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:For other senses of this word, see milk (disambiguation). Milk is the nutrient fluid produced 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. Humans, like other mammals, can consume mother's milk during their infancy, but many cultures consume the milk of domesticated ruminants as well, especially milk from cows, but also that from sheep, goats, yaks, water buffalo, horses 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, although these amounts are not large in comparison to other foods rich in them, including coconuts, fish, and kale respectively. 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. 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. History 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. 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. In the Western world today, cow's milk is extracted on an industrial scale for human consumption and industrial uses. It is the most commonly consumed form of milk. Commercial-scale dairy farming using automated milking equipment produces the vast majority of milk in many 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 are Holsteins, and 85% in Great Britain (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 USA and New Zealand. Other milk animals 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. 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.). Physical and chemical structure
Processing 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. Pasteurization and raw milk 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. 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, salmonella, diphtheria, polio, and escherichia coli. 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. 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. Additives 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. 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: 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): 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. 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 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. Distribution
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 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. 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. 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 for many months, unrefrigerated. The most durable form of milk is milk powder which is produced from milk by removing almost all water. See also | |||||||||||
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