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 the baby and can reduce the risk of many diseases in the baby.
Milk derived from cattle species is an important food. It has many nutrients. The precise nutrient composition of raw milk vary by species and by a number of other factors, but it contains significant amounts of saturated fat, protein and calcium as well as vitamin C. Cow's milk has a pH ranging from 6.4 to 6.8, making it slightly acidic.[1][2]
Milk is an important food for over 6 billion human beings of all ages, majority of them in developing countries. Over 750 million people live within dairy farming households. World's dairy farms produced over 710 million tons of milk in 2010. India is the world's largest producer and consumer of milk, yet neither exports nor imports milk. New Zealand, EU-15 and Australia are the world's three largest exporters of milk and milk products. China, Mexico and Japan are the world's largest importers of milk and milk products.
Milk is a key contributor to improving nutrition and food security particularly in developing countries. Improvements in livestock, dairy technology and milk quality may offer the most promise in reducing poverty and malnutrition in the world.[3]
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.
In almost all mammals, milk is fed to infants through breastfeeding, either directly or by expressing the milk to be stored and consumed later. Some cultures, historically or currently, continue to use breast milk to feed their children until they are seven years old.
Human infants sometimes are fed fresh goat milk. There are known risks in this practice, including those of developing electrolyte imbalances, metabolic acidosis, megaloblastic anemia, and a host of allergic reactions.[4]
In many cultures of the world, especially the Western world, humans continue to consume milk beyond infancy, using the milk of other animals (especially cattle, goats and sheep) as a food product. For millennia, cow's milk has been processed into dairy products such as cream, butter, yogurt, kefir, ice cream, and especially the more durable and easily transportable product, cheese. Modern industrial processes produce casein, whey protein, lactose, condensed milk, powdered milk, and many other food-additive and industrial products.
Humans are an exception in the natural world for consuming milk past infancy, despite the fact that many humans show some degree (some as little as 5%) of lactose intolerance, a characteristic that is more prevalent among individuals of African or Asian descent.[5] 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 intestines after birth and then begins a slow decline unless milk is consumed regularly.[6] On the other hand, those groups who do continue to tolerate milk often have exercised great creativity in using the milk of domesticated ungulates, not only of cattle, but also sheep, goats, yaks, water buffalo, horses, reindeers and camels. The largest producer and consumer of cattle and buffalo milk in the world is India.[7]
| Country | Milk (liters) | Cheese (kg) | Butter (kg) |
|---|---|---|---|
| Finland | 183.9 | 19.1 | 5.3 |
| Sweden | 145.5 | 18.5 | 1.0 |
| Ireland | 129.8 | 10.5 | 2.9 |
| Netherlands | 122.9 | 20.4 | 3.3 |
| Norway | 116.7 | 16.0 | 4.3 |
| Spain | 119.1 | 9.6 | 1.0 |
| Switzerland | 112.5 | 22.2 | 5.6 |
| United Kingdom | 111.2 | 12.2 | 3.7 |
| Australia | 106.3 | 11.7 | 3.7 |
| Canada | 94.7 | 12.2 | 3.3 |
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.[9] 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.[10]
The term milk is also used for white colored, non-animal beverages resembling milk in color and texture 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.[11] Dairy relates to milk and milk production, eg. dairy products.
The mammary gland is thought to have been derived from apocrine skin glands.[12] 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).[12][13][14] The original adaptive significance of milk secretions may have been nutrition[15] or immunological protection.[16][17] This secretion gradually became more copious and accrued nutritional complexity over evolutionary time.[12]
Humans first learned to regularly consume the milk of other mammals following the domestication of animals during the Neolithic Revolution[18] or the invention of agriculture. This development occurred independently in several places around the world from as early as 9000–7000 BC in Southwest Asia[19] to 3500–3000 BC in the Americas.[20] The most important dairy animals—cattle, sheep and goats—were first domesticated in Southwest Asia, although domestic cattle has been independently derived from wild auroch populations several times since.[21][22] 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 4th millennium BC.[23] Sherratt's model is not supported by recent findings, based on the analysis of lipid residue in prehistoric pottery, that show that dairying was practiced in the early phases of agriculture in Southwest Asia, by at least the 7th millennium BC.[24][25]
From Southwest Asia domestic dairy animals spread to Europe (beginning around 7000 BC but not reaching Britain and Scandinavia until after 4000 BC),[26] and South Asia (7000–5500 BC).[27] The first farmers in central Europe[28] and Britain[29] 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 4th millennium BC, and subsequently spread across much of the Eurasian steppe.[30] Sheep and goats were introduced to Africa from Southwest Asia, but African cattle may have been independently domesticated around 7000–6000 BC.[31] Camels, domesticated in central Arabia in the 4th millennium BC, have also been used as a dairy animal in North Africa and the Arabian peninsula.[32] 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 1863, French chemist and biologist Louis Pasteur invented pasteurization, a method of killing harmful bacteria in beverages and food products.[33]
In 1884, Doctor Hervey Thatcher, an American inventor from New York, invented the first glass milk bottle, called 'Thatcher's Common Sense Milk Jar', which was sealed with a waxed paper disk.[33] Later, in 1932, plastic-coated paper milk cartons were introduced commercially as a consequence of their invention by Victor W. Farris.[33]
All female mammals can by definition produce milk, but cow milk dominates commercial production. 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 neonates, babies with allergies, metabolic diseases, etc.).
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.[6] Other dairy cows in the United States include Ayrshire, Brown Swiss, Guernsey, Jersey, and Milking Shorthorn (Dairy Shorthorn).
Aside from cattle, many kinds of livestock provide milk used by humans for dairy products. These animals include camel, donkey, goat, horse, reindeer, sheep, water buffalo, and yak.
In Russia and Sweden, small moose dairies also exist.[34]
According to the US National Bison Association, American bison (also called American buffalo) are not milked commercially;[35] 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[36] and during the development of commercial Beefalo in the 1970s and 1980s.[37]
The largest producer of dairy products and milk is India followed by the United States,[38] Germany, and Pakistan.
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 investment 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 farmers.[39] 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.[40]
The table below shows the numbers for water buffalo milk production. Cattle milk is produced in a much wider range.
| Country | Production (tonnes) | Note |
|---|---|---|
| India | 59,210,000 | Unofficial/Semi-official/mirror data |
| Pakistan | 20,372,000 | official figure |
| People's Republic of China | 2,900,000 | FAO estimate |
| Egypt | 2,300,000 | |
| Nepal | 958,603 | official figure |
| Iran | 241,500 | FAO estimate |
| Myanmar | 220,462 | official figure |
| Italy | 200,000 | FAO estimate |
| Vietnam | 32,000 | |
| Turkey | 30,375 | official figure |
| World | 86,574,539 | Aggregate |
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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.[42] 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.
Milk is an emulsion or colloid of butterfat globules within a water-based fluid that contains dissolved carbohydrates.[43] Because it is produced as a food source for a neonate, all of its contents provide benefits to the growing young. The principle requirements of the neonate 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.[44]
Initially milk fat is secreted in the form of a fat globule surrounded by a membrane.[45] 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.[45]
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.[45] 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.[6]
Normal bovine milk contains 30–35 grams of protein per liter of which about 80% is arranged in casein micelles.
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 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% [44] 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.[6][46]
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 they 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.[6]
Minerals or milk salts, are traditional names for a variety of cations and anions within bovine milk. Calcium, phosphate, magnesium, sodium, potassium, citrate, and chlorine 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[44] 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.
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.[47]
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 dependant upon the type of milk as other carbohydrates can be present at higher concentrations that lactose in milks.[44]
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.[6]
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.[6] 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.[46]
In most Western countries, centralized dairy facilities process milk and products obtained from milk (dairy products), 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 is used to kill harmful microorganisms by heating the milk for a short time and then cooling it for storage and transportation. Pasteurized milk still is perishable, however, 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. The process destroys the vitamin C content of the raw milk.
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 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 95% killing of microorganisms in conventional pasteurization). The whey then is recombined with the pasteurized cream to reconstitute the original milk composition.
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.[6] 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.[48] 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.[6] 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.[49] Homogenized milk may be more digestible than unhomogenized milk.[50]
Kurt A. Oster, M.D., who worked during the 1960s through the 1980s, suggested a link between homogenized milk and arterosclerosis, due to damage to plasmalogen resulting from the release of bovine xanthine oxidase (BXO) from the milk fat globular membrane (MFGM) during homogenization. Oster's hypothesis has been widely criticized, however, and has not been generally accepted by the scientific community. No link has been found between arterosclerosis and milk consumption.[50]
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 than may vary.[8] For example:
Donkey and horse milk have the lowest fat content, while the milk of seals and whales may contain more than 50% fat.[52][53] High fat content is not unique to aquatic mammals. Guinea pig milk has an average fat content of 46%.[54]
| Constituents | Unit | Cow | Goat | Sheep | Water buffalo |
|---|---|---|---|---|---|
| Water | g | 87.8 | 88.9 | 83.0 | 81.1 |
| Protein | g | 3.2 | 3.1 | 5.4 | 4.5 |
| Fat | g | 3.9 | 3.5 | 6.0 | 8.0 |
| Carbohydrate | g | 4.8 | 4.4 | 5.1 | 4.9 |
| Energy | kcal | 66 | 60 | 95 | 110 |
| Energy | kJ | 275 | 253 | 396 | 463 |
| Sugars (lactose) | g | 4.8 | 4.4 | 5.1 | 4.9 |
| Cholesterol | mg | 14 | 10 | 11 | 8 |
| Calcium | mg | 120 | 100 | 170 | 195 |
| Saturated fatty acids | g | 2.4 | 2.3 | 3.8 | 4.2 |
| Monounsaturated fatty acids | g | 1.1 | 0.8 | 1.5 | 1.7 |
| Polyunsaturated fatty acids | g | 0.1 | 0.1 | 0.3 | 0.2 |
These compositions vary by breed, animal, and point in the lactation period.
| Cow breed | Approximate percentage |
|---|---|
| Jersey | 5.2 |
| Zebu | 4.7 |
| Brown Swiss | 4.0 |
| Holstein-Friesian | 3.6 |
The protein range for these four breeds is 3.3% to 3.9%, while the lactose range is 4.7% to 4.9%.[6]
Milk fat percentages may be manipulated by dairy farmers' stock diet formulation strategies. Mastitis infection can cause fat levels to decline.[57]
| Nutritional value per 100 g (3.5 oz) | |
|---|---|
| Energy | 252 kJ (60 kcal) |
| Carbohydrates | 5.26 g |
| - Sugars | 5.26 g |
| - Lactose | 5.26 g |
| Fat | 3.25 g |
| - saturated | 1.865 g |
| - monounsaturated | 0.812 g |
| - polyunsaturated | 0.195 g |
| Protein | 3.22 g |
| - Tryptophan | 0.075 g |
| - Threonine | 0.143 g |
| - Isoleucine | 0.165 g |
| - Leucine | 0.265 g |
| - Lysine | 0.140 g |
| - Methionine | 0.075 g |
| - Cystine | 0.017 g |
| - Phenylalanine | 0.147 g |
| - Tyrosine | 0.152 g |
| - Valine | 0.192 g |
| - Arginine | 0.075 g |
| - Histidine | 0.075 g |
| - Alanine | 0.103 g |
| - Aspartic acid | 0.237 g |
| - Glutamic acid | 0.648 g |
| - Glycine | 0.075 g |
| - Proline | 0.342 g |
| - Serine | 0.107 g |
| Water | 88.32 g |
| Vitamin A equiv. | 28 μg (4%) |
| Thiamine (vit. B1) | 0.044 mg (4%) |
| Riboflavin (vit. B2) | 0.183 mg (15%) |
| Vitamin B12 | 0.44 μg (18%) |
| Vitamin D | 40 IU (7%) |
| Calcium | 113 mg (11%) |
| Magnesium | 10 mg (3%) |
| Potassium | 143 mg (3%) |
| Sodium | 43 mg (3%) |
| 100 mL corresponds to 103 g.[58] Percentages are relative to US recommendations for adults. Source: USDA Nutrient Database |
|
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 the age, milk contains 8 grams of protein, and a number of other nutrients (either naturally or through fortification) including:
The amount of calcium from milk that is absorbed by the human body is disputed.[59] Calcium from dairy products has a greater bioavailability than calcium from certain vegetables, such as spinach, that contain high levels of calcium-chelating agents,[60] but a similar or lesser bioavailability than calcium from low-oxalate vegetables such as kale, broccoli, or other vegetables in the Brassica genus.[61][62]
A 2006 study found that for women desiring to have a child, those who consume full fat dairy products may slightly increase their fertility, while those consuming low-fat dairy products may slightly reduce their fertility.[63]
Numerous studies have found that conjugated linoleic acid, found mainly in milk, meat and dairy products, provides several health benefits including prevention of atherosclerosis, different types of cancer, and hypertension and improved immune function.[64][65][65]
There is recent evidence suggesting consumption of milk is effective at promoting muscle growth[66] and improving post exercise muscle recovery.[67]
In 2010, scientists at the Harvard School of Public Health identified a substance in dairy fat, trans-palmitoleic acid, that may substantially reduce the risk of type 2 diabetes. The researchers examined participants who have been followed for 20 years in an observational study to evaluate risk factors for cardiovascular diseases in older adults. During followup it was found that individuals with higher circulating levels of trans-palmitoleic acid had a much lower risk of developing diabetes, with about a 60% lower risk among participants in the highest quintile (fifth) of trans-palmitoleic acid levels.[68]
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/intolerance, is labeled C/T-13910.[69] 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 who excrete gas, a process known as 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.[70] 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.[71] Another locus of lactose tolerance is in Northern India.[69]
Lactose intolerance is a natural process and there is no reliable way to prevent or reverse it.
Some studies suggest that milk consumption may increase the risk of suffering from certain health problems. Cow milk allergy (CMA) is an immunologically mediated adverse reaction to one or more cow's milk proteins. Rarely is it severe enough to cause death.[72]
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,[73][74] 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.[74]
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.[75] The reason behind this is not fully understood, and it also remains unclear why there is less of a risk for women.[75][76] Several sources suggest a correlation between high calcium intake (2000 mg per day, or twice the US recommended daily allowance, equivalent to six or more glasses of milk per day) and prostate cancer.[77] A large study specifically implicates dairy, i.e., low-fat milk and other dairy to which vitamin A palmitate has been added.[78][79]
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 excessive dairy product consumption and prostate cancer.[80][81]
Medical studies also have shown a possible link between milk consumption and the exacerbation of diseases such as Crohn's disease,[82] Hirschsprung's disease–mimicking symptoms in babies with existing cow's milk allergies,[83] and the aggravation of Behçet's disease.[84]
Since November 1993, with FDA approval,[85] Monsanto has been selling recombinant bovine somatotropin (rbST), also called rBGH, to dairy farmers. 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,[86] however, Monsanto's own tests, conducted in 1987, demonstrated that statistically significant growth stimulating effects were induced in organs of adult rats by feeding IGF-1 at low dose levels for only two weeks. "Drinking rBGH milk would thus be expected to significantly increase IGF-1 blood levels and consequently to increase risks of developing breast cancer and promoting its invasiveness."[87]
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."[88] 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.[89] 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.[90] 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.[91][92]
In the European Union, rBGH is banned.[93]
Vegans and some other vegetarians do not consume milk for a variety of reasons. They may object to features of dairy farming including the necessity of killing 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.[94]
According to an article in The New York Times, 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 the US is flavored, causing some school districts to propose a ban because flavored milk has added sugars. The Boulder, Colorado school district banned flavored milk in 2009 and instead installed a dispenser that keeps the milk colder.[95]
Milk products are sold in a number of varieties based on types/degrees of
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.
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,[96] 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,[97] and the United States, where the company says ultrafiltration is used.[98]
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.[99] 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.[100]
Alternatively, a bacterium such as L. acidophilus may be added, which affects the lactose in milk the same way it affects the lactose in yogurt.
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.[101]
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.[102]
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.
In Australia and New Zealand, prior to "metrification", milk was generally distributed in 1 pint (568ml) glass bottles. In Australia and in 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.[103] 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.
In rural India, milk is delivered daily by a local milkman carrying bulk quantities in a metal container, usually on a bicycle, and in other parts of metropolitan India, milk is usually bought or delivered in a plastic bags or cartons via shops or supermarkets.
In Pakistan, milk is supplied in jugs. Milk has been a staple food, especially among the pastoral tribes in this country.
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. 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). 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.[104] By 2010, the estimated number of milkmen had dropped to 6,000.[105] 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).[106]
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.
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 .5 US pints (0.24 l) 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.
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:
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.
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 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 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.
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 African and Asian developing nations, 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.[108]
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 early 17th century the word was used to mean semen, or vaginal secretions, or to masturbate oneself or someone else. 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.[109]
Besides serving as a beverage or source of food, milk has been described as used by farmers and gardeners as an organic fungicide and foliage fertilizer.[110] 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.[111]
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