toddlers who have graduated from breast milk or infant formula.

In the United States and Canada, a blended mixture of half cream and half milk is often sold in small quantities and is called half-and-half. Half-and-half is used for creaming coffee and similar uses. In Canada, low-fat cream is available, which has half the fat content of half-and-half.

Organic Milk (in the United States) or Bio-Milk & Biologique Milk (in Europe) is milk produced without the use of chemical herbicides or pesticides, and generally with more natural fertilizers and higher standards for the animals, and is now easy to find on the shelves in many areas. Demeter Certified Milk is milk produced according to biodynamic farming methods and is similar in standards to organic milk and biological milk, with a few special farm procedures added that are specific to biodynamic agriculture.

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Spoilage

When raw milk is left standing for a while, it turns "sour". This is the result of fermentation: lactic acid bacteria turning the sugar inside the milk into lactic acid. This fermentation process is exploited in the production of various dairy products such as cheese and yogurt. There are four noted periods of milk decay:

Pasteurized cow's milk, on the other hand, spoils in a way that makes it unsuitable for consumption. This causes it to assume an unpleasant odor and pose a high danger of food poisoning if ingested. In raw milk, the naturally-occurring lactic acid bacteria, under suitable conditions, quickly produce large amounts of lactic acid. The ensuing acidity in turn prevents other germs from growing, or slows their growth significantly. Through pasteurization, however, these lactic acid bacteria are mostly destroyed, which means that other germs can grow unfettered and thus cause decomposition.

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 lose more nutrients and assume a still 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.

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Milk in language and culture

The importance of milk in human culture is attested to by the numerous expressions embedded in our languages, for example "the milk of human kindness", and the ways we have used it to name the visible world, for example the Milky Way.

Milk is drunk as an accompaniment to meals in North America, whereas Europeans, with the exception of North Europeans, do not habitually do so after childhood.

In African and Asian developing nations, butter is traditionally made from sour milk rather than cream. It can take several hours of churning to produce workable butter grains from fermented milk.[18]

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References

  1. FAO Food outlook: International dairy product prices are turning down: how far, how fast? FAO online publication, 1 June 2006
  2. Milk From Cows and Other Animals, web page by Washington Dairy Products Commission
  3. Diary Chemistry and Physics, webpage of University of Guelph
  4. Diary Chemistry and Physics, webpage of University of Guelph
  5. Raw Milk Vs. Pasteurized Milk. Reproduction from Armchair Science, London 1938.
  6. Homogenization of Milk and Milk Products, webpage of University of Guelph
  7. Density of milk
  8. Dairy's Role in Managing Blood Pressure, web page of the US National Diary Council
  9. Cohen, Rob. Your state's average pus count. Web page of the anti-diary Diary Education Board
  10. Greger, Michael. Paratuberculosis and Crohn's Disease: Got Milk? Pro-vegan online publication, January 2001
  11. 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.
  12. Pacher, M. et al., Impact of constitutive IGF1/IGF2 stimulation on the transcriptional program of human breast cancer cells. Carcinogenesis. 2006 Jun 14
  13. International Scientific Committee Warns of Serious Risks of Breast and Prostate Cancer from Monsanto's Hormonal Milk. Press release of the Cancer Prevention Coalition.
  14. Milk: Epstein, S., America's Health Problem. Web page of the Cancer Prevention Coalition.
  15. Giovannucci, E. et al., Calcium and fructose intake in relation to risk of prostate cancer., Cancer Res. 1998 Feb 1;58(3):442-7.
  16. 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)
  17. Journal of the American College of Nutrition: Milk Consumption Does Not Lead to Mucus Production or Occurrence of Asthma
  18. Crawford et al, part B, section III, ch. 1: Butter. Retrieved 28 November 2005.
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See also

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External links

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