Biochemistry of Casein Proteins

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Questions and Answers

What is the primary role of κ-casein in micelle structure?

It affects the taste of dairy products.

It contributes to the color of the micelles.

It stabilizes calcium-sensitive caseins. (correct)

It determines the shelf life of milk.

What condition can lead to the destabilization of micelles?

Low temperatures.

Alcohol and acetone exposure. (correct)

High concentrations of calcium.

Increased levels of proteinases.

What is the zeta potential of micelles at pH 6.7?

-20 mV (correct)

+20 mV

-40 mV

0 mV

How do whey proteins interact with κ-casein in milk?

They form a disulphide-linked complex. Signup and view all the answers

What is the average protein occupancy in the volume of a micelle?

25% Signup and view all the answers

What happens to β-casein as the temperature is lowered?

It dissociates from the micelles. Signup and view all the answers

What impact does freezing have on micelles?

It leads to destabilization through pH decrease and increased calcium. Signup and view all the answers

Where are κ-casein deficient submicelles located in the micelles?

In the interior of the micelles. Signup and view all the answers

What characteristic of caseins contributes to their role as effective emulsifiers?

Clusters of polar and non-polar residues Signup and view all the answers

Which type of casein contains carbohydrate, and what is one component of this carbohydrate?

κ-casein; galactose Signup and view all the answers

What is one critical role of phosphorus in caseins?

Facilitating casein coagulation Signup and view all the answers

What is a significant reason for the high stability of caseins when exposed to denaturing agents?

The lack of higher structures Signup and view all the answers

Why do caseins have relatively little secondary and tertiary structure?

Presence of high levels of proline residues Signup and view all the answers

Which type of casein is hydrolyzed by rennets during milk coagulation?

κ-casein Signup and view all the answers

What is one consequence of caseins being susceptible to proteolysis?

Enhanced flavor development in cheese Signup and view all the answers

What is the molecular weight range of caseins?

20 to 25 kDa Signup and view all the answers

Which of the following describes a benefit of caseins' open structure?

Enhanced adsorption at interfaces Signup and view all the answers

At what temperature does β-casein become soluble in high concentrations of Ca2+?

18°C Signup and view all the answers

What is a significant nutritional advantage of the proteolysis vulnerability of caseins?

Enhanced mineral binding capacity Signup and view all the answers

Which type of phosphorus is primarily bound to serine in caseins?

Covalent phosphorus Signup and view all the answers

What is the predominant composition of casein micelles on a dry matter basis?

94% protein, 6% low molecular weight species Signup and view all the answers

What is the average diameter range of casein micelles?

50 to 500 nm Signup and view all the answers

How do the micelles contribute to the white color of milk?

By scattering light Signup and view all the answers

What happens to casein micelles when subjected to ultracentrifugation?

They can be sedimented and re-dispersed Signup and view all the answers

Study Notes

Primary Structure of Casein

All caseins have both polar and non-polar amino acid residues
These residues are clustered, creating hydrophobic and hydrophilic regions
This clustering makes caseins good emulsifiers
An emulsion is a mixture of two or more immiscible liquids

Casein Carbohydrate

αs1-, αs2, and β-caseins lack carbohydrates
κ-casein contains about 5% carbohydrates
These carbohydrates include sialic acid, galactose, and N-acetylgalactosamine
The carbohydrates contribute to the high solubility and hydrophilicity of κ-casein

Casein Phosphorus

Milk contains approximately 900 mg of phosphorus per liter
Casein comprises about 0.85% phosphorus
Casein phosphorus includes inorganic (soluble and colloidal phosphates), organic (phospholipids, casein and sugar phosphates, nucleotides)
Phosphorus is nutritionally important because it binds to calcium, zinc, and other metals
It also enhances casein solubility and heat stability
Casein phosphorus is covalently bonded to the protein, primarily to serine
Phosphorylation of casein occurs mainly in the Golgi membrane

Secondary and Tertiary Structures of Casein

Caseins have limited secondary and tertiary structures
This is likely due to high proline residue content, particularly in β-casein
Proline residues disrupt α-helix and β-sheet structures

Lack of Secondary and Tertiary Structures

The lack of secondary and tertiary structures makes caseins readily susceptible to proteolysis
Caseins readily hydrolyze in cheese, contributing to flavor and texture
Caseins are good at adsorbing to air-water and oil-water interfaces due to their open structure and a high content of nonpolar amino acid residues
This quality makes them good emulsifiers and foamers in food

Molecular Size of Casein

Casein molecules are generally small, ranging from 20 to 25 kDa

Hydrophobicity of Casein

Casein molecules are often considered hydrophobic

Influence of Calcium on Casein

αs1- and αs2-caseins are insoluble in calcium-containing solutions
β-casein is soluble in high concentrations of calcium (0.4M) at temperatures below 18°C
κ-casein is soluble in calcium at all concentrations

Action of Rennits on Casein

κ-casein is the major casein hydrolyzed by rennets, occurring during the primary phase of milk coagulation
κ-casein exists largely in solution as disulfide-linked polymers at 4°C
β-casein exists as monomers at 4°C, but as polymers at 8.5°C
as1-casein polymerizes into tetramers at a molecular mass of 113 kDa
The presence of calcium promotes the formation of casein micelles

Casein Micelle Structure

Casein micelles are large colloidal particles comprising 94% protein and 6% low molecular weight substances (calcium, magnesium, phosphate and citrate)
They are typically spherical with diameters ranging from 50 to 500 nm (average 120 nm)
Casein micelles contain about 2.0 g of water per gram of protein
There are roughly 10^14-10^16 micelles per milliliter of milk
The white color of milk is due to light scattering by casein micelles

Stability of Casein Micelles

Casein micelles are stable at high temperatures and to common processing methods
They are stable to high calcium concentrations (up to 200 mM) at temperatures up to 50°C
They aggregate and precipitate when the pH is adjusted to the isoelectric point (pH 4.6)
Caseins are destabilized by 40% ethanol at pH 6.7 and by decreasing pH and increasing calcium levels

Principal Micelle Characteristics

κ-Casein, comprising about 15% of total casein, is crucial to micelle structure and stability
Micelle structure is porous, with protein occupying about 25% of the total volume
Chymosin and similar enzymes hydrolyze most of the к-casein in micelles
Heating with whey proteins forms a complex, potentially modifying rennet coagulability and heat stability
Alcohol and acetone can destabilize micelles
β-casein dissociates from micelles with decreasing temperature

Micelle Subunit Structure

The к-casein content of the submicelles varies, with k-casein-deficient submicelles located within the micelle interior and к-casein-rich submicelles concentrated on the outside
Some αs1-, αs2- and β-casein are exposed on the surface

Micelle Stabilization

Micelle stability is achieved by surface (zeta) potential and steric stabilization
Zeta potential (approximately -20mV at pH 6.7) is likely insufficient for colloidal stability, so steric stabilization from protruding к-casein hairs is important
Inter-micellar collisions are frequent, but micelles do not usually remain together after these collisions

Whey Protein Rich Product Preparation

Whey protein-rich products are prepared through various methods on a commercial scale
These methods include ultrafiltration, ion-exchange chromatography, demineralization, thermal evaporation, and thermal denaturation

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Description

Explore the complex structure and properties of casein proteins, including their amino acid composition, carbohydrate content, and phosphorus significance. This quiz delves into the roles these components play in creating emulsions and enhancing nutritional benefits. Test your understanding of casein's biochemistry and its applications in various fields.