Milk

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For other uses, see Milk (disambiguation).
A glass of cow's milk
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A glass of cow's milk

Milk most often means the nutrient fluid produced by the mammary glands of female mammals. It provides the primary source of nutrition for newborns before they are able to digest more diverse foods. It is also processed into dairy products such as cream, butter, yoghurt, ice-cream, gelato, cheese, casein, whey protein, lactose, condensed milk, dried milk, and many other food-additive and industrial products.

It can also be used to mean

Human milk is fed to infants through breastfeeding, either directly or by the female expressing her milk to be saved and fed later. 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 mother and baby.

Contents

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Composition and nutrition

The composition of milk differs widely from species to species and a little within species. Factors such as the lactose content, the proportion of and size of the butterfat globule, and the strength of the curd (formed by the human enzymes digesting the milk) can differ from breed to breed and mammal to mammal. For example:

Lactose in milk is digested with the help of the enzyme lactase produced by the bodies of infants. In humans, production of lactase falls off towards adulthood, leading to an inability to digest milk, this is known as lactose intolerance. Some human populations (most notably Europeans) retain the ability to digest lactose into adulthood.

Whole cow's milk has around 634 Calories (2650kJ) per litre.

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Cow's milk

In the western world cow's milk is most often extracted on an industrial scale for human consumption and industrial uses. It is the most commonly consumed form of milk. Dairy farming has become such a large business that in many countries the process is highly automated; with farmers using machines that attach directly to the teats of the cow's udder, to speed milking, and breeds of cattle, such as Holstein, specially bred for increased milk production.

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Commercial processing of milk

a cow-milking machine in action
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a cow-milking machine in action

In North America a dairy facility processes milk and products obtained from milk (dairy products), such as cream, butter, cheese and so on. Most dairies there are local companies (as opposed to large nationwide companies, such as they are in the southern hemisphere).

Upon standing, fresh milk has a tendency to separate into two layers; some rises to a high-fat cream layer on top of the, larger, skim-milk layer. Cream is often sold as a separate product with its own uses. In the United States, a blended mixture of half cream and half milk is often sold in smaller quantities and is called half and half; half and half is used for creaming coffee and similar uses.

Milk produced for commercial consumption usually undergos several processes. Pasteurization is heating milk for a short time, to kill harmful micro-organisms, followed by cooling for storage and transportation. Such milk is still perishable and must be stored cold by both suppliers and consumers. Milk is sold by certain expiration dates written on each container by the dairy. To improve public health it is illegal to sell milk, in many countries, that is not pasteurized.

A complementary process for commercial milk is homogenization, which produces milk in a single phase (or layer). This is accomplished by mechanically reducing the fat globules to a size that stabilizes them in solution.

Milk, sold commercially in countries where the cattle (and often the people) live inside, commonly has vitamin D added to it to make up for the lack of UVB radiation. Milk often has flavoring added to it for better taste or as a means of improving sales. For many years chocolate-flavored milk has been sold but recently other flavors of milk and cream have become available.

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


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Support

Cow's milk is a generally healthy source of protein and calcium in human diets. It is also a good source of a number of vitamins. 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 - essential for bone health
  • Iodine - a mineral essential for thyroid function
  • Vitamin B12 and Riboflavin - necessary for cardiovascular health and energy production
  • Biotin and Pantothenic Acid - B vitamins important for energy production
  • Vitamin A - critical for immune function
  • Potassium and Magnesium - for cardiovascular health
  • Selenium - cancer-preventive trace mineral
  • Thiamin - B-vitamin important for cognitive function, especially memory
  • Conjugated Linoleic Acid - beneficial fatty acid that inhibits several types of cancer in mice, has been shown to kill human skin cancer, colorectal cancer and breast cancer cells in in vitro studies, and may help lower cholesterol and prevent atherosclerosis; only available in milk from grass-fed cows

Milk supporters point out that 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. [1]

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Criticism

Cow's milk is also argued to be unhealthy primarily due to its fat and cholesterol content, as well as the toxicity of its protein. The following studies are used to support this position:

  • Some milk is rich in saturated fat, which studies have linked to increased risk of atherosclerosis and coronary heart disease. Low-fat and non-fat forms of milk may mitigate any such risk.
  • Up to 70% of humans have an incomplete ability to digest milk, lactose intolerance. For those individuals, milk may induce symptoms such as cramping, bloating, gas, and diarrhea. Certain ethnic groups may be more susceptible to these effects.
  • Critics dispute the claim that drinking large amounts of milk can reduce the risk of bone fractures, especially in the elderly. Studies have failed to associate high calcium intakes with lower risk of hip and forearm fractures in men[2] or women[3].
  • Critics of milk claim that plant-based sources of calcium are preferable, on the grounds that animal proteins in milk causes leaching or excretion of calcium from bones.[4] Such critics refute the claim that milk prevents osteoporosis and make the counterclaim that milk, in fact, contributes to that disease.
  • A study published in June 2005 suggests that consumption of milk by 9- to 14-year-old children is associated with weight gain, although the researchers identify that excessive calorie intake is the cause rather than dairy specific factors. Researchers were surprised by their conclusion that weight gain was associated with dietary calcium and low-fat or skim milk, but not dairy fat.[5]
  • A February 2005 study found a positive association between acne and the consumption of whole milk, skim milk, and other dairy products in high-school-age women.[6].
  • Critics also make the claim that the protein content of cow’s milk can act to block the absorption of calcium and cause the human body to produce antibodies that are believed to damage the pancreas, leading to the development of type 1 diabetes.
  • In children, cow’s milk consumption has allegedly been linked to anemia, colic, allergies, and asthma. In adults, cow's milk consumption has been related to breast cancer.
  • Two studies show a correlation between high galactose consumption, and high rates of ovarian cancer. [7][8]
  • A study suggests a correlation between high calcium intake and prostate cancer.[9]. There is no evidence that any such problem is specific to milk. A review published by the World Cancer Research Fund and the American Institute for Cancer Research states that at least 11 human population studies have linked dairy product consumption and prostate cancer.
  • Scientific evidence has also been unable to support the claim that the consumption of cow’s milk as a source of calcium reduces the risk of osteoporosis. On the contrary, epidemiological research has linked the countries with the highest dairy consumption rates (for example, the United States, Sweden and Finland) to the incidence of osteoporosis. But no studies have shown the same in New Zealand, which has the highest per capita consumption.
  • The A1 β-casein in cow's milk has been reportedly linked to ischaemic heart disease, type I diabetes, and to a lesser extent, schizophrenia and autism. Some milk contains higher levels of the A2 β-casein, which has been claimed to not lead to these diseases. Milk with the "A2 milk" trademark has been tested to be high in the A2 β-casein.
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Recombinant bovine somatotropin

Recombinant bovine somatotropin (rbST), a synthetic non-steroidal growth hormone approved by the FDA, is used in one-third of US dairy cows. All mammal's milk contains hormones, including, in cows, naturally occuring bST. Recombinant bST is not added to milk. The only hormone added to any milk is the misnamed Vitamin D3.

Supplemental bST increases the amount of milk produced per cow by 15 percent but, according to the FDA and others, it does not alter the milk itself. The FDA also asserts that cows receiving supplemental bST remain as healthy as cows that do not receive the supplement. FDA reports that there has been no increase in antibiotic use linked to cows receiving supplemental bST. For consumers who prefer milk that does not come from bST-supplemented cows, organic production prohibits the use of supplemental bST.

The nutritional makeup of cow's milk is much different from human milk. One cup of human milk has 70% less protein, 38% more fat, and 47% more carbohydrates than cow's milk. Vitamin C, folic acid, sodium, iron and calcium levels also differ significantly.

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Distribution

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

Because of the perishable nature of milk, expeditious distribution is desirable. Milk used to be delivered to households daily, but this is no longer economically feasible. 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 milk man 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 cheaper prices. In New Zealand, in some urban areas, bottled milk is still delivered to customers' homes.

In the United States bottles were replaced with milk cartons, 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 bottles made to fit in automobile cup holders.

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

Milk preserved by the UHT process is sold in boxes often called a "brick" that lack the peak of the traditional milk carton.

Glass containers are rare these days. Most people purchase milk in plastic jugs or bags or in waxed-paper cartons. 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 using thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better.

In the United States, milk is commonly sold in gallon, half-gallon and quart containers (U.S. customary units) of rigid plastic or waxed cardboard. The US single-serving size is usually the half-pint (about 500 ml). In Canada, a 1 1/3 litre plastic bag (sold as 4 litres in 3 bags) is the most common, while 2 litre, 1 litre, 500 millilitre, and 250 millilitre cartons are also available. In Europe, sizes of 500 millilitres, 1 litre (the most common), 2 litres and 3 litres are commonplace (in the UK, some stores still stock the equivalents of old Imperial sizes: 568 ml (1 pint), 1.136 l (2 pints), 2.273 l (4 pints) or, rarely, a combination including both metric and imperial sizes). In Australia and New Zealand a 250 ml cardboard container of flavoured milk is marketed as a common breakfast meal. For refrigerator use milk comes in 1, 2 and 3 litre plastic screw-top bottles. Most UHT-milk is packed in 1 litre paper containers with a sealed plastic spout.

Condensed milk is distributed in metal cans, 250 and 125 ml paper containers and 100 and 200 ml squeeze tubes, and powdered milk (skim and whole) is distributed in boxes or bags.

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Varieties and brands

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

  • full cream (or "whole" in North America, 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 percent" (reduced fat)
  • "1 percent" (low fat)
  • "1/2 percent" (low fat)
  • "skim" (very low fat)

Note: In Canada "whole" milk refers to unhomogenized milk. "Homogenized" milk (or "Homo milk" in short) 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 1/2 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 Friesian- 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.

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Other milk animals

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

In Russia and Sweden, small moose dairies also exist [10]. Donkey and horse milk have the lowest fat content, while the milk of seals contains more than 50% fat. [11]

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Curdling

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

Pasteurized cow's milk, on the other hand, spoils in a way that makes it unsuitable for consumption, causing it to assume an unpleasant odor and pose a high danger of food poisoning if ingested. The naturally-occurring lactic acid bacteria in raw milk, 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. 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 for many months, unrefrigerated. The most durable form of milk is milk powder which is produced from milk by removing almost all water.


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See also

Wikibooks
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External links

Retrieved from "http://en.wikipedia.org/wiki/Milk"
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