Milk is a complex secrete from the mammary gland of animals designated to feeding the new-born. The chemical composition and the relationship between the different components determine the specific properties, the high nutrition and biological values of the milk. Milk contains about 250 different substances, thus almost everything necessary for maintaining life and growth of human body. The nutrition value of milk consists of fats, proteins, carbohydrates, vitamins, enzymes, micro- and macro-elements, hormones. Some enzymes, antibiotic and antitoxic substances built up in milk when it is fermented by lactic acid bacteria.
Milk is produced in the mammary gland by substances coming from blood. In order 12 liters of milk to be produced about 6,000 liters of blood should pass through the cow’s udder.
Milk is considered as an international product and it is used for nutrition purposes in lots of countries in the world. Rae milk is directly received milk which has not gone through heat processing.
Chemical composition of milk
The chemical composition of milk varies depending on the period of lactation, the breed, age, health condition of the animal, the conditions of raising, the quality of fodder and they type of feeding and other factors.
The largest part of water (84.1-84.5 %) is in a free state and is easily removed by evaporating and drying milk. Another part of water (3-3.5 %) is in a connected state, i.e. it is retained by molecular forces near the surface of the particles (proteins, phosphatides and polysaccharides). Water contained in milk is formed by the blood plasma in the mammary gland and contains the components of solid substances in colloid state.
In milk there can be distinguished swelling water, free water, connected water and crystallizing water. The free water is a solvent for the organic and inorganic compounds of milk. The connected water is due to the hydrophilic groups of the protein substances, phosphatides and polysaccharides. The crystallizing water is connected with the crystals of the lactic sugar substances. On average milk contains 87 % of water and 13 % of dry residue.
It is considered the most valuable component, so the price of the product depends on the fat content (2.8-5%). Milk fat is characterized with the highest energy value and various biological activity. It is a source of a group of vitamins, tocopherols, phosphates and other important components. In milk fat is in the form of suspension consisting of small fat globules with a size from 0.1 to 20 microns.
In 1 ml cow milk there are 1.0 – 12.0 billions of fat globules the temperature of melting of milk fat is 25-28 oC, the density at 20 oC is 0.918-0.924, number of refraction is 42-45, index of refraction is 1.453-1.455, iodine number is 25-28.
The content of fats in cow milk varies from 2.8 to 6.0 %, which depends on the way and conditions of milking& the first third of the milk supply there are more fats than in the last part of the milk. The content of excessive amount of carbon dioxide, ammonia, hydrogen sulphide and other harmful gases in the air reduce fat content by 0.3-0.5 %. The fat in milk is in the form of fat globules surrounded by protecting lecithin protein coatings with a size of 2-5 microns. The protecting coating does not permit the globules to merge into conglomerates, due to which the milk fat represents an emulsion in water. The destabilization of the emulsion (appearance of free fat, unprotected by the coating) is the reason for oxidative spoilage of the product. Milk fat easily breaks down into glycerol and free fatty acids and it oxidizes through saturation of unsaturated fatty acids.
Milk proteins consist mainly of casein, albumin and globulin. The content of proteins in milk varies from 2.8 to 4.6 % from which the casein represents about 82 %, albumin – 12 % and globulin – 6 % from the total amount proteins. Milk protein ratio varies depending on the period of lactation, feeding of animals, various diseases and other factors.
Milk proteins are highly fulfilling since they contain all necessary for people and animals amino acids (about 20), among which there are irreplaceable and replaceable amino acids, which up to 80 % are considered easily absorbed. During the first hours after birth milk proteins are absorbed in the body of the new-born animals without any change, which ensures protection against unfavorable conditions of the environment. Milk contains more irreplaceable amino acids than meat, fish, legumes and other foods.
Casein is complex milk protein which belongs to phosphor proteins and its composition is a complex of several fractions (ɑ, ß, y, k) differing from each other in their amino acid composition and the content of residues of phosphor acid. In milk casein is contained in the form of a complex casein-calcium phosphate composition, which in milk is in the form of micelles. Casein contains 18 amino acids whose ratio varies depending on the seasons and the periods of lactation. Casein settles down (acid, rennet, calcium coagulation) when interacting with acids, salts and rennet enzymes; this property is used when preparing fermented milk products.
Milk contains 0.2-0.6 % albumin. This is useful protein, easily absorbed by the human body and it does not contain phosphor in its molecule. It contains more vital amino acids, which valuable for the nutrition of children and young animals. It does not coagulate by rennet enzyme and due to this it is used when preparing milk creams, pastes, cheese. In milk there is albumin in the amount of 10-12 % due to which milk is considered valuable protective antitoxic product for the new-born.
Globulins in milk are in a dissolved state in the amount of 0.05-0.2 %. They are antibacterial and antitoxic specific antibodies. Globulins are contained in the colostrum in the amount of 8-15 %. The rennet enzyme does not coagulate the globulins so they stay in the whey during the production of cheese. Globulins and albumin coagulate at heating up to 75 oC, and they are firstly denatured and then coagulated.
Milk sugar (lactose) is disaccharide which breaks down into glucose and galactose during hydrolysis. Milk contains 4.6-5.5 % lactose, which makes it slightly sweet. Lactose is 5-6 times less sweet than sugar from beetroot so despite its high contain milk does not have sweet taste. Under the action of dilute acids and enzymes produced by certain lactic acid bacteria and yeasts, lactose is hydrolyzed to form glucose and galactose. Milk contains small amounts of phosphate sugars – galactose and fructose.
When milk sours, lactose breaks down into lactic acid, alcohol, esters, volatile acids and other compounds. Heating milk to a temperature above 95 oC leads to a slightly brown coloring due to a reaction between lactose and some free amino acids, which leads to formation of melanoidins with prominent caramel flavor. Milk sugar though it is due to fermentation in the intestines is absorbed almost fully.
The composition of milk practically includes all elements of Mendeleev’s periodic table. Despite this the calcium and phosphor ate most spread. Most of the macronutrients can be found in milk in the form of inorganic salts. Milk is very rich in microelements, from which manganese, nickel, cobalt, fluorine, bromine and iodine are really important.
Overall, milk contains 0.6-0.85 % minerals including macro elements (calcium, phosphor, sodium, potassium, magnesium, chloride, sulfur) and more than 20 microelements (iron, manganese, lead, cobalt, copper, iodine, zinc, helium, silver, tin, chromium, nickel).
Minerals in milk are contained in ionic and molecular dispersed state, also in combination with proteins, vitamins, enzymes, hormones. They ensure the exchange of salts in the body, regulate the osmotic pressure of blood and tissues, activate the activity of enzymes and ensure hematopoiesis. The content of minerals in milk depend on the animal diet during lactation and on biogeochemical characteristics of soil in which the fodder has been grown.
Milk contains a large number of vitamins. Their content in milk depend on the conditions of feeding and raising the milk cows. For enriching the milk with vitamins it is really important that the cows get juicy and green fodder was well as regular walks of the animals.
Milk of cows and goats is characterized with the largest variety regarding the content of vitamins.
From the group of fat-soluble vitamins, vitamins A (retinol), B (calciferol), E (tocopherol), K, P, etc. are found in milk. From the group of water-soluble vitamins, vitamin C (ascorbic acid), B (thiamine), B2 (riboflavin), B5 (pantothenic acid), B6 (pyridoxine), B12 (cobalamin), PP (niacin), H (biotin ), folic acid (folicin), etc.
Vitamins A, P are preserved when processing the milk, vitamin B and C are partially destroyed and their loss is 10-30 %, and carotene does not exceed 10-13 %. In winter the content of vitamins A and B in milk reduces.
In addition to this milk contains enzymes (esterases, carbohydrates, proteases, peroxidases, catalases, etc.) and hormones (prolactin, oxybonyl, tyroliberin, estrogens, progesterone, thyroxine, prostaglandins).
Milk does not have specific enzymes. They come into it during milking from the cells of the mammary gland and are formed by the micro flora developing in milk. Of these, lipase, phosphatase, catalase, peroxidase and reductase deserve the most attention. Some of these enzymes are widely used for sanitary and hygiene evaluation of milk.
In total 20 real (native) enzymes and enzymes of microbe character are isolated from milk. Among the latter is the reductase which builds up in milk when bacteria grow in it. Its activity increases directly proportional to the number of microorganisms. Reductase analysis is used for determining the total bacterial pollution of milk. Peroxidase is native enzyme of milk, which is characterized with thermal stability and breaks down at a temperature of 8 oC. The peroxidase reaction monitors the effectiveness of milk pasteurization. Catalase is brought into milk from the tissue of the mammary gland. Its content is increased in milk in animals with mastitis. Phosphatase catalyzes the hydrolysis of phosphoric acid esters. It breaks down at 63 oC for 30 minutes, which is used to determine the pasteurization and sterilization methods of milk.
Animal milk contains hormones as well as oxygen and carbon dioxide in gaseous state.
For the last years the presence of foreign substances in milk, many of which toxic for people (antibiotics, pesticides, heavy metals, radioactive isotopes, nitrates, mycotoxins etc.) has become a serious problem.
Milk can contain different microorganisms, including such causing infectious diseases (tuberculosis, brucellosis, listeriosis, leukemia).
During the first days after calving (up to 10-11 days), the milk is called colostrum. It differs significantly from normal milk by almost all indicators: taste, color, texture, density, high content of protein (up to 15-23 %), fats (up to 6%) and milk sugar (up to 4 %). It contains more albumin, globulin and leucocytes. Milk coagulates easily at heating and is weakly active when interacts with rennet.
During the last 7-14 days of lactation before insemination of the cow, milk changes drastically. The content of fats increases to 5-19 %, of proteins – to 4-5,3 %, acidity decreases to 12-13o T, fat globules become very small and poorly separate from butter. Milk gets bitter-salty taste. With the increase of the cow’s age up to 6 years, the content of fats in milk increases and after that it slowly decreases. Milk from 3-7 lactations is more fulfilling regarding the content of nutrients compared to the milk of the first heifers and older cows.
For measurement of milk compounds different analytical laboratory methods and tools are used. The determined standards and methods are applied in compliance to the laws of a certain country or region.
Ultrasonic milk analyzers of the company Milkotester Ltd.
Lactomat and Master determine the content of the milk compounds with exceptional precision when determining its quality: fat content, dry fat content, proteins, lactose, salts, and also density, added water, freezing point, sample temperature. With their use, determining the quality of raw milk becomes fast, easy and precise.