Proteins are considered as the third class of macro components of all living systems, and also that of the food stuffs .Proteins are polymers with highly complex structures and their molecular weight ranges from 10,000 to several million. The structure of protein is quite straightforward; they are made up of monomeric units called as Amino acids. These amino acids are mostly linked by a single peptide bond.The range of amino acids is highly limited in number and mostly common to all proteins. Mostly all proteins are mixtures of 20 standard amino acids. The polypeptide bonds of proteins are never branched. The uniqueness of protein lies in the diversity of variation in its structure and function. All proteins have their own sequence of amino acids of defined length. All amino acids that occur in protein have a general formula:




Proteins are generally made up of one of the four structure .They may be primary, secondary, tertiary or quaternary structure.

Primary structure:

They are the sequence of a chain of amino acids.

Secondary protein structure:

When the sequence of a chain of amino acids is linked by an amino acid. E.g. either alpha helix or pleated sheet.

Tertiary protein structure: Three dimensional structure of a protein molecule formed by the spatial arrangement of the secondary structures like alpha helix and pleated sheet.


Quaternary protein structure:

If a protein has more than one amino acid chain then its called quaternary protein structure.

Milk proteins (casein):

Milk of cow is the important source of protein for man and most importantly for children. Milk is a mixture of aqueous solution of proteins, lactose, minerals and certain vitamins which carry emulsified fat globules and casein micelles which have protein along with phosphate, citrate, and calcium. The fat removed milk is called skimmed milk. If the pH of the skim milk is reduced to 4.6 at 20C, the casein is precipitated and the residue is called whey or serum. The proportion of different casein in milk varies and so does the whey. The content of casein vary in different types of milk like in cows milk it accounts for 80% where as in human milk its just 40%.

The various proportion of casein in skimmed milk is shown in the below table:

Casein proteins (80%) Skimmed milk protein%
α-Casein 40
β-Casein 24
χ-Casein 12
γ-Casein 4
  • Source: Coultate,T.P.1989.Food-The Chemistry of its Components.2nd ed.London: The Royal society of Chemistry

The Greek letters that are used in casein proteins are used on basis of the protein mobility in electrophoresis. The‘s’ in α s-casein refers to its sensitivity to precipitation by the calcium ion. Where as α, β, χ, and γ define particular protein species, it has been discovered that there are lots of different versions of each that only differ slightly in their amino acid sequences. For example, α s1C casein is different from that of α s1B casein by having glycine residue instead of glutamic acid residue at position 192 in the polypeptide chain. With these advancements one can even identify the breed of cow from which the sample milk was obtained by examining the proportions of these variants.

Heterogeneity of caseins:

Heterogeneity is seen in caseins. For example, in bovine casein there are four major types of casein .They are α s1, α s2, β and κ-casein. Each of these at least exhibit micro heterogeneity in one of the below reasons:

  • Variation in the degree of phosphorylation.
  • Variation in the degree of glycosylation in κ-casein alone.
  • Genetically controlled substitution in the amino acids, which results in the genetic polymorphism.
  • Proteolysis of the indigenous proteinases.

Molecular properties of casein:

Casein are well characterized proteins and indeed they are quite small proteins, with a molecular mass say around 20-25kDa .Which is the main reason for their high stability. For example β-casein has high level of proline say like 35 out of the 209 amino acids are proline which is equally distributed. Due to the presence of high level of proline the alpha helix, beta-sheets and beta-turns are absent.

Casein are generally hydrophobic .Apart from this it is also said that caseins have very low secondary and tertiary structures .As they lack secondary structures its been told that they are very flexible and unstable which makes it ‘rheomorphic'(they are so flexible in solution that they can adopt any structure dictated by the environment).The lack of stable secondary and tertiary structures also make them stable against denaturing agents like heat and urea. This latter property also confers good foaming and emulsifying property on casein. This also makes casein readily susceptible to proteolysis. These properties of casein make them unique in dietary applications like in that of cheese ripening etc.

Properties of the principal caseins in cows’ milk:

protein molecular mass Amino acids proline residues cysteine residues PO4 group concentration(g/L) glycoprotein genetic variants
α s1 -casein 23164 199 17 0 8 10 no A, B, C, D, E, F, G, H
α s2-Casein 25388 207 10 2 10 2.6 no A, B, C, D
β-casein 23983 209 35 0 5 9.3 no A1, A2, A3, B, C, D, E, F, G
κ-casein 19038 169 20 2 1 10.3 yes A, B, C, E, FS, FI, GS, GS

Table2: Properties of caseins present in cows Milk.

Source:Yada, Rickey Yoshio, 1954-, Proteins in food processing ,pg29-62,Knovel firm

Role of casein in dairy products:

In western countries, Dairy products account for about 30% of the daily dietary energy, lipids and protein. And even some of the vitamins are also got from dairy products like riboflavin and vitamin B12.Minerals also come into this list, followed by calcium it is also estimated that 80% of calcium is obtained from dairy products.

Some of the functions of caseins in dairy products are given below:

Pasteurized liquid milk Appearance, heat stability, mouth feel and flavor.
fermented milks Gel formation, mouth feel, flavor, syneresis of the gel and rheology
Creams Emulsion stability, rheology (body) and whippability.
ice creams Emulsion stability, rheology (body) and whippability
Milk powders wettability , dispersibility,solubility,flavor,color and other uses which depend upon the end use of the product
Cheese texture, body and other rheological properties; functional properties

such as meltability, stretchability, sliceability, adhesiveness, water-binding

Properties; (off-) flavors.

  • Coultate, T.P. Food-the Chemistry of Its Components. second ed. london: The Royal society of Chemistry 1989

The precipitation of casein is the basic process involved in cheese making .In case of yogurt and some cottage cheese the precipitation is brought up by low pH.

Functional food properties of casein (dairy products):

Functional food is nothing but if a food ingredient or food provides some kind of health benefit which doesn’t include the traditional nutrient it contains then it can be called as a functional food. Some of the common functional food characteristics of dairy products are Anti-microbial – including control of gut micro flora, Anti-viral, Binding of E coli and cholera enter toxins, Anti-cancer, Immunomodulation, Anti-oxidative, Opioid effects

Retard osteoporosis. The bioactive peptides of casein are responsible for most of these functional properties like Casein glycomacropeptide which is responsible in Cancer Prevention, Immunomodulation, Diet Suppression, and Cardiovascular Effects (cholesterol reduction).

Fractionation of casein:

Its possible to fractionate whole casein into its component proteins from 1940 but it is limited to laboratory .It is done by exploiting the difference in solubility in the solution of urea pH4.6 or cacl.Its is also done by various chromatography methods but unfortunately none of these methods are suitable for industrial scale production of individual casein.

But there is a huge scope or opportunity for this because:

  • The Beta casein has high surface activity hence they can be used has good emulsifying or good foaming agents.
  • Human milk contains beta casein and k-casein .Alpha casein is almost absent in human milk hence the beta casein will be an attractive ingredient of bovine milk based infant formulae.
  • As k-casein is responsible for the stability of the casein micelle it can be used in some of the milk products.
  • Latest discoveries have revealed that fortification of milk with Beta-casein improves cheese making properties.

One of the successful methods of separation of beta casein is from skimmed milk by using rennet.



The Casein micelle

The Casein in milk exist as colloidal particles of 50-500nm, which are called as casein micelles. The casein micelles of milk are roughly spherical particles. Milk has about 1015 micelles dm-3.A casein micelle contain approximately about 2×104 casein molecules. Of all structure models slattery and evards model comes near to accounting all the observed properties of casein. According to them micelle is an aggregate of sub micelles each consisting of 25-30 molecules of all types of casein in roughly similar proportions to those in milk as a whole.

Figure : A casein micelle; A: a submicelle; B: protruding chain; C: Calcium phosphate; D: κ-casein; E: phosphate groups


Casein micelles are stable to:

-Compaction-Pellet that was recovered from ultracentrifugation can be readily redispersed.

-Homogenization at normal or high pressure

-Also to high Ca2+ concentrations, up to 200mM, at temperatures upto 50C

However number things can destabilize, redisperse or affect the casein micelle hence they are usually destabilsed in certain food products like cheese etc.


As its clear that casein has a great potential as a functional food more research should be carried out in this field. More studies should be undertaken in bioactive peptides of casein which are released by the enzymatic hydrolysis which have a huge role in cancer prevention, cardiovascular disease prevention etc.As they are good emulsifying and foaming agents they can be extensively used in food industry.


  • “Milk Proteins.” Wageningen University,
  • Coultate, T.P. Food-the Chemistry of Its Components. second ed. london: The Royal society of Chemistry 1989
  • Harper, W. James. “Functional Aspects of Dairy Foods “, 2003.
  • Yada, Rickey Yoshio. “Proteins in Food Processing.” Place Published, 1954.
  • Hurley, Walter L. “Milk Composition Proteins.”

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