From Academic Kids
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, dried milk, and many other food-additive and industrial products.
It can also be used to mean
- the white juice and the processed meat of the coconut in, more or less, liquid form, used especially in Thai, Indian (Kerala), and Polynesian cuisine.
- a non-animal substitute such as soya milk, rice milk, and almond milk.
Human milk is often fed to infants through breastfeeding, either directly or by the female expressing her milk to be saved and fed later. As colostrum, it carries the mother's antibodies and intestinal bacteria to the baby.
Composition and nutrition
The composition of milk varies greatly among different mammals.
- Human breast milk is thin and high in lactose, its primary sugar.
- Cow's milk, in contrast, is lower in sugar and higher in protein, and is composed of about 3.5% to 6.5% milkfat, 4% to 8.5% milk solids and about 88% water. Its main protein (80%) is casein, while whey proteins make up most of the rest.
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 (depending on the person's ethnic origin), in many cases to the point where lactose becomes indigestible, leading to lactose intolerance a gastrointestinal condition that afflicts many.
There is some controversy over whether consumption of cow's milk is good for adult humans. While milk is often touted as healthy for its significant amount of calcium, required for healthy bone growth and nerve function, there is some disputed research to suggest that proteins in milk interfere with the use of its calcium to form bones by increasing the acidity level of the blood and triggering a response which balances that acidity level by leeching calcium that is presently in bones. However, the composition of milk differs widely from species to species; cow's milk is a completely different substance than goat's milk for instance, which any person who tastes both will recognize. Such factors 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.
Cow's milk is most often produced on an industrial scale for human consumption. 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. Farmers even use machines which attach to the udder of the cow and milk it automatically.
Varieties and brands
Cow's milk is generally available in several varieties. In some countries these are:
- full cream (or "whole" in North America)
- 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.
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.
There are many brands of milk currently; most milk brands vary little from each other. These brands include:
- Anchor (a brand from Fonterra)
- Beatrice (http://www.beatrice.ca/) (a subsidiary of Parmalat in Canada - since March of 1997 (http://www.beatrice.ca/about/index.asp))
- Dairy Farmers
- H.P. Hood
- Leche Caparra
- Leche Suiza
- Lock City Dairies (http://www.lockcitydairies.com)
- Pauls (a subsidiary of Parmalat in Australia)
- Shamrock Milks
Other milk animals
In addition to cows, the following animals provide milk for dairy products:
- Camels (including the South American camelids)
- Water buffalo
In Russia and Sweden, small moose dairies exist  (http://www.cnn.com/2004/WORLD/europe/06/23/sweden.moosecheese.ap/index.html). Donkey and horse milk have the lowest fat content, while the milk of seals contains more than 50% fat.  (http://www.havemilk.com/content/contentid/1706#contentbyspecies)
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.
UHT milk is very popular in Europe, whereas in North America, most of the milk sold is pasteurized.
Prior to the widespread use of plastics, milk was usually commercially distributed to consumers in glass bottles. In the UK, milk can be delivered daily by a milk man who travels his local milk round (route) on an electric 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 milk is still delivered to customers' homes.
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 vessels are starting to use thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better. (However, few people have ever tasted fresh, unprocessed, milk straight from the cow.)
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 U.S. single serving size is usually the half-pint. In much of 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, metric sizes of 500 millilitres, 1 litre (the most common), 2 litres and 3 litres are commonplace (in the UK, some stores instead 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, such as a choice of 568 ml, 1 l, 2 l and 3 l containers).
Some nutritionists, animal-rights activists, and others criticize the widespread consumption of cow's milk by humans. They generally challenge the nutritional benefits of milk or raise ethical issues associated with its consumption.
Critics of milk claim that it can have adverse health effects that outweigh any benefits. They point to scientific studies suggesting links between milk and some health problems:
- Two studies suggested that galactose, which is produced in the digestion of lactose in milk, can be a cause of ovarian cancer. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=2510499) (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=2567871) Other studies failed to show such a link, however.
- One study suggests an association between high calcium intake and prostate cancer. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9458087) There is no evidence that any such problem is specific to milk.
- 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.
- As many as 70% of humans may suffer from some degree of lactose intolerance. For those individuals, milk may induce symptoms such as cramping, bloating, gas, and diarrhea.
- Critics question the claim that drinking large amounts of milk can reduce the risk of bone fractures, especially in the elderly. Some studies have failed to associate high calcium intakes with lower risk of hip and forearm fractures in men (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9278560) or women (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9224182).
- Recent studies show that milk may cause acne (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15692464).
- Some critics of milk claim that plant-based sources of calcium are preferable because, they claim, animal proteins in milk may "leach" calcium from bones. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=8198060) Such critics challenge the mainstream belief that milk lowers the risk of osteoporosis.
- 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. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15939853)
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. (http://www.nationaldairycouncil.org/NationalDairyCouncil/Nutrition/Reducing/DairysRoleManagingBP.htm)