Milk

For other senses of this word, see milk (disambiguation).

File:Milk l de.jpg

A glass of cow milk

Milk is the nutrient fluid produced by the mammary glands of female mammals. 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. Humans, like other mammals, consume mother's milk during their infancy, but many human societies consume the milk of domesticated ruminants as well, especially milk from cows, but also that from sheep, goats, yaks, water buffalo, and camels. Milk can be processed into dairy products such as cream, butter, yogurt, ice cream, cheese, casein, whey protein, lactose, condensed milk, powdered milk, and many other food-additive and industrial products. Milk contains significant amounts of saturated fat, protein and calcium.

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

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

Composition and nutrition

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.[1] 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.

The structure of milk

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.

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 threories 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. (McGee 19–20) [2]

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. [3]

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)

Lactose intolerance

Main article: 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: the majority of humans stop producing significant amounts of lactase sometime between the ages of two and five. A relatively recent genetic change caused some populations, including many Northern Europeans, to continue producing lactase into adulthood; these lactose-tolerant populations are the exception and not the rule.

Cow milk allergy

Main article: Milk allergy

Cow milk allergy (CMA) is as an immunologically mediated adverse reaction to one or more cow's milk proteins.

Cow's milk

Holstein cattle, the dominant breed in industrialized dairying today.

In the Western world, cow's milk is 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 the highly automated. With farmers using machines that attach directly to teats of the cow's udder to speed milking, and breeds of cattle, such as Holstein, specially bred for increased milk production. According to McGee, 90% of the dairy cows in the United States are Holsteins, and 85% in Great Britain. (McGee 12)

History

Animal milk was first used as beverage at the beginning of animal domestication. Goats and sheep were domesticated in the Middle East in 9000 BC. Goats and sheep were one of the first animals to be domesticated. Around the year 7000 BC, cattle were being herded in parts of Africa and Turkey. Milk was also consumed in the British Isles during the Neolithic period. The use of cheese and butter spread in Europe, parts of Asia and parts of Africa. Cattle were then introduced to European colonies after the Age of exploration.

Commercial processing of milk

A milking machine in action.

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

Upon standing, 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. In the United States, 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.

Milk produced for commercial consumption usually undergoes several processes. Pasteurization kills many 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 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.

Additionally, commercial milk is often homogenized. This mechanically reduces the size of the fat globules, so that they will not separate out into a cream layer. Creamline milk is unhomogenized; it may or may not have been pasteurized.

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 recently by such other flavors as strawberry and banana.

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.

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. Raw Milk Versus Pasteurized Milk

Nutritional benefits

Milk began containing differing amounts of fat during the 1950's. 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.
• 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, and obesity. Overweight individuals who drink milk may benefit from decreased risk of insulin resistance and type 2 diabetes. [4]

Nutritional/physiological detriments

Consumption of cow's milk and its byproducts (hereafter referred to as "milk") are reported as 'unhealthy' primarily due to their high saturated fat and cholesterol content. The following additional issues are often cited as warranting consideration:

• Lactose intolerance, discussed above.
• Milk contains white blood cells; controversy surrounds whether these are simply somatic cells or, in an alternate form, [pus]] [5]. In the United States, one to seven drops of these cells are in every eight-ounce glass of milk, varying by state, according to guidelines set up by the FDA and statistics reported by the dairy industry [6]. Only one state out of all fifty, Hawaii, has a cell count lower than the dairy industry's recommendations; seventeen states produce milk that would be illegal to sell based on somatic cell limits in Europe.
• Since 1994, with FDA approval, Monsanto has been selling recombinant bovine somatotropin (rbST)--or rBGH--to dairy farmers. Bovine growth hormone is administered to cattle in order to increase their milk production, though the hormone also fosters liver production of insulin-like growth factor 1 (IGF1) and the deposit, thereof, in the milk of rBGH-affected cattle. Elevated levels of IGF1 in humans have been linked to increased rates of breast, colon, and prostate cancer by stimulating their growth [7] [8]. In addition, the cows frequently contract an udder infection known as mastitis, partly responsible for the aforementioned prevalence of blood cells in dairy products [9]. Milk from rBGH-affected cattle is banned in Canada, Australia, New Zealand, and Japan; the European Union also disallows the sale of this milk. 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 [10]."
• Milk that has not received a fat content reduction is rich in saturated fat and cholesterol, which numerous sources have suggested as contributing to an increased risk of atherosclerosis and coronary heart disease. Low-fat and non-fat forms of milk may mitigate this risk.
• Several sources suggest a correlation between high calcium intake and prostate cancer [11]. At least two large studies specifically implicate dairy [12], including one conducted by the European Commission [13], though diet is not the only factor. 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 dairy product consumption and prostate cancer.
• Consumption of milk is reported to increase the risk of bone fractures, due to animal protein's effect on intensifying urinary calcium excretion. It is because of this that milk may in fact contribute to osteoporosis, rather than preventing it as is commonly thought [14] [15] [16] [17] [18] [19].
• A study published in June 2005 of 9- to 14-year-old children found that children who reported drinking the most glasses of milk per day gained the most weight. However overall calorie intake was a better predictor of weight gain. Researchers were surprised by their conclusion that weight gain was associated with dietary calcium and low-fat or skim milk, but not dairy fat. A limitation of this study was that it was based on self-reported dietary intake, a method which can be inaccurate even when administered to adults [20] [21].

Distribution

Glass milk bottles used for home delivery service

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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 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 boxes 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.

Glass milk 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 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:

• United States: Commonly sold in gallon, half-gallon and quart containers (U.S. customary units) of rigid plastic or, occasionally for sizes less than a gallon, waxed cardboard. The US single-serving size is usually the half-pint (about 240 ml).
• Canada: A 1 1/3 litre plastic bag (sold as 4 litres in 3 bags) were common as the bags are most economical [22], although they have become less prevalent as the 4 litre plastic jug has supplanted it. Common packaging sizes are cartons of 2 litres, 1 litre, 500 millilitre, and 250 millilitre, as well as 4 litre, 1 litre, and 500 millilitre plastic jugs.
• Parts of Europe: Sizes of 500 millilitres, 1 litre (the most common), 2 litres and 3 litres are commonplace.
• United Kingdom: Most stores stock the equivalents of old Imperial sizes: 568 ml (1 pint), 1.136 l (2 pints), 2.273 l (4 pints), 3.408 l (6 pints) or, rarely, a combination including both metric and imperial sizes. Glass milk bottles delivered by the milkman are pint-sized.
• Australia and New Zealand: Distributed in a variety of sizes, most commonly in Tetra Paks for up to 1 litres, and plastic screw-top bottles beyond that. Most UHT-milk is packed in 1 litre paper containers with a sealed plastic spout.
• South Africa: Commonly sold in 1 litre bags. The bag is then placed in a plastic jug and the corner cut off before the milk is poured.
• China: Sweetened milk is a drink popular with students of all ages and is often sold in small plastic bags complete with straw. Adults not wishing to drink at a banquet often drink milk served from cartons or milk tea.

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.

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 % (reduced fat)
• 1 % (low fat)
• 1/2 % (very low fat)
• Skim (nearly no fat)

Note: 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 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.

Other milk animals

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

• Sheep
• Goats
• Horses
• 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 seals contains more than 50% fat. [23]

Whale's milk, not generally 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.

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 infants, infants with allergies, metabolic diseases, etc.).

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. There are four noted periods of milk decay:

• Rancid (also called "on the turn". Milk is still consumable at this stage)
• Curdling (separation of curd and whey will occur but may still be consumable)
• Coagulation (beyond use. A period of aromatic decay sets in accompanied by mould)
• Dry (beyond use. The milk has dehydrated and become hard and chalky)

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. The naturally-occurring lactic acid bacteria in raw milk, and 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.

References

• McGee, Harold (2004). On Food and Cooking (Revised Edition). Scribner. ISBN 0-684-80001-2.
• Introduction to Dairy Science and Technology: Milk History, Consumption, Production, and Composition
• Cow's Milk-Vegetarian or Non-vegetarian
• Milk
• How is Milk Made
• Milk Notes

See also

• Bovine somatotropin
• Babcock test - determines the butterfat content of milk.
• Grain milk
• Milk bottle
• Raw milk

External links

• Milking in the early days
• Advances in processing milk
• Year-round grazing of dairy cows in the United States
• Boning Up on Osteoporosis
• Horse milk
• Harvard School of Public Health: Calcium and Milk: describes claims of milk supporters and critics
• USDA National Nutrient Database, Calcium contents of selected foods
• USDA Dietary Reference Intakes: Elements
• Is cow's milk "the worst beverage on earth?" - straightdope.com response to anti-milk websites.
• MilkSucks.Com - a site run by PETA extolling the alleged benefits of going Dairy-free
• [24] National Dairy Council
• Act Against Allergy
• Milk Factsheet