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Questions and Answers
What is the primary reason for the high stability of caseins against denaturing agents?
What is the primary reason for the high stability of caseins against denaturing agents?
Which type of casein is hydrolyzed by rennets during milk coagulation?
Which type of casein is hydrolyzed by rennets during milk coagulation?
At what temperature does β-casein exist as monomer chains?
At what temperature does β-casein exist as monomer chains?
What is the main protein content percentage in casein micelles on a dry matter basis?
What is the main protein content percentage in casein micelles on a dry matter basis?
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What characteristic of casein micelles contributes to the white color of milk?
What characteristic of casein micelles contributes to the white color of milk?
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How do casein micelles respond to ultracentrifugation?
How do casein micelles respond to ultracentrifugation?
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What is the average diameter of casein micelles?
What is the average diameter of casein micelles?
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What percentage of low molecular weight species is found in casein micelles?
What percentage of low molecular weight species is found in casein micelles?
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What factor primarily destabilizes micelles when ethanol is present?
What factor primarily destabilizes micelles when ethanol is present?
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Which casein is crucial for stabilizing the majority of caseins in micelles?
Which casein is crucial for stabilizing the majority of caseins in micelles?
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What is the surface potential of micelles at pH 6.7?
What is the surface potential of micelles at pH 6.7?
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Upon heating normal milk in the presence of whey proteins, what complex is formed?
Upon heating normal milk in the presence of whey proteins, what complex is formed?
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What happens to β-casein as the temperature is lowered?
What happens to β-casein as the temperature is lowered?
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Which interaction contributes to the stabilization of micelles?
Which interaction contributes to the stabilization of micelles?
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Where are κ-casein deficient submicelles predominantly found?
Where are κ-casein deficient submicelles predominantly found?
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What is the primary role of chymosin in relation to micelles?
What is the primary role of chymosin in relation to micelles?
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What is a key characteristic of casein's primary structure?
What is a key characteristic of casein's primary structure?
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Which type of casein contains carbohydrates and what are they?
Which type of casein contains carbohydrates and what are they?
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What percentage of phosphorus is contained in casein?
What percentage of phosphorus is contained in casein?
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Which aspect of phosphorus in casein is NOT mentioned as significant?
Which aspect of phosphorus in casein is NOT mentioned as significant?
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Why do caseins have relatively little secondary and tertiary structure?
Why do caseins have relatively little secondary and tertiary structure?
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What is one advantage of the caseins' open structure?
What is one advantage of the caseins' open structure?
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Which of the following is NOT a factor that contributes to caseins' emulsifying properties?
Which of the following is NOT a factor that contributes to caseins' emulsifying properties?
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What role does phosphorylation play in casein?
What role does phosphorylation play in casein?
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Study Notes
Primary Structure of Casein
- All caseins contain both polar and non-polar amino acid residues
- These residues are clustered, creating hydrophobic and hydrophilic regions
- This clustering makes casein good emulsifiers
- An emulsion is a mixture of two or more immiscible liquids
Casein Carbohydrate
- αs1-, αs2-, and β-caseins have no carbohydrates
- κ-casein contains about 5% carbohydrate
- Carbohydrates include sialic acid, galactose, and N-acetylgalactosamine
- These carbohydrates increase solubility and hydrophilicity of κ-casein
Casein Phosphorus
- Milk contains approximately 900 mg phosphorus per liter
- Casein contains about 0.85% phosphorus
- Phosphorus exists in inorganic (soluble and colloidal phosphates), organic (phospholipids, casein, and sugar phosphates, nucleotides) forms
- Phosphorus is nutritionally important because it binds to Ca2+, Zn2+, and other metals
- Phosphorus increases casein solubility and heat stability
- Phosphorus is covalently bound to serine residues in the protein
- Phosphorylation occurs in the Golgi membrane
Secondary and Tertiary Structures of Casein
- Casein has relatively little secondary and tertiary structure
- This is likely due to high levels of proline residues, which disrupt α-helices and β-sheets, especially in β-casein
Lack of Secondary and Tertiary Structure Significance
- Caseins are easily broken down by proteolysis (which is beneficial for digestion).
- Caseins' open structures make them easily adsorb to air-water and oil-water interfaces
- This makes caseins good emulsifiers and foamers, which are useful in food industries.
- The lack of higher structures likely explains the high stability of caseins to denaturing agents, including heat.
Molecular Size and Hydrophobicity of Casein
- Casein molecules are relatively small, ranging in molecular weight from 20 to 25 kDa
- Caseins are often considered to be hydrophobic
Influence of Calcium on Caseins
- 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 Rennin on Casein
- κ-casein is the only major casein hydrolyzed by rennet during milk coagulation
- κ-casein exists in solutions as disulfide-linked polymers
- At 4 °C, β-casein exists as monomers; at 8.5 °C, multiple β-casein monomers are associated.
- αs1-casein forms tetramers with a molecular mass of 113 kDa
- These caseins interact to form casein micelles in the presence of calcium
Casein Micelle Structure
- Casein micelles are 95% casein and 5% low-molecular-weight substances (calcium, magnesium, phosphate, citrate)
- Casein micelles are roughly spherical in shape, ranging in diameter from 50 to 500 nm; average diameter is 120 nm
- They are hydrated, binding about 2 g of water per gram of protein.
- The casein micelles have a hydrophobic core surrounded by a layer of κ-casein
Stability of Casein Micelles
- Casein micelles are stable to normal processing, ultracentrifugation, homogenization and 50 C, up to certain calcium concentration levels.
- They aggregate and precipitate at their isoelectric point (pH 4.6)
- Proteinases help to catalyze hydrolysis of certain bonds in κ-casein
- The micelles can destabilize from alcohols, acetone and electrostatic interactions.
Principal Micelle Characteristics
- κ-Casein, which makes up 15% of total casein, is critical to micelle structure and stability
- Micelle structure has pores where proteins occupy about 25% of its total volume
- Chymosin and similar proteinases hydrolyze most κ-casein
- When heated in milk, κ-casein interacts with other proteins (β-lactoglobulin) and forms disulphide-linked complexes.
- Change in pH and temperatures can modify the structure and stability of casein micelles
Micelle Subunit Structure
- κ-casein content in submicelles varies
- Deficient κ-casein submicelles are located inside the micelle and concentrated κ-casein submicelles are located on the surface
- Other caseins (αs1, αs2, and β) can be exposed on the surface of the micelles as well
Micelle Stability Mechanisms
- Frequent collisions between closely-packed micelles do not normally cause them to separate since they are stabilized by
- Surface charge (-20 mV at pH 6.7): this is not sufficient for maintaining stability on its own
- Steric stabilization: protruding κ-casein "hairs" prevent aggregation
Casein Preparation Methods
- Whey protein products are prepared via
- Separation through ultrafiltration,
- Ion-exchange chromatography,
- Demineralization (electrodialysis and ion exchange),
- Thermal evaporation,
- Crystallization,
- Thermal denaturation and precipitation using filtration/centrifugation/spray-drying
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Description
Explore the primary structure of casein, including its amino acid residues, carbohydrate content, and phosphorus significance. This quiz covers the features that make casein effective as an emulsifier and its nutritional importance. Test your understanding of these essential components in dairy science.
