Milk . It is an opaque whitish-colored nourishing secretion produced by the mammary glands of females (sometimes also males) of mammals (including monotremes). This capacity is one of the characteristics that define mammals.

The main function of milk is to nurture children until they are able to digest other foods. It also fulfills the functions of protecting the pups’ gastrointestinal tract against pathogens, toxins and inflammation, and contributes to metabolic health by regulating the processes of obtaining energy, especially the metabolism of glucose and insulin.

It is the only fluid ingested by the young of mammals (from the infant in the case of humans) until weaning. The milk of domestic mammals is part of the current human diet in the vast majority of civilizations: mainly from cattle , but also from sheep , goats , mares, camels.


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  • 1 Base of various products
  • 2 History
  • 3 Biology of milk
  • 4 Milk producing animals
  • 5 Milking
    • 1 Milking techniques
  • 6 General characteristics
    • 1 Physical properties
  • 7 Composition of milk
    • 1 Lactose
    • 2 Lipids or fats
  • 8 Nutritional properties
  • 9 Industrial processes
  • 10 Debugging
  • 11 Thermal treatments
  • 12 Sources

Base of various products

Milk is the base of many dairy products, such as butter , cheese , yogurt , among others.

The use of milk derivatives is very frequent in the agri-food, chemical and pharmaceutical industries in products such as condensed milk, powdered milk, casein or lactose.

Cow’s milk is also used in animal feed . It is mainly composed of water , ions ( salt , minerals and calcium ), carbohydrates (lactose), fat and proteins .

The milk of marine mammals, such as whales, is much richer in fat and nutrients than that of land mammals. The liquid is produced by the secretory cells of the mammary or mammary glands (called “breasts” among many other forms, in the case of women, and “udders”, in the case of domestic mammals). The milk secretion from a female days before and after giving birth is called colostrum.


Milking a cow traditionally Human consumption of milk of animal origin began around 11,000 years ago with the domestication of livestock during the so-called climate optimum.

This process occurred especially in the Middle East, promoting the Neolithic revolution. The first animal that was domesticated was the cow, from Bos primigenius, then the goat, at approximately the same dates, and finally the sheep, between 9000 and 8000 BC. C. (see next section).

There are hypotheses, such as that of the sparing genotype, which affirm that this meant a fundamental change in the eating habits of the hunter-gatherer populations, which went from eating abundant but sporadic intakes to receiving daily carbohydrate contributions.

According to the theory, this change made Euro-Asian populations more resistant to type 2 diabetes and more lactose tolerant compared to other human populations that only more recently knew about livestock products.

However this hypothesis has not been verified and even its own author, James V Neel has refuted it, claiming that the observed differences in human populations could be due to other environmental factors.

Regarding the ability of adults to tolerate unfermented dairy products, especially milk, several hypotheses have been put forward. One of them is that the gene responsible for lactase (an enzyme that hydrolyzes lactose), a rare and rare gene in European Neolithic populations, has possibly been conserved as a consequence of including dairy products in human nutrition. It would have appeared 7,500 years ago in an area centered around present-day Hungary , and although this gene would compensate for the poor synthesis of Vitamin D in northern latitudes, this does not seem an essential factor for its appearance.

During the Ancient and Middle Ages , milk was very difficult to preserve and, for this reason, it was consumed fresh or in the form of cheeses. Over time other dairy products such as butter were added. The industrial revolution in Europe , around 1830 , brought the possibility of transporting fresh milk from rural areas to large cities thanks to improvements in transport.

Over time, new instruments have appeared in the milk processing industry. One of the best known is that of pasteurization, suggested for milk for the first time in 1886 by the German agricultural chemist Franz von Soxhlet. These innovations have made milk look healthier, have more predictable shelf life and more hygienic processing.

Milk biology

The production of milk to nourish the young could have been an evolutionary trait associated with the hormone prolactin. Some species of fish in the Symphysodon genus are known to nourish their young with a fluid similar to milk.

The so-called “milk of crop” is present in various groups of birds, such as pigeons, flamingos and penguins. From the biological point of view it is a true milk, secreted by specialized glands. However, where this evolutionary adaptation becomes characteristic is in mammals.

These are believed to come from a group close to the tritelodontides of the late Triassic period. These same sources believe that they already showed signs of lactation.

Among the many existing theories, it has been proposed that milk production arose because the synapsid ancestors of mammals had soft-shelled eggs , like the current monotremes, which caused their rapid drying. Milk would thus be a modification of the secretion of the sweat glands intended to transfer water to the eggs .

Other authors, in a theory that may be complementary to the previous one, believe that the mammary glands come from the innate immune system and that lactation would be, in part, an inflammatory response to tissue damage and infection. Although there are difficulties, several approaches approximate the date of appearance in evolutionary history: The evolutionary need to feed the young is satisfied in the production of milk typical of mammals.

  • First, casein has a vitellogenin-like function, behavior, and even structural motives. Casein appeared between 200 and 310 ma ago. It is observed that, although this protein still exists in monotremes, it was progressively replaced by casein, allowing a smaller size of the eggs and finally their intrauterine retention.
  • On the other hand, anatomical modifications are observed in advanced cynodonts that are only explained by the appearance of lactation, such as small body size, epipubic bones and low level of dental replacement.

The oldest fossil of placental mammals discovered to date is Eomaia scansoria, a small animal that outwardly resembled today’s rodents and lived 125 million years ago during the Cretaceous period.

This animal almost certainly produced milk like today’s placental mammals. They suck the suckers. The difference is that the nipple will be completely cleaned with this solution to close the lactiferous duct. This prevents the nipple from becoming infected. If the sucker generated a wound in the animal, since it has very sensitive skin, the iodine will avoid a later infection.

Milk producing animals

Currently, the milk that is most used in the production of dairy derivatives is cow’s milk (due to the properties it possesses, the quantity obtained, a pleasant taste, easy digestion, as well as the large quantity of derivatives obtained). However, it is not the only one that is exploited. There is also goat’s milk, donkey, mare, camel, among others. The consumption of certain types of milk depends on the region and the type of animals available.

Goat’s milk is ideal for making dulce de leche (also called cajeta) and whale’s milk is used in arctic regions. Donkey and mare’s milk contain the least fat, while seal milk contains more than 50% of that.

Human milk is neither produced nor distributed on an industrial scale. However, it can be obtained through donations. There are banks of milk that are responsible for collecting it to provide it to premature or allergic children who cannot receive it in any other way. Globally, there are several species of animals from which milk can be obtained: sheep, goat, mare, donkey, camel (and other camelids, such as llama or alpaca), yaka , buffalo , the queen and the moose .

Milk from the cow (Bos taurus) is the most important for the human diet and the one with the most industrial applications. Holstein cow’s milk is the one most frequently used on dairy farms.

  • The European and Indica cow (Bos taurus) began to be domesticated 11,000 years ago with two different maternal lines, one for European cows and one for Indica cows. The ancestor of the current Bos taurus was called Bos primigenius. It was a broad-horned bovine that was domesticated in the Middle East, spread across Africa, and gave rise to the famous Central Asian zebu breed. Zebu is valued for its meat intake and for its milk. The European variant of Bos primigenius has the shortest horns and is adapted for stable breeding. It is the one that has ended up giving a greater set of dairy breeds such as Holstein, Guernsey, Jersey, etc.
  • The buffalo: The so-called water buffalo (Bubalus bubalis) was domesticated in 3000 BC. C. in Mesopotamia. This animal is very sensitive to heat and its name denotes the habit it has of going into the water to protect itself from it. In general, it is little known in the West. The Arabs brought it to the Middle East during the Middle Ages (700 BC). Its use in certain areas of Europe dates back to that time. For example, in the elaboration of the famous Italian buffalo mozzarella. Products made from buffalo milk are beginning to replace cow’s milk in some communities.
  • The yak : scientifically called Bos grunniens, is a long-haired bovine that contributes fundamentally to feeding the populations of Tibet and Central Asia. It has a milk rich in protein and fat (its concentration is higher than its beef equivalent). Tibetans make butter and different fermented dairy products from it. One of the best known is salty butter tea.
  • The sheep: it was domesticated in the Mediterranean east, mainly from Ovis aries. From archaeological evidence, five mitochondrial lines produced between 9000 and 8000 BC have been identified. C. Sheep’s milk is richer in fat content than buffalo milk and is even richer in protein content. It is highly valued in Mediterranean cultures.
  • The goat: it began to be domesticated mainly in the Euphrates valley and the Zagros mountains from Capra hircus at around the same time as the cows (10,500 years old). It has a milk with a strong flavor and aroma. Goat milk is somewhat different from sheep’s milk, mainly in terms of taste, it contains a higher amount of chlorides, which gives it a slightly salty taste. It is also “thicker” in cream content (caseinates), and has higher calcium levels. Goat cheese is made from the fat of this milk
  • The camel is an animal far from bovids and ovicáprids (goats and sheep). It was domesticated in 2500 a. C. in Central Asia. Its milk is highly appreciated in arid climates where some cultures constantly use it, for example, the gastronomy of Northwest Africa.
  • Llama and alpaca: They are common animals in the Andean highlands in South America. Its dairy production is mainly aimed at local consumption and has no major industrial projection.
  • Cervids: In various populations close to the Arctic, the consumption of the milk of cervids is frequent, such as reindeer (Rangifer tarandus) and alcesa (Alces alces). The latter is marketed in Russia and Sweden. Some studies suggest that it can protect children against gastrointestinal diseases.
  • Equidae: The production of mare’s milk is very important for many populations of the steppes of Central Asia, especially for the production of a fermented derivative called kumis, since consumed raw it has a powerful laxative effect. This milk has a higher carbohydrate content than that of goat or cow and is therefore more suitable for alcoholic fermentation. It is estimated that in Russia there are some 230,000 horses dedicated to the production of Kumis. Donkey’s milk is one of the most similar to human in composition. Successful studies have been carried out to supply it as food to children allergic to cow’s milk. There are also farms in Belgium that produce donkey milk for cosmetic uses. One of the so-called “extreme long-lived” people, the Ecuadorian María Ester Capovilla, who died at the age of almost 117 years, alleged that the secret of his longevity was the daily consumption of this type of milk. Zebra milk has become a popular item for eccentric millionaires.

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Milking machine that works by vacuum suction. Note that the suckers reach the top of the nipple to prevent milk from leaving the metal container while avoiding damage to the nipple.

Milking techniques

  • Manual: It is necessary to clean the udder of the animal in an aseptic way (that is, with a special soap and always using potable water) to avoid infecting the animal with mastitis.

Then, the milker’s face must always look directly at the cow’s belly, position the right hand on one teat of the udder, while the other hand grasps another, located in the same plane of the hand, but in the posterior plane of the udder, and then constantly invert it.

This means that each hand will milk a pair of teats; while one hand grasps the anterior of a pair, the other pulls the posterior of the other.


Image where manual milking is observ

Image where manual milking is observed


  • Mechanics: Uses a sucker that milks the cow in the same order as manual milking. Extract the milk by vacuuming. The difference is that it does it in less time and without risk of damaging the udder tissue. It is used in industries and on some farms where dairy cattle are very large. Suckers should be cleaned with a 4% iodine solution.


Image where mechanical milking is obse


When milking, two tasks should always be performed:

  1. Disinfect the nipple with distilled water: This is done with a mesh made with a sky blanket (a white fabric made with fine thread). When firing a trickle of milk towards it, it must be observed if the milk comes out smooth, since this can mean that the cow has mastitis.
  2. Seal the nipple: It is done with the same solution with which the suckers are cleaned. The difference is that the nipple will be completely cleaned with this solution to close the lactiferous duct. This prevents the nipple from becoming infected. If the sucker generated a wound in the animal, since it has very sensitive skin, the iodine will avoid a later infection.

General characteristics

Not all milk from mammals have the same properties. As a general rule, milk can be said to be a slightly viscous, matt white liquid, the composition and physical-chemical characteristics of which vary considerably according to the animal species, and even according to the different breeds. These characteristics also vary in the course of the lactation period, as well as in the course of its treatment.

Physical properties

Cow’s milk has an average density of 1,032 g / ml. It is a complex and heterogeneous mixture made up of a three-phase colloidal system:

  • Solution: minerals as well as carbohydrates are dissolved in water.
  • Suspension: Protein substances are found in water in suspension.
  • Emulsion: fat in water is presented as an emulsion.

It contains a significant proportion of water (about 87%). The rest constitutes the dry extract that represents 130 grams (g) per l and in which there are 35 to 45 g of fat.

Other main components are lactose carbohydrates, proteins and lipids. Organic components (carbohydrates, lipids, proteins , vitamins ), and mineral components (Ca, Na, K, Mg, Cl). Milk contains different groups of nutrients. Organic substances (carbohydrates, lipids, proteins) are present in more or less equal amounts and constitute the main source of energy. These nutrients are divided into building elements, proteins, and energy compounds, carbohydrates and lipids.

Milk composition

Immediately after giving birth, the female of the mammal begins to produce mammary secretions; during the first two or three days it produces colostrum. After this period, the animal properly synthesizes milk throughout the lactation period, which varies from 180 to 300 days (depending on many factors), with a very fluctuating average daily production that ranges from 3 to 25 liters.

Milk is synthesized primarily in the mammary gland, but a large part of its constituents come from the blood serum. Its chemical composition is very complex and complete, reflecting its great importance in feeding the young. The composition of milk depends on the needs of the species during the breeding period.


Lactose is a disaccharide present only in milk, representing the main and only carbohydrate. However, small amounts of glucose, galactose, sucrose, cerebrosides, and hexosamine-derived amino sugars have been identified .

Lactose is synthesized in the mammary gland by an enzymatic system in which a-lactoalbumin intervenes and then segregates in milk. It is 15% less sweetening than sucrose and contributes, together with salts, to the overall flavor of the food.

There are certain sectors of the population (especially blacks and Latin American mestizos) that do not tolerate milk due to its lactose content. This is because the lining of the small intestine does not synthesize lactase, which is the enzyme that hydrolyzes the glycosidic bond and separates the sugar into glucose and galactose.

When lactose reaches the colon, it ferments and produces hydrogen , carbon dioxide, and lactic acid, which irritate this organ; In addition, water is absorbed from the intestine to balance the osmotic pressure. All of this can result in diarrhea , flatulence and abdominal cramps.

To remedy this biochemical anomaly that affects some sectors of the world population, the producers add an enzyme to the permeate (serum), the α-lactase that hydrolyzes the disaccharide in its two monosaccharides and is thus tolerated by the lactose allergic groups .

Lactose is produced since the baby begins to breastfeed, and its production begins to decrease with growth, since biologically, humans do not necessarily require milk in their basic diet after childhood, as demonstrated by 70 or 80% of adults do without it.

Lipids or fats

The properties of milk are a reflection of the fatty acids it contains. Thus we have several groups of lipids present in milk: triacylglycerides , diacylglycerides , monoacylglycerides , phospholipids , free fatty acids, sterols and their esters, and some carbohydrates.

Nutritional properties

Its diversified composition, which includes fats (where triglycerides are the majority fraction with 98% of the total lipid and whose fatty acids that form them are mostly saturated), proteins (casein, albumin and serum proteins) and carbohydrates ( lactose, specific sugar in milk), make it a complete food.

Furthermore, whole cow’s milk is an important source of vitamins ( vitamins A, B, D3, E). Vitamin D is what fixes calcium phosphate to teeth and bones, making it especially recommended for children. Colostrum is a yellowish liquid, rich in proteins and antibodies, essential for the immunization of the newborn. Despite this, it has no industrial application.

Industrial processes

Raw milk or raw milk would not be suitable for commercialization and consumption without being subjected to certain industrial processes that ensure that the microbiological load is within safe limits.Thus, a milk with guarantees of health must have been milked with modern methods. and suction toilets in which there is no physical contact with the milk. After milking, it has to be cooled and stored in a stirred milk tank and transported in isothermal cisterns to the processing plants.

In these plants, the milk must be analyzed before unloading to see that it meets optimal characteristics for consumption.

Among the analyzes, there are physicochemicals to see their composition in fat and dry matter, among other parameters, to detect possible fraud by watering, organoleptics, to detect strange flavors and bacteriological ones, which detect the presence of pathogenic bacteria and antibiotics. These pass to the milk coming from the cow in veterinary treatment and in turn pass to the consumer. Milk that does not meet the quality requirements must be rejected.

Once its optimal condition has been verified, it is stored in large-capacity tanks and ready for commercial packaging.


Milk , depending on the commercial application that is going to be given, can go through a large number of processes, known as purification processes. These ensure the sanitary quality of milk, and are listed below:

  • Filtration: it is used to separate the protein from the whey and thus remove impurities such as blood, hair, straw, manure. A filter or screen is used.
  • Homogenization: also called homogenization. This physical process is used, which consists of continuous agitation (pneumatic or mechanical) either with a pump, a homogenizer or a clarifier, and whose purpose is to reduce the fat globule before heating it and thus prevent cream from forming. This should be 1µm (micrometer) in diameter.

When milk is standardized or fat content is regularized, it is mixed with homogenization, avoiding subsequent phase separation. It is performed at 50 ° C to avoid denaturation. Homogenization, after pasteurization, stabilizes the fat in small particles that prevent cremation during fermentation and generates a better texture since the interaction between caseins and fat globules becomes favorable to make dairy derivatives that require fermentation.

  • Standardization: when a milk does not pass the fat content test to make a certain product, milk powder or vegetable fat is used. It is carried out in two ways: first mathematically (with procedures such as Pearson’s ² test or Matter Balance) and the other practice, measuring the masses and mixing them.

Before milk goes into any process, it must be 3.5% fat. This process is also used when the milk, once heat treated, lost some type of components, which is more commonly done with milk that loses calcium and to which new nutrients are reincorporated.

  • Deodorization: it is used to remove odors that could permeate the milk during its extraction (manure, for example). For this, a vacuum chamber is used, where odors are completely eliminated. Milk should smell sweet or sour.
  • Bactofugation: eliminates bacteria by centrifugation. The machine designed for this function is called a bactofuge. It generates a centrifugal rotation that causes the bacteria to die and separate from the milk. The milk must have 300,000 CFU / mL (colony forming units for each milliliter). Before performing a bactofugation, a culture of the bacteria in the milk must be carried out and identified, this is very important since it allows determining the most effective procedure to eliminate a specific bacterium. It is usually taken as a standard that 1800 seconds heating to 80 ° C eliminates coliforms, tuberculosis bacillus and spores; as well as the inhibition of the enzymes alkaline phosphatase and peroxidase. But this is just a highly variable standard that depends on many conditions.
  • Clarification: used to separate unnecessary solids and sediments from milk (such as dust or dirt, very small particles that cannot be filtered). A clarifier is used, where the process can be carried out in two ways: heating the milk to 95 ° C and allowing it to stir for 15 minutes, or heating it to 120 ° C for 5 minutes.

Thermal treatments

Package with UHT milk from the Normanoir brand, from France, of the semi-skim type. The data in the upper part allows to observe the characteristics of the batch and the product.

Once the purification has been carried out, the milk can be treated for human consumption by applying heat for the partial or total elimination of bacteria.

According to the required objective, thermization, pasteurization, ultrapasteurization or sterilization will be used.

  • Thermization: with this procedure the enzymatic activity is reduced or inhibited.
  • Pasteurization (Slow High Temperature, SHT): with this procedure the milk is heated to certain temperatures for the elimination of specific pathogenic microorganisms: mainly the one known as Streptococcus thermophilus. It inhibits some other bacteria.
  • Ultrapasteurization (Ultra High Temperature, UHT): this procedure uses a higher temperature than pasteurization. Eliminate all bacteria except lactic bacteria. Does not require subsequent cooling.
  • Sterilization: the high temperature used of 140 ° C for 45 s eliminates any microorganism present in the milk. It is not subsequently refrigerated; This milk is also called sanitized. This process does not apply to flavored or reformulated milks as they undergo caramelization.

Sterilization can occur in online autoclaves called Barriquands. The white milks treated in this way are packed in tetrabrik or special cardboard boxes sanitized and coated internally with a satin film.

After a heat treatment, refrigeration may be dispensable because it is not necessary to lower the temperature in all cases, only when the milk still has microorganisms.

According to the outgoing microbial quality, refrigeration is considered; Hence, the thermization has forced refrigeration and the sterilized does not. If there are no bacteria or enzymatic activity, the milk will not change at room temperature; If we leave any milk in a glass and uncovered then oxygen will do the same as an oxidizing agent, but not due to internal activities of the milk.


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