Milk

Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to its young and can reduce the risk of many diseases. Milk contains many other nutrients and the carbohydrate lactose.

As an agricultural product, milk is extracted from mammals during or soon after pregnancy and is used as food for humans. Worldwide, dairy farms produced about 730 million tonnes of milk in 2011, from 260 million dairy cows. India is the world's largest producer of milk, and is the leading exporter of skimmed milk powder, yet has little to no other milk product exports. The ever increasing rise in domestic demand for dairy products and a large demand-supply gap could lead to India being a net importer of dairy products in the future. New Zealand, the European Union's 28 member states, Australia, and the United States are the world's largest exporters of milk and milk products. China and Russia are the world's largest importers of milk and milk products.

Throughout the world, there are more than six billion consumers of milk and milk products. Over 750 million people live within dairy farming households.

Types of consumption
There are two distinct types of milk consumption: a natural source of nutrition for all infant mammals and a food product for humans of all ages that is derived from other animals.

Nutrition for infant mammals
In almost all mammals, milk is fed to infants through breastfeeding, either directly or by expressing the milk to be stored and consumed later. The early milk from mammals is called colostrum. Colostrum contains antibodies that provide protection to the newborn baby as well as nutrients and growth factors. The makeup of the colostrum and the period of secretion varies from species to species.

For humans, the World Health Organization recommends exclusive breastfeeding for six months and breastfeeding in addition to other food for at least two years. In some cultures it is common to breastfeed children for three to five years, and the period may be longer.

Fresh goats' milk is sometimes substituted for breast milk. This introduces the risk of the child developing electrolyte imbalances, metabolic acidosis, megaloblastic anemia, and a host of allergic reactions.

Food product for humans


In many cultures of the world, especially the West, humans continue to consume milk beyond infancy, using the milk of other animals (especially cattle, goats and sheep) as a food product. Initially, the ability to digest milk was limited to children as adults did not produce lactase, an enzyme necessary for digesting the lactose in milk. Milk was therefore converted to curd, cheese and other products to reduce the levels of lactose. Thousands of years ago, a chance mutation spread in human populations in Europe that enabled the production of lactase in adulthood. This allowed milk to be used as a new source of nutrition which could sustain populations when other food sources failed. Milk is processed into a variety of dairy products such as cream, butter, yogurt, kefir, ice cream, and cheese. Modern industrial processes use milk to produce casein, whey protein, lactose, condensed milk, powdered milk, and many other food-additives and industrial products.

Whole milk, butter and cream have high levels of saturated fat. The sugar, lactose, is found only in milk, forsythia flowers, and a few tropical shrubs. The enzyme needed to digest lactose, lactase, reaches its highest levels in the small intestine after birth and then begins a slow decline unless milk is consumed regularly. Those groups who do continue to tolerate milk, however, often have exercised great creativity in using the milk of domesticated ungulates, not only of cattle, but also sheep, goats, yaks, water buffalo, horses, reindeer and camels. The largest producer and consumer of cattle and buffalo milk in the world is India.

Terminology
The term milk is also used for white colored, non-animal beverages resembling milk in color and texture (milk substitutes) such as soy milk, rice milk, almond milk, and coconut milk. In addition, a substance secreted by pigeons to feed their young is called "crop milk" and bears some resemblance to mammalian milk, although it is not consumed as a milk substitute. Dairy relates to milk and milk production, e.g. dairy products. Milk can be synthesized in a laboratory, from water, fatty acids and proteins.

Evolution of lactation
The mammary gland is thought to have derived from apocrine skin glands. It has been suggested that the original function of lactation (milk production) was keeping eggs moist. Much of the argument is based on monotremes (egg-laying mammals). The original adaptive significance of milk secretions may have been nutrition or immunological protection. This secretion gradually became more copious and accrued nutritional complexity over evolutionary time.

History
Humans first learned to regularly consume the milk of other mammals following the domestication of animals during the Neolithic Revolution or the development of agriculture. This development occurred independently in several places around the world from as early as 9000–7000 BC in Southwest Asia to 3500–3000 BC in the Americas. The most important dairy animals—cattle, sheep and goats—were first domesticated in Southwest Asia, although domestic cattle had been independently derived from wild aurochs populations several times since. Initially animals were kept for meat, and archaeologist Andrew Sherratt has suggested that dairying, along with the exploitation of domestic animals for hair and labor, began much later in a separate secondary products revolution in the fourth millennium BC. Sherratt's model is not supported by recent findings, based on the analysis of lipid residue in prehistoric pottery, that shows that dairying was practiced in the early phases of agriculture in Southwest Asia, by at least the seventh millennium BC.

From Southwest Asia domestic dairy animals spread to Europe (beginning around 7000 BC but not reaching Britain and Scandinavia until after 4000 BC), and South Asia (7000–5500 BC). The first farmers in central Europe and Britain milked their animals. Pastoral and pastoral nomadic economies, which rely predominantly or exclusively on domestic animals and their products rather than crop farming, were developed as European farmers moved into the Pontic-Caspian steppe in the fourth millennium BC, and subsequently spread across much of the Eurasian steppe. Sheep and goats were introduced to Africa from Southwest Asia, but African cattle may have been independently domesticated around 7000–6000 BC. Camels, domesticated in central Arabia in the fourth millennium BC, have also been used as dairy animals in North Africa and the Arabian Peninsula. The earliest Egyptian records of burn treatments describe burn dressings using milk from mothers of male babies. In the rest of the world (i.e., East and Southeast Asia, the Americas and Australia) milk and dairy products were historically not a large part of the diet, either because they remained populated by hunter-gatherers who did not keep animals or the local agricultural economies did not include domesticated dairy species. Milk consumption became common in these regions comparatively recently, as a consequence of European colonialism and political domination over much of the world in the last 500 years.

In the Middle Ages, milk was called the "virtuous white liquor" because alcoholic beverages were more safe to consume than water.

Industrialization
The growth in urban population coupled with the expansion of the railway network in the mid-19th century, brought about a revolution in milk production and supply. Individual railway firms began transporting milk from rural areas to London from the 1840s and 1850s. Possibly the first such instance was in 1846, when St Thomas's Hospital in Southwark contracted with milk suppliers outside London to provide milk by rail. The Great Western Railway was an early and enthusiastic adopter, and began to transport milk into London from Maidenhead in 1860, despite much criticism. By 1900, the company was transporting over 25 million gallons annually. The milk trade grew slowly through the 1860s, but went through a period of extensive, structural change in the 1870s and 1880s. Urban demand began to grow, as consumer purchasing power increased and milk became regarded as a required daily commodity. Over the last three decades of the 19th century, demand for milk in most parts of the country doubled, or in some cases, tripled. Legislation in 1875 made the adulteration of milk illegal - this combined with a marketing campaign to change the image of milk. The proportion of rural imports by rail as a percentage of total milk consumption in London grew from under 5% in the 1860s to over 96% by the early 20th century. By that point, the supply system for milk was the most highly organized and integrated of any food product.

The first glass bottle packaging for milk was used in the 1870s. The first company to do so may have been the New York Dairy Company in 1877. The Express Dairy Company in England began glass bottle production in 1880. In 1884, Hervey Thatcher, an American inventor from New York, invented a glass milk bottle, called 'Thatcher's Common Sense Milk Jar', which was sealed with a waxed paper disk. Later, in 1932, plastic-coated paper milk cartons were introduced commercially.

In 1863, French chemist and biologist Louis Pasteur invented pasteurization, a method of killing harmful bacteria in beverages and food products. He developed this method while on summer vacation in Arbois, to remedy the frequent acidity of the local wines. He found out experimentally that it is sufficient to heat a young wine to only about 50 - 60 °C for a brief time to kill the microbes, and that the wine could be nevertheless properly aged without sacrificing the final quality. In honor of Pasteur, the process became known as "pasteurization". Pasteurization was originally used as a way of preventing wine and beer from souring. Commercial pasteurizing equipment was produced in Germany in the 1880s, and the process had been adopted in Copenhagen and Stockholm by 1885.

Sources of milk
The females of all mammal species can by definition produce milk, but cow's milk dominates commercial production. In 2011, FAO estimates 85% of all milk worldwide was produced from cows.

Human milk is not produced or distributed industrially or commercially; however, human milk banks collect donated human breastmilk and redistribute it to infants who may benefit from human milk for various reasons (premature neonates, babies with allergies, metabolic diseases, etc.) but who cannot breastfeed.

In the Western world, cow's milk is produced on an industrial scale and 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. Dairy cattle such as the Holstein have been bred selectively for increased milk production. About 90% of the dairy cows in the United States and 85% in Great Britain are Holsteins. Other dairy cows in the United States include Ayrshire, Brown Swiss, Guernsey, Jersey and Milking Shorthorn (Dairy Shorthorn).

Sources aside from cows
Aside from cattle, many kinds of livestock provide milk used by humans for dairy products. These animals include buffalo, goat, sheep, camel, donkey, horse, reindeer and yak. The first four respectively produced about 11%, 2%, 1.4% and 0.2% of all milk worldwide in 2011.

In Russia and Sweden, small moose dairies also exist.

According to the US National Bison Association, American bison (also called American buffalo) are not milked commercially; however, various sources report cows resulting from cross-breeding bison and domestic cattle are good milk producers, and have been used both during the European settlement of North America and during the development of commercial Beefalo in the 1970s and 1980s.

Production worldwide
In 2009, the largest producer of milk and milk products was the European Union followed by India, the United States, China, Germany, Brazil, and Russia. All European Union members together produced about 138 million tonnes of milk in 2011.

Increasing affluence in developing countries, as well as increased promotion of milk and milk products, has led to a rise in milk consumption in developing countries in recent years. In turn, the opportunities presented by these growing markets have attracted investments by multinational dairy firms. Nevertheless, in many countries production remains on a small scale and presents significant opportunities for diversification of income sources by small farms. Local milk collection centers, where milk is collected and chilled prior to being transferred to urban dairies, are a good example of where farmers have been able to work on a cooperative basis, particularly in countries such as India.

Production yields
FAO reports Israel dairy farms are the most productive in the world, with a yield of 12,546 kg milk per cow per year. This survey over 2001 and 2007 was conducted by ICAR (International Committee for Animal Recording ) across 17 developed countries. The survey found that the average herd size in these developed countries increased from 74 to 99 cows per herd between 2001 to 2007. A dairy farm had an average of 19 cows per herd in Norway, and 337 in New Zealand. Annual milk production in the same period increased from 7,726 to 8,550 kg per cow in these developed countries. The lowest average production was in New Zealand at 3,974 kg per cow. The milk yield per cow depended on production systems, nutrition of the cows, and only to a minor extent different genetic potential of the animals. What the cow ate made the most impact on the production obtained. New Zealand cows with the lowest yield per year grazed all year, in contrast to Israel with the highest yield where the cows ate in barns with an energy-rich mixed diet.

The milk yield per cow in the United States, the world's largest cow milk producer, was 9,954 kg per year in 2010. In contrast, the milk yields per cow in India and China – the second and third largest producers – were respectively 1,154 kg and 2,282 kg per year.

Price
It was reported in 2007 that with increased worldwide prosperity and the competition of bio-fuel production for feed stocks, both the demand for and the price of milk had substantially increased worldwide. Particularly notable was the rapid increase of consumption of milk in China and the rise of the price of milk in the United States above the government subsidized price. In 2010 the Department of Agriculture predicted farmers would receive an average of $1.35 per US gallon of cow's milk (35 cents per liter), which is down 30 cents per gallon from 2007 and below the break-even point for many cattle farmers.

Grading
In the United States, there are two grades of milk, with grade A primarily used for direct sales and consumption in stores, and grade B used for indirect consumption, such as in cheese making or other processing.

The differences between the two grades are defined in the Wisconsin administrative code for Agriculture, Trade, and Consumer Protection, chapter 60. Grade B generally refers to milk that is cooled in milk cans, which are immersed in a bath of cold flowing water that typically is drawn up from an underground water well rather than using mechanical refrigeration.

Physical and chemical properties of milk
Milk is an emulsion or colloid of butterfat globules within a water-based fluid that contains dissolved carbohydrates and protein aggregates with minerals. Because it is produced as a food source for the young, all of its contents provide benefits for its growth. The principal requirements are energy (lipids, lactose, and protein), biosynthesis of non-essential amino acids supplied by proteins (essential amino acids and amino groups), essential fatty acids, vitamins and inorganic elements, and water.
 * TriglycerideDairyButter.png (fat) formed from fatty acids such as myristic, palmitic, and oleic acids.]]

Lipids
Initially milk fat is secreted in the form of a fat globule surrounded by a membrane. Each fat globule is composed almost entirely of triacylglycerols and is surrounded by a membrane consisting of complex lipids such as phospholipids, along with proteins. These act as emulsifiers which keep the individual globules from coalescing and protect the contents of these globules from various enzymes in the fluid portion of the milk. Although 97–98% of lipids are triacylglycrols, small amounts of di- and monoacylglycerols, free cholesterol and cholesterol esters, free fatty acids, and phospholipids are also present. Unlike protein and carbohydrates, fat composition in milk varies widely in the composition due to genetic, lactational, and nutritional factor difference between different species.

Like composition, fat globules vary in size from less than 0.2 to about 15 micrometers in diameter between different species. Diameter may also vary between animals within a species and at different times within a milking of a single animal. In unhomogenized cow's milk, the fat globules have an average diameter of two to four micrometers and with homogenization, average around 0.4 micrometers. The fat-soluble vitamins A, D, E, and K along with essential fatty acids such as linoleic and linolenic acid are found within the milk fat portion of the milk.

Proteins
Normal bovine milk contains 30–35 grams of protein per liter of which about 80% is arranged in casein micelles.

Caseins
The largest structures in the fluid portion of the milk are "casein micelles": aggregates of several thousand protein molecules with superficial resemblance to a surfactant micelle, bonded with the help of nanometer-scale particles of calcium phosphate. Each casein micelle is roughly spherical and about a tenth of a micrometer across. There are four different types of casein proteins: αs1-, αs2-, β-, and κ-caseins. Collectively, they make up around 76–86% 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, k-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.

Milk contains dozens of other types of proteins beside the caseins including enzymes. These other proteins are more water-soluble than the caseins and do not form larger structures. Because the 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 approximately 20% of the protein in milk, by weight. Lactoglobulin is the most common whey protein by a large margin.

Salts, minerals, and vitamins
Minerals or milk salts, are traditional names for a variety of cations and anions within bovine milk. ,, , , , citrate, and are all included as minerals and they typically occur at concentration of 5–40 mM. The milk salts strongly interact with casein, most notably calcium phosphate. It is present in excess and often, much greater excess of solubility of solid calcium phosphate. In addition to calcium, milk is a good source of many other vitamins. Vitamins A, B6, B12, C, D, K, E, thiamine, niacin, biotin, riboflavin, folates, and pantothenic acid are all present in milk.

Calcium phosphate structure
For many years the most accepted theory of the structure of a micelle was that it was composed of spherical casein aggregates, called submicelles, that were held together by calcium phosphate linkages. However, there are two recent models of the casein micelle that refute the distinct micellular structures within the micelle.

The first theory attributed to de Kruif and Holt, proposes that nanoclusters of calcium phosphate and the phosphopeptide fraction of beta-casein are the centerpiece to micellular structure. Specifically in this view, unstructured proteins organize around the calcium phosphate giving rise to their structure and thus no specific structure is formed.

The second theory proposed by Horne, the growth of calcium phosphate nanoclusters begins the process of micelle formation but is limited by binding phosphopeptide loop regions of the caseins. Once bound, protein-protein interactions are formed and polymerization occurs, in which K-casein is used as an end cap, to form micelles with trapped calcium phosphate nanoclusters.

Some sources indicate that the trapped calcium phosphate is in the form of Ca9(PO4)6; whereas, others say it is similar to the structure of the mineral brushite CaHPO4 -2H2O.

Carbohydrates and miscellaneous contents
Milk contains several different carbohydrate including lactose, glucose, galactose, and other oligosaccharides. The lactose gives milk its sweet taste and contributes approximately 40% of whole cow's milk's calories. Lactose is a disaccharide composite of two simple sugars, glucose and galactose. Bovine milk averages 4.8% anhydrous lactose, which amounts to about 50% of the total solids of skimmed milk. Levels of lactose are dependent upon the type of milk as other carbohydrates can be present at higher concentrations that lactose in milks.

Other components found in raw cow's milk are living white blood cells, mammary gland cells, various bacteria, and a large number of active enzymes.

Appearance
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 (such as Guernsey and Jersey cattle) 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 sometimes can be discerned in skimmed milk or whey products. Fat-free skimmed milk has only the casein micelles to scatter light, and they tend to scatter shorter-wavelength blue light more than they do red, giving skimmed milk a bluish tint.

Processing
In most Western countries, centralized dairy facilities process milk and products obtained from milk, such as cream, butter, and cheese. In the US, these dairies usually are local companies, while in the Southern Hemisphere facilities may be run by very large nationwide or trans-national corporations such as Fonterra.

Pasteurization
Pasteurization is used to kill harmful microorganisms by heating the milk for a short time and then immediately cooling it. The standard high temperature short time (HTST) process produces a 99.999% reduction in the number of bacteria in milk, rendering it safe to drink for up to three weeks if continually refrigerated. Dairies print expiration dates on each container, after which stores remove any unsold milk from their shelves.

A side effect of the heating of pasteurization is that some vitamin and mineral content is lost. Soluble calcium and phosphorus decrease by 5%, thiamin and vitamin B12 by 10%, and vitamin C by 20%. Because losses are small in comparison to the large amount of the two B-vitamins present, milk continues to provide significant amounts of thiamin and vitamin B12. The loss of vitamin C is not nutritionally significant, as milk is not an important dietary source of vitamin C.

A newer process, ultrapasteurization or ultra-high temperature treatment (UHT), heats the milk to a higher temperature for a shorter amount of time. This extends its shelf life and allows the milk to be stored unrefrigerated because of the longer lasting sterilization effect.

Microfiltration
Microfiltration is a process that partially replaces pasteurization and produces milk with fewer microorganisms and longer shelf life without a change in the taste of the milk. In this process, cream is separated from the whey and is pasteurized in the usual way, but the whey is forced through ceramic microfilters that trap 99.9% of microorganisms in the milk (as compared to 99.999% killing of microorganisms in standard HTST pasteurization). The whey then is recombined with the pasteurized cream to reconstitute the original milk composition.

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 often is sold as a separate product with its own uses. Today the separation of the cream from the milk usually is 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's milk much more quickly 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. 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, resulting in a slower separation of cream from these milks.

Milk often is homogenized, a treatment that 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 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. Creamline (or cream-top) milk is unhomogenized. It may or may not have been pasteurized. Milk that has undergone high-pressure homogenization, sometimes labeled as "ultra-homogenized," has a longer shelf life than milk that has undergone ordinary homogenization at lower pressures.

Nutrition and health
The composition of milk differs widely among 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 that may vary. 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, 0.7% minerals and supplies 66 kcal of energy per 100 grams. See also Nutritional value further on

Donkey and horse milk have the lowest fat content, while the milk of seals and whales may contain more than 50% fat.

Cow's milk
These compositions vary by breed, animal, and point in the lactation period.

The protein range for these four breeds is 3.3% to 3.9%, while the lactose range is 4.7% to 4.9%.

Milk fat percentages may be manipulated by dairy farmers' stock diet formulation strategies. Mastitis infection can cause fat levels to decline.

Nutritional value
Processed cow's milk was formulated to contain differing amounts of fat during the 1950s. One cup (250 ml) of 2%-fat cow's milk contains 285 mg of calcium, which represents 22% to 29% of the daily recommended intake (DRI) of calcium for an adult. Depending on its age, milk contains 8 grams of protein, and a number of other nutrients (either naturally or through fortification) including:
 * Biotin
 * Iodine
 * Magnesium
 * Pantothenic acid
 * Potassium
 * Riboflavin
 * Selenium
 * Thiamine
 * Vitamin A
 * Vitamin B12
 * Vitamins D
 * Vitamin K

The amount of calcium from milk that is absorbed by the human body is disputed. Calcium from dairy products has a greater bioavailability than calcium from certain vegetables, such as spinach, that contain high levels of calcium-chelating agents, but a similar or lesser bioavailability than calcium from low-oxalate vegetables such as kale, broccoli, or other vegetables in the Brassica genus.

Milk as a calcium source has been questioned in media, but scientific research is lacking to support the hypothesis of acidosis induced by milk. The hypothesis in question being that acidosis would lead to leeching of calcium storages in bones to neutralize pH levels (also known as acid-ash hypothesis). Research has found no link between metabolic acidosis and consumption of milk.

Recommended consumption
The U.S. federal government document Dietary Guidelines for Americans, 2010 recommends consumption of three glasses of fat-free or low-fat milk for adults and children 9 and older (less for younger children) per day. This recommendation is disputed by some health researchers who call for more study of the issue, given that there are other sources for calcium and vitamin D. The researchers also claim that the recommendations have been unduly influenced by the American dairy industry, and that whole milk may be better for health due to its increased ability to satiate hunger.

Medical research
There is recent evidence suggesting consumption of milk is effective at promoting muscle growth. Some studies have suggested that conjugated linoleic acid, which can be found in dairy products, is an effective supplement for reducing body fat.

Lactose intolerance
Lactose, the disaccharide sugar component of all milk, must be cleaved in the small intestine by the enzyme, lactase, in order for its constituents, galactose and glucose, to be absorbed. The production of the enzyme lactase declines significantly after weaning in all mammals. Consequently, many humans become unable to digest lactose properly as they mature. There is a great deal of variance, with some individuals reacting badly to even small amounts of lactose, some able to consume moderate quantities, and some able to consume large quantities of milk and other dairy products without problems. The gene in humans that controls lactase production, and hence lactose tolerance and intolerance, is labeled C/T-13910. An individual who consumes milk without producing sufficient lactase may suffer diarrhea, intestinal gas, cramps and bloating, as the undigested lactose travels through the gastrointestinal tract and serves as nourishment for intestinal microflora that excretes gas in processes known as fermentation and anaerobic respiration.

It is estimated that 30 to 50 million Americans are lactose intolerant, including 75% of Native Americans and African Americans, and 90% of Asian Americans. Lactose intolerance is less common among those descended from northern Europeans. Other genetic groups that have a lower prevalence of lactose intolerance are the Tuareg of the Sahara, the Fulani of the West African Sahel, and the Beja and Kabbabish of Sudan, as well as possibly the Tutsi population of the Uganda–Rwanda area. Another locus of lactose tolerance is in Northern India.

Lactose intolerance is a natural process and there is no reliable way to prevent or reverse it. "Having lactose intolerance doesn't mean saying goodbye to dairy forever. Some dairy foods are easier to digest than others, including yogurt and aged cheese. Lactose-intolerant people vary in how much lactose they can tolerate, but dairy and aged cheese are easier to digest because processing has already broken down some of the lactose. If you take supplemental lactase, which is an enzyme that breaks down lactose, you can generally enjoy these foods and other dairy products without experiencing the unpleasant side effects."

Possible harms
Some studies suggest that milk consumption may increase the risk of suffering from certain health problems. Cow's milk allergy (CMA) is an immunologically mediated adverse reaction, rarely fatal, to one or more cow's milk proteins. Milk from any mammal contains amino acids and microRNA which influence the drinker's metabolism and growth; this "programming" is beneficial for milk's natural consumers, namely infants of the same species as the milk producer, but post-infancy and trans-species milk consumption affects the mTORC1 metabolic pathway and may promote diseases of civilization such as obesity and diabetes.

Milk contains casein, a substance that breaks down in the human stomach to produce casomorphin, an opioid peptide. In the early 1990s it was hypothesized that casomorphin can cause or aggravate autism spectrum disorders, and casein-free diets are widely promoted. Studies supporting these claims have had significant flaws, and the data are inadequate to guide autism treatment recommendations.

A study demonstrated that men who drink a large amount of milk and consume dairy products were at a slightly increased risk of developing Parkinson's disease; the effect for women was smaller. The reason behind this is not fully understood, and it also remains unclear why there is less of a risk for women.

The most recent assessment by the World Cancer Research Fund and the American Institute for Cancer Research found that most individual epidemiological studies showed increased risk of prostate cancer with increased intake of milk or dairy products. "Meta-analysis of cohort data produced evidence of a clear dose-response relationship between advanced/aggressive cancer risk with milk intake, and between all prostate cancer risk and milk and dairy products." Possible mechanisms proposed included inhibition of the conversion of vitamin D to its active metabolite, 1,25- dihydroxy vitamin D3 by calcium (which some evidence suggests increases cell proliferation in the prostate), and elevation of levels of insulin-like growth factor-1 (IGF-1). Several sources suggest a correlation between high calcium intake from milk, in particular, and prostate cancer,  consistent with a calcium/vitamin D based mechanism. Overall, the WCRF/AICR panel concluded that "The evidence is inconsistent from both cohort and case-control studies. There is limited evidence suggesting that milk and dairy products are a cause of prostate cancer."

Medical studies also have shown a possible link between milk consumption and the exacerbation of diseases such as Crohn's disease, Hirschsprung's disease–mimicking symptoms in babies with existing cow's milk allergies, and the aggravation of Behçet's disease.

Flavored milk in US schools
Milk must be offered at every meal if a United States school district wishes to get reimbursement from the federal government. A quarter of the largest school districts in the US offer rice or soy milk and almost 17% of all US school districts offer lactose-free milk. Seventy-one percent of the milk served in US school cafeterias is flavored, causing some school districts to propose a ban because flavored milk has added sugars. (Though some flavored milk products use artificial sweeteners instead.) The Boulder, Colorado, school district banned flavored milk in 2009 and instead installed a dispenser that keeps the milk colder.

Bovine growth hormone supplementation
Since November 1993, recombinant bovine somatotropin (rbST), also called rBGH, has been sold to dairy farmers with FDA approval. Cows produce bovine growth hormone naturally, but some producers administer an additional recombinant version of BGH which is produced through a genetically engineered E. coli because it increases milk production. Bovine growth hormone also stimulates liver production of insulin-like growth factor 1 (IGF1). Monsanto has stated that both of these compounds are harmless given the levels found in milk and the effects of pasteurization.

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." Milk from cows given rBST may be sold in the United States, and the FDA stated that no significant difference has been shown between milk derived from rBST-treated and that from non-rBST-treated cows. Milk that advertises that it comes from cows not treated with rBST, is required to state this finding on its label.

Cows receiving rBGH supplements may more frequently contract an udder infection known as mastitis. Problems with mastitis have led to Canada, Australia, New Zealand, and Japan banning milk from rBST treated cows. Mastitis, among other diseases, may be responsible for the fact that levels of white blood cells in milk vary naturally.

rBGH is also banned in the European Union.

Criticism
Vegans and some other vegetarians do not consume milk for reasons mostly related to animal rights and environmental concerns. They may object to features of dairy farming including the necessity of keeping dairy cows pregnant, the killing of almost all the male offspring of dairy cows (either by disposal soon after birth, for veal production, or for beef), the routine separation of mother and calf soon after birth, other perceived inhumane treatment of dairy cattle, and culling of cows after their productive lives.

Some have criticized the American government's promotion of milk consumption. The main concern is the financial interest that the American government has taken in the dairy industry, promoting milk as the best source of calcium. All United States schools that are a part of the federally funded National School Lunch Act are required by the federal government to provide milk for all students. The Office of Dietary Supplements recommends that healthy adults between ages 19 and 50 get about 1,000 mg of calcium per day, but studies show that the human body can only retain about 550 mg of calcium per day. Milk also contains more excess calories, sugar, and fat than many other sources of calcium.

There is also some skepticism of the idea that large doses of calcium provide for healthier bones and teeth. This is a commonly held belief, but there have been some studies that show there is "no connection between the intake of calcium (in any form) and the reduced risk of bone fractures in women aged 34-71 years". Another study suggests that calcium supplements do not contribute to bone gain when a daily intake of 800 mg is surpassed.

In 2011, The Journal of Bone and Mineral Research published a meta-analysis examining whether milk consumption might protect against hip fracture in middle-aged and older adults. Studies could find no association between drinking milk and lower rates of fractures. In 2014, JAMA Pediatrics published a report after monitoring almost 100,000 men and women for more than two decades. Subjects were asked to report on how much milk they had consumed as teenagers, and were followed to see if there is any association with a reduced chance of hip fractures later in life, it found there was not any. A study published in The BMJ that followed more than 45,000 men and 61,000 women in Sweden age 39 and older had similar results. Milk consumption in adults was associated with no protection for men, and an increased risk of fractures in women. The risk of any bone fracture increased 16 percent in women who drank three or more glasses daily, and the risk of a broken hip increased 60 percent. It was also associated with an increased risk of death in both sexes.

Varieties and brands
Milk products are sold in a number of varieties based on types/degrees of
 * additives (e.g., vitamins),
 * age (e.g., cheddar),
 * coagulation (e.g., cottage cheese),
 * farming method (e.g., organic, grass-fed).
 * fat content (e.g., half and half),
 * fermentation (e.g., buttermilk),
 * flavoring (e.g., chocolate and strawberry),
 * homogenization (e.g., cream top),
 * reduction or elimination of lactose,
 * mammal (e.g., cow, goat, sheep),
 * packaging (e.g., bottle),
 * pasteurization (e.g., raw milk),
 * water content (e.g., dry milk)

Milk preserved by the UHT process does not need to be refrigerated before opening and has a longer shelf life than milk in ordinary packaging. It is typically sold unrefrigerated in the UK, US, Europe, Latin America, and Australia.

Reduction or elimination of lactose
Lactose-free milk can be produced by passing milk over lactase enzyme bound to an inert carrier. Once the molecule is cleaved, there are no lactose ill effects. Forms are available with reduced amounts of lactose (typically 30% of normal), and alternatively with nearly 0%. The only noticeable difference from regular milk is a slightly sweeter taste due to the generation of glucose by lactose cleavage. It does not, however, contain more glucose, and is nutritionally identical to regular milk.

Finland, where approximately 17% of the Finnish-speaking population has hypolactasia, has had "HYLA" (acronym for hydrolysed lactose) products available for many years. Lactose of low-lactose level cow's milk products, ranging from ice cream to cheese, is enzymatically hydrolysed into glucose and galactose. The ultra-pasteurization process, combined with aseptic packaging, ensures a long shelf life. In 2001, Valio launched a lactose-free milk drink that is not sweet like HYLA milk but has the fresh taste of ordinary milk. Valio patented the chromatographic separation method to remove lactose. Valio also markets these products in Sweden, Estonia, Belgium, and the United States, where the company says ultrafiltration is used.

In the UK, where an estimated 15% of the population are affected by lactose intolerance, Lactofree produces milk, cheese, and yogurt products that contain only 0.03% lactose.

To aid digestion in those with lactose intolerance, milk with added bacterial cultures such as Lactobacillus acidophilus ("acidophilus milk") and bifidobacteria ("a/B milk") is available in some areas. Another milk with Lactococcus lactis bacteria cultures ("cultured buttermilk") often is used in cooking to replace the traditional use of naturally soured milk, which has become rare due to the ubiquity of pasteurization, which also kills the naturally occurring Lactococcus bacteria.

Additives and flavoring
In areas where the cattle (and often the people) live indoors, commercially sold milk commonly has vitamin D added to it to make up for lack of exposure to UVB radiation.

Reduced-fat milks often have added vitamin A palmitate to compensate for the loss of the vitamin during fat removal; in the United States this results in reduced fat milks having a higher vitamin A content than whole milk.

Milk often has flavoring added to it for better taste or as a means of improving sales. Chocolate milk has been sold for many years and has been followed more recently by strawberry milk and others. Some nutritionists have criticized flavored milk for adding sugar, usually in the form of high-fructose corn syrup, to the diets of children who are already commonly obese in the US.

Distribution


Due to the short shelf life of normal milk, it used to be delivered to households daily in many countries; however, improved refrigeration at home, changing food shopping patterns because of supermarkets, and the higher cost of home delivery mean that daily deliveries by a milkman are no longer available in most countries.

Australia and New Zealand
In Australia and New Zealand, prior to metrication, milk was generally distributed in 1 pint (568ml) glass bottles. In Australia and Ireland there was a government funded "free milk for school children" program, and milk was distributed at morning recess in 1/3 pint bottles. With the conversion to metric measures, the milk industry were concerned that the replacement of the pint bottles with 500ml bottles would result in a 13.6% drop in milk consumption; hence, all pint bottles were recalled and replaced by 600 mL bottles. With time, due to the steadily increasing cost of collecting, transporting, storing and cleaning glass bottles, they were replaced by cardboard cartons. A number of designs were used, including a tetrahedron which could be close-packed without waste space, and could not be knocked over accidentally. (slogan: No more crying over spilt milk.) However, the industry eventually settled on a design similar to that used in the United States.

Milk is now available in a variety of sizes in cardboard cartons (250 mL, 375 mL, 600 mL, 1 liter and 1.5 liters) and plastic bottles (1, 2 and 3 liters). A significant addition to the marketplace has been "long-life" milk (UHT), generally available in 1 and 2 liter rectangular cardboard cartons. In urban and suburban areas where there is sufficient demand, home delivery is still available, though in suburban areas this is often 3 times per week rather than daily. Another significant and popular addition to the marketplace has been flavored milks – for example, as mentioned above, Farmers Union Iced Coffee outsells Coca-Cola in South Australia.

India
In rural India, milk is home delivered, daily, by local milkmen carrying bulk quantities in a metal container, usually on a bicycle. In other parts of metropolitan India, milk is usually bought or delivered in plastic bags or cartons via shops or supermarkets.

The current milk chain flow in India is from milk producer to milk collection agent. Then it is transported to a milk chilling center and bulk transported to the processing plant, then to the sales agent and finally to the consumer.

A 2011 survey by the Food Safety and Standards Authority of India found that nearly 70 per cent of samples had not conformed to the standards set for milk. The study found that due to lack of hygiene and sanitation in milk handling and packaging, detergents (used during cleaning operations) were not washed properly and found their way into the milk. About eight per cent of samples in the survey were found to have detergents, which are hazardous to health.

Pakistan
In Pakistan, milk is supplied in jugs. Milk has been a staple food, especially among the pastoral tribes in this country.

United Kingdom
Since the late 1990s, milk-buying patterns have changed drastically in the UK. The classic milkman, who travels his local milk round (route) using a milk float (often battery powered) during the early hours and delivers milk in 1 pint glass bottles with aluminium foil tops directly to households, has almost disappeared. Two of the main reasons for the decline of UK home deliveries by milkmen are household refrigerators (which lessen the need for daily milk deliveries) and private car usage (which has increased supermarket shopping). Another factor is that it is cheaper to purchase milk from a supermarket than from home delivery. In 1996, more than 2.5 billion liters of milk were still being delivered by milkmen, but by 2006 only 637 million liters (13% of milk consumed) was delivered by some 9,500 milkmen. By 2010, the estimated number of milkmen had dropped to 6,000. Assuming that delivery per milkman is the same as it was in 2006, this means milkmen deliveries now only account for 6–7% of all milk consumed by UK households (6.7 billion liters in 2008/2009).

Almost 95% of all milk in the UK is thus sold in shops today, most of it in plastic bottles of various sizes, but some also in milk cartons. Milk is hardly ever sold in glass bottles in UK shops.

United States
In the United States, glass milk bottles have been replaced mostly with milk cartons and plastic jugs. Gallons of milk are almost always sold in jugs, while half gallons and quarts may be found in both paper cartons and plastic jugs, and smaller sizes are almost always in cartons.

The "half pint" .5 USpt milk carton is the traditional unit as a component of school lunches, though some companies have replaced that unit size with a plastic bottle, which is also available at retail in 6- and 12-pack size.

Packaging
Glass milk bottles are now rare. Most people purchase milk in bags, plastic bottles, or plastic-coated paper cartons. Ultraviolet (UV) light from fluorescent lighting can alter the flavor of milk, so many companies that once distributed milk in transparent or highly translucent containers are now using thicker materials that block the UV light. Milk comes in a variety of containers with local variants:
 * Australia and New Zealand
 * Distributed in a variety of sizes, most commonly in aseptic cartons for up to 1.5 liters, and plastic screw-top bottles beyond that with the following volumes; 1.1 L, 2 L, and 3 L. 1 liter milk bags are starting to appear in supermarkets, but have not yet proved popular. Most UHT-milk is packed in 1 or 2 liter paper containers with a sealed plastic spout.


 * Brazil
 * Used to be sold in cooled 1 liter bags, just like in South Africa. Today the most common form is 1 liter aseptic cartons containing UHT skimmed, semi-skimmed or whole milk, although the plastic bags are still in use for pasteurized milk. Higher grades of pasteurized milk can be found in cartons or plastic bottles. Sizes other than 1 liter are rare.


 * Canada
 * 1.33 liter plastic bags (sold as 4 liters in 3 bags) are widely available in some areas (especially the Maritimes, Ontario and Quebec), although the 4 liter plastic jug has supplanted them in western Canada. Other common packaging sizes are 2 liter, 1 liter, 500 mL, and 250 mL cartons, as well as 4 liter, 1 liter, 250 mL aseptic cartons and 500 mL plastic jugs.


 * Chile
 * Distributed most commonly in aseptic cartons for up to 1 liter, but smaller, snack-sized cartons are also popular. The most common flavors, besides the natural presentation, are chocolate, strawberry and vanilla.


 * 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.


 * Colombia
 * Sells milk in 1 liter plastic bags.


 * Croatia, Bosnia and Herzegovina, Serbia, Montenegro
 * UHT milk (trajno mlijeko/trajno mleko/трајно млеко) is sold in 500 mL and 1 L (sometimes also 200 ml) aseptic cartons. Non-UHT pasteurized milk (svježe mlijeko/sveže mleko/свеже млеко) is most commonly sold in 1 L and 1.5 L PET bottles, though in Serbia one can still find milk in plastic bags.


 * Parts of Europe
 * Sizes of 500 mL, 1 liter (the most common), 1.5 liters, 2 liters and 3 liters are commonplace.


 * Finland
 * Commonly sold in 1 L or 1.5 L cartons, in some places also in 2 dl and 5 dl cartons.


 * Germany
 * Commonly sold in 1-liter cartons. Sale in 1-liter plastic bags (common in the 1980s) now rare.


 * Hong Kong
 * Milk is sold in glass bottles (220 mL), cartons (236 mL and 1 L), plastic jugs (2 liters) and aseptic cartons (250 mL).


 * India
 * Commonly sold in 500 mL plastic bags and in bottles in some parts like in west. It is still customary to serve the milk boiled, despite pasteurization. Milk is often buffalo milk. Flavored milk is sold in most convenience stores in waxed cardboard containers. Convenience stores also sell many varieties of milk (such as flavored and ultra-pasteurized) in different sizes, usually in aseptic cartons.


 * Indonesia
 * Usually sold in 1 liter cartons, but smaller, snack-sized cartons are available.


 * Israel
 * Non-UHT milk is most commonly sold in 1 liter waxed cardboard boxes and 1 liter plastic bags. It may also be found in 0.5 L and 2 L waxed cardboard boxes, 2 L plastic jugs and 1 L plastic bottles. UHT milk is available in 1 liter (and less commonly also in 0.25 L) carton "bricks".


 * Japan
 * Commonly sold in 1 liter waxed paperboard cartons. In most city centers there is also home delivery of milk in glass jugs. As seen in China, sweetened and flavored milk drinks are commonly seen in vending machines.


 * South Korea
 * Sold in cartons (180 mL, 200 mL, 500 mL 900 mL, 1 L, 1.8 L, 2.3 L), plastic jugs (1 L and 1.8 L), aseptic cartons (180 mL and 200 mL) and plastic bags (1 L).


 * Pakistan
 * Milk is supplied in 500 mL Plastic bags and carried in Jugs from rural to cities and sell


 * Philippines
 * Milk is supplied in 1000 mL Plastic bottles and delivered from factories to cities and sell


 * Poland
 * UHT milk is mostly sold in aseptic cartons (500 mL, 1 L, 2 L), and non-UHT in 1 L plastic bags or plastic bottles. Milk, UHT is commonly boiled, despite being pasteurized.


 * South Africa
 * Commonly sold in 1 liter bags. The bag is then placed in a plastic jug and the corner cut off before the milk is poured.


 * Sweden
 * Commonly sold in 0.3 L, 1 L or 1.5 L cartons and sometimes as plastic or glass milk bottles.


 * Turkey
 * Commonly sold in 500 mL or 1L cartons or special plastic bottles. UHT milk is more popular. Milkmen also serve in smaller towns and villages.


 * United Kingdom
 * Most stores stock imperial sizes: 1 pint (568 mL), 2 pints (1.136 L), 4 pints (2.273 L), 6 pints (3.408 L) or a combination including both metric and imperial sizes. Glass milk bottles delivered to the doorstep by the milkman are typically pint-sized and are returned empty by the householder for repeated reuse. Milk is sold at supermarkets in either aseptic cartons or HDPE bottles. Supermarkets have also now begun to introduce milk in bags, to be poured from a proprietary jug and nozzle.


 * United States
 * Commonly sold in gallon (3.78 L), half-gallon (1.89 L) and quart (0.94 L) containers of natural-colored HDPE resin, or, for sizes less than one gallon, cartons of waxed paperboard. Bottles made of opaque PET are also becoming commonplace for smaller, particularly metric, sizes such as one liter. The US single-serving size is usually the half-pint (about 240 mL). Less frequently, dairies deliver milk directly to consumers, from coolers filled with glass bottles which are typically half-gallon sized and returned for reuse. Some convenience store chains in the United States (such as Kwik Trip in the Midwest) sell milk in half-gallon bags, while another rectangular cube gallon container design used for easy stacking in shipping and displaying is used by warehouse clubs such as Costco and Sam's Club, along with some Wal-Mart stores.


 * Uruguay
 * Commonly sold in 1 liter bags. The bag is then placed in a plastic jug and the corner cut off before the milk is poured.

Practically everywhere, condensed milk and evaporated milk are 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.

Spoilage and fermented milk products


When raw milk is left standing for a while, it turns "sour". This is the result of fermentation, where lactic acid bacteria ferment the lactose in the milk into lactic acid. Prolonged fermentation may render the milk unpleasant to consume. This fermentation process is exploited by the introduction of bacterial cultures (e.g. Lactobacilli sp., Streptococcus sp., Leuconostoc sp., etc.) to produce a variety of fermented milk products. The reduced pH from lactic acid accumulation denatures proteins and causes the milk to undergo a variety of different transformations in appearance and texture, ranging from an aggregate to smooth consistency. Some of these products include sour cream, yogurt, cheese, buttermilk, viili, kefir, and kumis. See Dairy product for more information.

Pasteurization of cow's milk initially destroys any potential pathogens and increases the shelf life, but eventually results in spoilage that makes it unsuitable for consumption. This causes it to assume an unpleasant odor, and the milk is deemed non-consumable due to unpleasant taste and an increased risk of food poisoning. In raw milk, the presence of lactic acid-producing bacteria, under suitable conditions, ferments the lactose present to lactic acid. The increasing acidity in turn prevents the growth of other organisms, or slows their growth significantly. During pasteurization, however, these lactic acid bacteria are mostly destroyed.

In order to prevent spoilage, milk can be kept refrigerated and stored between 1 and 4 C 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 but has a characteristic "cooked" 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 powdered milk, which is produced from milk by removing almost all water. The moisture content is usually less than 5% in both drum- and spray-dried powdered milk.

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". In ancient Greek mythology, the goddess Hera spilled her breast milk after refusing to feed Heracles, resulting in the Milky Way.

In many African and Asian countries, butter is traditionally made from fermented milk rather than cream. It can take several hours of churning to produce workable butter grains from fermented milk.

Holy books have also mentioned milk. The Bible contains references to the 'Land of Milk and Honey'. In the Qur'an, there is a request to wonder on milk as follows: 'And surely in the livestock there is a lesson for you, We give you to drink of that which is in their bellies from the midst of digested food and blood, pure milk palatable for the drinkers.'(16-The Honeybee, 66). The Ramadan fast is traditionally broken with a glass of milk and dates.

Abhisheka is conducted by Hindu and Jain priests, by pouring libations on the image of a deity being worshipped, amidst the chanting of mantras. Usually offerings such as milk, yogurt, ghee, honey may be poured among other offerings depending on the type of abhishekam being performed.

To milk someone, in the vernacular of many English-speaking countries, is to take advantage of the person.

The word "milk" has had many slang meanings over time. In the 19th century, milk was used to describe a cheap alcoholic drink made from methylated spirits mixed with water. The word was also used to mean defraud, to be idle, to intercept telegrams addressed to someone else, and a weakling or 'milksop'. In the mid-1930s, the word was used in Australia meaning to siphon gas from a car.

Other uses
Besides serving as a beverage or source of food, milk has been described as used by farmers and gardeners as an organic fungicide and fertilizer,  however, its effectiveness is debated. Diluted milk solutions have been demonstrated to provide an effective method of preventing powdery mildew on grape vines, while showing it is unlikely to harm the plant.