1.Introduction
Bottling up all the goodness of a cow into a creamy, nutrient-rich elixir is nature's gift to humankind, and it has been a staple of the human diet for thousands of years. What makes this white liquid gold a potion of rich nutrition is the composition of its milk proteins. A good share of milk's goodness can be attributed to its protein reserve. Milk protein is an excellent source of essential amino acids that are necessary for the proper functioning of the human body. These proteins, in the form of ninja warriors, implicitly work towards preventing chronic diseases such as diabetes, muscle loss, sarcopenia, atherosclerosis, high blood pressure, osteoporosis, and some cardiovascular chronic diseases.
The milk protein family has two very important siblings: casein and whey protein, each possessing a unique set of characteristics. Slow and steady Casein is the older sibling found in milk and other dairy products. This slow-digesting protein is soluble in milk, but once it takes the slow lane to reach the stomach, it forms insoluble curds, more like gel-like substances, inhibiting digestive enzymes from working their magic and ultimately slowing down the absorption of nutrients. Slower digestion delays the release of the essential amino acids into the bloodstream, making casein an ideal protein source for building muscle mass and improving overall health.
On the other hand, the fast and furious whey protein takes the cake for being the fast-digesting younger sibling of the protein family. It allows faster and more efficient absorption of essential amino acids into the body, helping to speed up muscle recovery and growth after a tough workout. And not only does whey protein help to keep those muscles bulging, but it can also promote weight loss by keeping one feeling full and satisfied.
Preventing the diary discussion from diluting, this article cuts to the cheese and gives casein its moment in the spotlight while saving the story of whey protein for another day. The article reviews curdled protein and makes an attempt to dig into the many flavours of casein. Let's begin milking the topic of casein.
2.Definition
Defining caseins is no easy feat, as it can be tricky to identify all the proteins that fall under this category while excluding others. However, there is one key characteristic that all caseins share: a low solubility at a pH level of 4.6, at least for bovine milk. This means most caseins, excluding some proteolytic derivatives, will precipitate at this pH level. As caseins have lower solubility compared to whey proteins, it is possible to manipulate the pH to separate them from other proteins. By adjusting the pH, we can quickly identify and isolate this important class of proteins to better understand their properties and functions and learn how they contribute to the nutritional benefits of milk.
Hence, casein can be defined as a type of protein found in milk that exhibits low solubility at a pH level of 4.6, causing it to precipitate and allowing for separation from other proteins through pH manipulation. This protein is categorised into:
● αs1-Caseins
● αs2-Caseins
● β-Caseins
● γ-Caseins
● κ-Caseins
Hydrophobic cluster analysis of (a) αs1 casein, (b) αs2 casein, (c) β casein and (d) κ casein
Some other characteristics of casein are:
● Separating casein components with careful control of their molecular interactions can be achieved by ion exchange chromatography using DEAE-cellulose or hydroxyapatite columns, leading to a yield of satisfactory fractions.
● All the casein polypeptide chains have at least one group of ester-bound phosphate per molecule.
● The types αs1 and β-caseins lack cysteine residues, while αs2 and κ-caseins each have two cysteine residues.
● Caseins have high proline contents which hinder the formation of common secondary structural motifs.
● Analytical studies reveal that caseins exhibit short lengths of α-helix or β-sheet structure and have accessible ionizable groups for titration and other reactions.
● Denaturing agents and heating have no effect on the secondary structure of caseins, which appear like denatured globular proteins.
● The four types of casein differ in charge distribution and tendency to aggregate in the absence and presence of Ca2+ ions.
Casein micelle models (a) by Waugh in 1958, (b) by Schmidt in 1982, (c1) model proposed by Walstra in 1990 & (c2)1999, (d1) Dual binding model by Horne in 2003, (d2) interpretation of Schmidt’s model in 2005.
3.Types
3.1. αs1-Caseins
αs1-Casein may sound like a mouthful, but it's a natural source of energy to fuel our body. The structure of this protein consists of two hydrophobic regions and a highly charged polar zone. Considering its conformation, it's a loose and flexible chain that can self-associate depending on various factors like concentration, pH, ionic strength, and the type of ions present.
Physical measurements like light scattering, sedimentation, and viscosity show that αs1-casein behaves differently under various conditions. At pH 12, it dissociates into a flexible chain of little monomers holding a molecular weight of 24,000. While at neutral pH and higher ionic strength, the protein binds to calcium ions. But at very low concentrations of calcium ions, it aggregates and precipitates. You may have observed peptides called A-casein, also found in milk, originating from the proteolysis of αs1-casein.
3.2. αs2-Caseins
αs2-casein has a unique dipolar structure with negative charges near the N-terminus, or start, of the protein and positive charges near the C-terminus, or towards the end. Although not as well studied as other caseins, it certainly binds to calcium strongly and is even more sensitive to precipitation by calcium ions than in αs1-casein. At neutral pH, in the absence of calcium ions, the proteins stick together, with the extent of association depending on the ionic strength of the solution but not more than 0.2.
3.3. β-Caseins
With a negatively charged head and an uncharged hydrophobic tail, the structure of β-casein bears a stark resemblance to that of anionic detergents. Interestingly, the association of β-casein is strongly dependent on temperature, both in the presence and absence of Ca2+. What is really unique about β-casein is its behaviour at different temperatures. At room temperature, large polymers consisting of 20–24 monomers are formed, while only monomers are formed at 4°C.
Furthermore, the removal of the 20-residue C-terminal sequence destroys the ability of β-casein to associate, suggesting that specific hydrophobic interactions are involved in the process. β-casein also binds tightly to about 5 calcium ions per mole, which is consistent with its ester phosphate content. Overall, the unique properties and structure of β-casein make it a fascinating subject for further study.
3.4. γ-Caseins
Did you know that a group of caseins are actually formed by cleaving β-casein with the enzyme plasmin? This process creates a whole new set of protein fragments, which are collectively known as γ-caseins. The presence of β-casein and γ-casein in milk suggests that there is a genetic basis for the polymorphism of γ-casein, indicating that variations in the γ-casein protein are likely due to differences in genes. Additionally, the fact that β-casein and γ-casein are found together in milk also suggests a close relationship between the synthesis of these two proteins. This close relationship is thought to occur because β-casein serves as the precursor to produce γ-casein. Thus, the production of γ-casein is dependent on the presence and synthesis of β-casein. Overall, the coexistence of β-casein and γ-casein in milk points to a complex and interrelated system of protein synthesis that is influenced by genetic factors.
3.5. κ-Caseins
Milk may seem plain white and simple, but its protein κ-casein is anything but! One-third of its molecules have no carbohydrates, and it's made up of multiple components. When isolated, κ-casein forms polymers that link together via disulfide bonds. It also rapidly breaks down with the help of chymosin, resulting in fragments like para-κ-casein and macro peptides. The protein's unique solubility is just another reason for setting it apart from the other caseins. All in all, κ-casein is a fun but complex protein that adds to the richness of milk!
3D Model of Casein Proteins
4.Conclusion
Caseins are marvelous milk proteins that play an essential role in nutrition and food functionality. From their ability to form micelles and bind calcium ions to their diverse and complex structures, their unique properties continue to intrigue scientists and researchers. Understanding the properties and functions of casein is crucial for the dairy industry. As for spilling the dairy secrets, the casein proteins certainly have a lot to reveal about the magic behind milk and its many products. So, the next time you are enjoying a glass of milk or a creamy dairy product, remember that there's a whole world of proteins that make it so much more than just a simple beverage!
5.References
1. http://rb.gy/gvckvx
2. http://rb.gy/wjb7ew
3. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=3911
4. https://doi.org/10.1533/9780857095725.2.417
5. https://www.sciencedirect.com/science/article/abs/pii/0003986168905730
6. https://www.mdpi.com/2304-8158/10/8/1965
7. https://www.mdpi.com/2073-4360/13/18/3164
8. https://www.scientificamerican.com/article/a-milk-curdling-activity/
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