Milking Every Drop of Casein and Whey Types
1.Introduction
Despite being a superstar in the realm of nourishment, milk is often relegated to the sidelines like an overlooked gem in a sea of beverage options! No matter how many times a mom tries to push the milk agenda by pleading and begging her child to finish their milk, some kids seem to have an innate aversion to the white liquid gold, and trying to convince them is like trying to find a needle in a haystack.
Milk plays a vital role in maintaining a well-balanced, nutritious diet. It comprises a variety of beneficial components that provide valuable nutrition for the body. The widely acclaimed health benefits of milk can be attributed to its proteins, which are valuable not only for their nutritive value but also for their biological properties. In a world where milk is often taken for granted, the proteins it contains are like hidden treasures waiting to be discovered. Scientific evidence suggests that these proteins exhibit anticarcinogenic and antimicrobial activities, antihypertensive properties, immune system modulation, and other metabolic features.
2.Milk Proteins & their Types
There is a total of 3.3% protein content found in milk, along with the presence of all nine essential amino acids required by humans. Synthesised in the mammary gland, 60% of the amino acids to build the proteins are obtained from a cow's diet. The milk protein content does vary with different cow breeds and individual animal genetics.
Bovine milk is a powerhouse of two major proteins: casein and whey protein. Casein, which makes up a whopping 80% of milk's protein content, is a complex family of four components that mainly consists of calcium phosphate-micelle complexes. Whey protein, on the other hand, accounts for the remaining 20%. It is also known as serum protein and doesn't contain phosphorus. Whey is a group of globular proteins rich in amino acids with unique alpha-helix structures.
Cryo-SEM image of fat globules from milk with different proportions of protein to fat.
The family of milk proteins also includes MFGM proteins and some indigenous milk enzymes.
3.Classification
Initially, proteins were classified based on solubility and composition. However, with advanced analytical techniques, it was established that what was once considered individual proteins were mixtures of several fractions in reality. Today, milk proteins are classified based on their fractionation, behaviour during electrophoresis, solubility in various solutions, differences in their sedimentation rate, etc.
Pie Chart Showing Distribution of Milk Proteins
A common method for the classification and distribution of milk proteins derived from Genus Bos (30-35 g/L), is expressed in the following manner:
3.1. Caseins (24-28 g/L)
3.1.1. αs1-Caseins (12-15 g/L)
● αs1-Casein X-8P (X = Genetic variants - A, B, C, D-9P, and E)
● αs1-Casein X-9P (X = Genetic variants - A, B, C, D-10P, and E)
● αs1-Casein fragments
Consisting of 199 amino acid residues with a high proline content that prevents them from folding into a specific structure, αs1-Caseins appear as loose and flexible polypeptide chains, making them useful for their biological function.
3.1.2. αs2-Caseins (3-4 g/L)
● αs2-Casein X-10P (X = Genetic variants - A, B, C-9P, and D-7P)
● αs2-Casein X-11P (X = Genetic variants - A, B, C-10P, and D-8P)
● αs2-Casein X-12P (X = Genetic variants - A, B, C-11P, and D-9P)
● αs2-Casein X-13P (X = Genetic variants - A, B, C-12P, and D-10P)
αs2-Caseins consist of 207 amino acid residues that bind strongly to calcium, making it more sensitive to precipitation by calcium ions than αs1-Caseins. Additionally, when αs2-Caseins undergo proteolysis, the fragments exhibit antimicrobial activity. Due to the high number of phosphorylation sites, they also have high solubility.
3.1.3. β-Caseins (9-11 g/L)
● β-Casein X-5P (X = Genetic variants - A1, A2, A3, B, C-4P, D-4P, and E)
● β-Casein X-lP (f 29-209) (X= Genetic variants - A1, A2, A3, and B)
● β-Casein X-(f 106-209) (X = Genetic variants - A2, A3, and B)
● β-Casein X-(f 108-209) (X = Genetic variants - [Fragments from A1, A2, A3] and B)
● β-Casein X-4P (f 1-28)
● β-Casein X-5P (f 1-105)
● β-Casein X-5P (f 1-107)
● β-Casein X-lP (f 29-105)
● β-Casein X-lP (f 29-107)
β-Caseins are proteins consisting of 209 amino acid residues with strong, negatively charged N-terminal portions. The overall structure of this protein is similar to that of an anionic detergent.
3.1.4. κ-Caseins (2-4 g/L)
● κ-Casein X-lP (X = Genetic variants - A and B)
● Minor κ-caseins X-l, -2, -3, etc. (X = Genetic variants - A and B)
With 169 amino acid residues and a glutamic acid N-terminal residue, κ-Casein is rapidly hydrolyzed to form para-κ-Casein with cysteine residues. A naturally occurring mixture of bovine κ-Casein variants can be polymerised through disulfide linkage. These proteins are found on the surface of casein micelles.
Casein Micelle
3.2. Whey Proteins (5-7 g/L)
3.2.1. β-Lactoglobulin (2-4 g/L)
● β-Lactoglobulin X (X = Genetic variants - A, B, C, D, Dr, E, F, and G)
The most abundant whey protein in bovine milk, this protein consists of 162 amino acid residues exhibiting an isoelectric point of 5.1. β-Lactoglobulin exists naturally as a covalently linked dimer of two monomeric subunits, which are folded into a globular structure with a hydrophobic core and hydrophilic surface.
3.2.2. α-Lactalbumin (0.6-1.7 g/L)
● α-Lactalbumin X (X = Genetic variants - A and B)
● Minor α-Lactalbumins
A significant protein in breast milk, α-Lactalbumin contains 123 amino acid residues and has an isoelectric point between 4.2 and 4.5.
3.2.3. Bovine Serum Albumin (0.2-0.4 g/L)
Bovine Serum Albumin is a major component of blood serum in cows and is synthesised in the liver. With a single polypeptide chain of 582 amino acid residues, 17 disulfide linkages, and one free thiol group, its conformation allows it to neatly hold the protein in a multi-loop structure.
Structure of Whey Protein
3.2.4. Immunoglobulins (0.5-1.8 g/L)
3.2.4.1 IgG Immunoglobulins
● IgG1 Immunoglobulins
● IgG2 Immunoglobulins
● IgG fragments
3.2.4.1 IgM Immunoglobulins
3.2.4.1 IgA Immunoglobulins
● IgA Immunoglobulins
● Secretory IgA Immunoglobulins
3.2.4.1 IgE Immunoglobulins
3.2.4.1 J-chain Component
3.2.4.1 Free secretory component
Immunoglobulins are antibodies produced in response to stimulation by macromolecular antigens or foreign substances. They are made up of two types of polypeptide chains, light and heavy, and are classified into five major types: IgA, IgG, IgM, IgE, and IgD, with each type having a different function to help protect against different types of antigens.
Structure of Immunoglobulin
3.3. Milk Fat Globule Membrane (MFGM) Proteins
● XDH/XD, CD36, BTN, ADPH, PAS 6/7, and FABP (stained with Coomassie Brilliant Blue).
● Muc-1 and PAS III (stained by Periodic Acid/Schiff [PAS] or Silver Stain).
MFGM is a complex mixture of proteins that accounts for approximately 1% of the total protein content in milk. They are difficult to resolve analytically.
Structure of MFGM Proteins
3.4. Minor Proteins
● Serum transferrin
Serum transferrin is a minor protein in milk that transports iron in the body. Genetic variants of this protein have not been specifically identified.
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4.Conclusion
β-Lactoglobulin exists in two forms, A and B, which indicate genetic polymorphism in milk proteins. AA, BB, or AB indicate the presence of β-Lactoglobulin A, B, or both in milk. This phenomenon, arising from genetic code changes in the protein, influences milk yield and composition, including protein percentage and cheese yield.
So, the next time your mom urges you to finish your glass of milk, think of the milk proteins as a squad of superheroes, working together to protect your body and keep you feeling your best.
5.References
1. http://rb.gy/rrssmd
2. http://milkfacts.info/Milk%20Composition/Protein.htm
3. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=3909
4. https://doi.org/10.1002/9781118860588.ch16
5. http://rb.gy/pp1vry
6. http://rb.gy/0cfbtb
7. https://pubs.rsc.org/en/content/articlehtml/2014/ra/c4ra06171c
8. https://en.wikipedia.org/wiki/Milk_fat_globule_membrane
9. http://rb.gy/nol06e
10. https://honehealth.com/edge/nutrition/casein-vs-whey-protein/
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