Milk is a complex mixture of various constituents. On keeping at room temperature milk separates into fat as cream, curd or casein and whey. Curd and whey separates on curdling of milk. However, the constituents of milk include water, fat, proteins,lactose, minerals, enzymes and vitamins. Distribution of constituents of cow milk is given in the Fig.
A detailed study of the constituents of milk is essential to understand the nature of milk, its physico-chemical characteristics and nutritive value. The constituents have an important role in the preparation of milk products. Study of each constituent helps in understanding its chemical microbial and technological behavior.
Physical States of Constituents of milk
The main constituents of milk, namely, fat, sugar, protein, minerals, vitamins and other minor constituents are present in three states. The three physical states are true solution, colloidal dispersion and fat in oil-type emulsion follows:
There are always a state of equilibrium between colloidal state and true solution in normal milk.
i. Milk Proteins
Proteins are present in milk in colloidial state. Important proteins found in milk are casein and whey proteins. Description of these
proteins is presented here:
i)Casein: Casein is the major proteins which contribution at 80% protein. It is present in milk in colloidal state in the form of Calcium caseinate phosphate complex. The micellar particles of casein are spherical in nature and exist in milk as distinct particles. Each micelle of casein is bridged with various submicelles. The submicelles within casein micelle are held together by calcium phosphate by an ester linkage with the serine hydroxyl group. The calcium phosphocaseinate are rather large in size, as seen by electron microscope ranging in size between 30-300 millimicrons in diameter.Definition of Casein: Casein may be defined as phosphoprotein obtained from raw milk at pH 4.6 at 20 0 C. Casein is made up of amino acids forming a polymeric chain through a peptide bond ( − CO − NH 2 − ) . This linkage is established by reaction of an amino and a carboxyl group.
A detailed study of the constituents of milk is essential to understand the nature of milk, its physico-chemical characteristics and nutritive value. The constituents have an important role in the preparation of milk products. Study of each constituent helps in understanding its chemical microbial and technological behavior.
Physical States of Constituents of milk
The main constituents of milk, namely, fat, sugar, protein, minerals, vitamins and other minor constituents are present in three states. The three physical states are true solution, colloidal dispersion and fat in oil-type emulsion follows:
There are always a state of equilibrium between colloidal state and true solution in normal milk.
i. Milk Proteins
Proteins are present in milk in colloidial state. Important proteins found in milk are casein and whey proteins. Description of these
proteins is presented here:
i)Casein: Casein is the major proteins which contribution at 80% protein. It is present in milk in colloidal state in the form of Calcium caseinate phosphate complex. The micellar particles of casein are spherical in nature and exist in milk as distinct particles. Each micelle of casein is bridged with various submicelles. The submicelles within casein micelle are held together by calcium phosphate by an ester linkage with the serine hydroxyl group. The calcium phosphocaseinate are rather large in size, as seen by electron microscope ranging in size between 30-300 millimicrons in diameter.Definition of Casein: Casein may be defined as phosphoprotein obtained from raw milk at pH 4.6 at 20 0 C. Casein is made up of amino acids forming a polymeric chain through a peptide bond ( − CO − NH 2 − ) . This linkage is established by reaction of an amino and a carboxyl group.
With difunctional units such as the amino acids, long chain polymers can be built in this way.
Heterogeneity of Casein : Based on its primary structure, electrophoresis,chromatography and solubility in salts such as urea, casein is classified in the form of its fractions namely,
Heterogeneity of Casein : Based on its primary structure, electrophoresis,chromatography and solubility in salts such as urea, casein is classified in the form of its fractions namely,
a s1 – Casein
a s2 – Casein
b – Casein and
k – Casein
These main fractions are themselves variable because of genetic polymorphism involving deletion or substitution of amino acids within the polypeptide chain of each fraction.
Elementary Composition: Casein contains the following elements (in percent) –Carbon 52.6-54.0, Hydrogen 6.75-7.10, Nitrogen 15.51-15.91, Sulphur 0.71-0.83 and Phosphorus 0.71-0.85.
Since casein is a phosphoprotein the phosphorus is bound chiefly if not entirely in ester linkge with the - OH group of amino acid serine or threonine
a s2 – Casein
b – Casein and
k – Casein
These main fractions are themselves variable because of genetic polymorphism involving deletion or substitution of amino acids within the polypeptide chain of each fraction.
Elementary Composition: Casein contains the following elements (in percent) –Carbon 52.6-54.0, Hydrogen 6.75-7.10, Nitrogen 15.51-15.91, Sulphur 0.71-0.83 and Phosphorus 0.71-0.85.
Since casein is a phosphoprotein the phosphorus is bound chiefly if not entirely in ester linkge with the - OH group of amino acid serine or threonine
Electrical Properties: The isoelectric point of casein is pH 4.6 i.e. at this pH it has a zero net change. Above this pH it carries a negative charge while below this pH it is positive in nature. Thus at the pH of milk (pH 6.6 to 6.8), casein carries negative charge and, therefore, migrates towards anode in an electric field.
Solubility: Both α -and -casein are insoluble at isoelectric point (pH 4.7 and 4.9) but dissolve readily on either side of pH, a-casein is less soluble at pH 4.2 at 2 0 C while b - casein is more soluble at pH 4.2 at 2 0 C causing their separation. Casein also binds calcium ions above isoelectric point causing aggregation of casein, which lowers its solubility.
Solubility: Both α -and -casein are insoluble at isoelectric point (pH 4.7 and 4.9) but dissolve readily on either side of pH, a-casein is less soluble at pH 4.2 at 2 0 C while b - casein is more soluble at pH 4.2 at 2 0 C causing their separation. Casein also binds calcium ions above isoelectric point causing aggregation of casein, which lowers its solubility.
Other Properties : Casein also posseses other properties such as optical rotation,suggesting globular nature, oxidation and reduction due to its amino acids, and reacts with formalin/ formaldehyde. This reaction with formalin takes place through amino or amide groups of amino acids of casein. During this reaction with formalin an equivalent amount of protons (H + ions) are released which is then titrated against an alkali. The amount of proton released are equivalent to the amount of protein present. This is the basis for estimating protein in milk quickly by titration with NaOH.
ii) Whey Proteins: Lactalbumin and globulin fractions of whey proteins are two proteins together which form about one-eighth of the total protein of milk and are described as soluble protein. Both albumin and globulin are soluble in milk whey but they differ in that while albumin is soluble in water, globulin is insobule in water but soluble in dilute salt solutions. Both are coagulated by heat, the extent of which is governed by temperature and time of holding, salt concentration and pH of the solution (e.g., pH 4.5). Whey proteins are the native proteins present in whey or serum and are a constituent of whey, which is prepared after removal of casein through acidification at pH 4.6 or by rennet action during preparation of cheese. b-lactoglobulin is the major whey protein of milk. b-lactoglobulin is globular in nature. It has a molecular weight of 36,000 daltons with an iso-electric point of 5.2. It is a heat labile protein and gets denatured on heating. It is soluble in salt solution. It has a high nutritive value. It possess free – SH group in its structure through amino acid cysteine.
Alpha-lactalbumin is the second most abundant whey protein after b lactoglobulin.It is rich in human milk. This protein is involved in the bio-synthesis of lactose. It has a molecular weight of 16,000 daltons with an iso-electric point of 4.2. It contains no phosphorus or carbohydrate. This protein is very rich in the amino acid tryptophan and sulphur containing amino acids cystine and cysteine. Due to the presence of essential amino acids this protein is nutritionaly important whey protein.
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| Properties of whey proteins |
iii. Milk Fat
In milk, fat is present in the form of fat globules with an average size of 3 micron.If cool raw milk is kept for sometime without mixing there is tendency for the fat globules to cluster and rise at the surface forming cream layer. However, these globules are independent due to the presence of a protective fat globule membrane layer. Fat is distributed in continuous aqueous phase as an oil-in-water type emulsion.The membrane prevents the merging of fat globules and formation of a continuous fat phase.
Nature of protective fat globule membrane layer: On examination of the protective fat globules layer it is found that a variety of substances are present in it. These include protein, phospholipids, phosphatase, complex of vitamin B 12 binding proteins and a complex of riboflavin- phosphoric acid protein. There are also metals like copper, iron and zinc. In the presence of copper it causes oxidation of fat.
Physico-chemical nature of milk lipids: Milk lipids are present in three phases in milk, namely, the fat globule, the membrane surrounding the globule called fat globule membrane (FGM) and the milk serum. Fat globules are entirely made up of triglycerides coated with FGM containing complex lipids, phospholipids and proteins(Table).
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| Salt contents in Milk |
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| Various lipid material present in milk |
Milk fat has a specific gravity range of 0.936-0.946 at 15 0 C. Milk fat has a refractive index of 1.459-1.462 at 15 0 C. It is slightly soluble in ethanol but is readily soluble in hot amyl alcohol.
Chemical Properties of Milk Fat: True fats are glyceride esters of higher fatty acids.During hydrolysis of fat it is resolved into its components i.e. glycerol and the corresponding fatty acids. This hydrolysis occur naturally by the enzyme lipase.
The hydrolysis can be brought about also by superheated steam or alcoholic NaOH.
iv. Milk Salts
In milk, salts are represented by the “non-combustible matter” known as “ash”.The ash constitute the mineral matter of milk which is present to the extent of 0.7 per cent. The ash content is normally constant in milk. A higher value indicate an abnormal condition of the mammary gland. The important salt constituent present in milk ash are calcium, phosphorus, magnesium, sodium, potassium chloride, citrate and sulphur. Calcium and phosphorus are the pre-requisite for bone formation in the body. Milk is a very rich source of calcium. Average content of milk salts present in milk is presented below: (Table).
Chemical Properties of Milk Fat: True fats are glyceride esters of higher fatty acids.During hydrolysis of fat it is resolved into its components i.e. glycerol and the corresponding fatty acids. This hydrolysis occur naturally by the enzyme lipase.
The hydrolysis can be brought about also by superheated steam or alcoholic NaOH.
iv. Milk Salts
In milk, salts are represented by the “non-combustible matter” known as “ash”.The ash constitute the mineral matter of milk which is present to the extent of 0.7 per cent. The ash content is normally constant in milk. A higher value indicate an abnormal condition of the mammary gland. The important salt constituent present in milk ash are calcium, phosphorus, magnesium, sodium, potassium chloride, citrate and sulphur. Calcium and phosphorus are the pre-requisite for bone formation in the body. Milk is a very rich source of calcium. Average content of milk salts present in milk is presented below: (Table).
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| Salt contents in Milk |
v. Milk Vitamins
Milk is a very rich source of most of the vitamins except vitamin C. Almost all the vitamins are present is milk. Milk contains both fat soluble and water soluble vitamins (Table).
Milk is a very rich source of most of the vitamins except vitamin C. Almost all the vitamins are present is milk. Milk contains both fat soluble and water soluble vitamins (Table).
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| Quantity of vitamins present in milk |
a) Fat soluble vitamins
i)Vitamin A
ii) Vitamin D
iii) Vitamin E
iv) Vitamin K
b) Water soluble vitamins
i)B 1 (Thiamine)
ii) B 2 group- This group consists of Riboflavin, Nicotinic acid, Pyridoxine,Pantothenic acid, Biotin, Vitamin B 12 and Folic acid
iii) Vitamin C or Ascorbic acid.
Though all the above mentioned vitamins are present in milk, but those present in appreciable amounts to make milk a valuable source are vitamin A and B 2 group as far as human nutrition is concerned.
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