24 Questions
What is milk produced by?
The mammary glands of female mammals
What is the primary source of carbohydrates in milk?
Lactose
What happens to lactose during souring or aging of cheese?
It is converted to lactic acid
What is the characteristic of milk fat?
Low density
What is the percentage of milk fat in whole milk?
3-4%
What is another source of milk besides cows?
Sheep
What is the primary component of milk?
Water
What is an example of a milk product that can be digested by lactose-intolerant individuals?
Aged cheese
What is the primary component of milk fat?
Triglyceride
What is the function of homogenization in milk processing?
To redistribute the fat molecules
What is the primary protein in milk?
Casein
What is the characteristic of α s- and β-casein fractions?
They are calcium-sensitive
What is the weight ratio of the four fractions of casein proteins?
3:3:1:0.8
What is the definition of nonfat milk according to the FDA?
Skim milk, concentrated skim milk, reconstituted skim milk, and nonfat dry milk
What is the percentage of milk composition that is protein?
3-4%
What is the characteristic of kappa-casein?
It contains only one phosphate group and is not calcium-sensitive
What is the main characteristic of k-casein?
Presence of acidic carbohydrate section
Where are the hydrophobic α s- and β-casein fractions mainly found in casein micelles?
In the interior of the micelles
What is the role of k-casein in casein micelles?
Provides stability in milk under normal handling conditions
What is the property of casein fractions that makes them good emulsifiers?
Presence of both hydrophobic and hydrophilic sections
Why do casein fractions not denature at neutral pH unless the temperature is extremely high for a prolonged time period?
Due to their heat stability
What is the function of lipase in milk?
Hydrolyze triglycerides
Which vitamin is naturally present in the fat component of whole milk?
Vitamin A
Why is vitamin D often added to whole milk?
Because it is naturally present in small amounts
Study Notes
Casein Micelles
- Casein fractions associate to form stable spherical structures known as casein micelles.
- Hydrophobic α s- and β-casein fractions exist mainly in the interior of the micelles.
- Hydrophilic k-casein exists mainly on the micelle surface.
- k-casein gives the micelles their stability in milk under normal handling conditions.
Properties of Casein
- Coagulation
- Heat Stable: do not denature at neutral pH unless the temperature is extremely high for prolonged time period.
- Emulsifiers: contain both hydrophobic and hydrophilic sections plus high proportions of proteins, making them more flexible and forming a stable film, preventing coalescence of emulsion droplets.
Composition of Milk
- Fat
- Protein
- Sugar (CHO)
- Vitamins
- Minerals
- Water
Milk Fat Composition
- Contains traces of fatty acids; vitamins A, D, E, and K; and enzymes.
- More than 400 different fatty acids, with predominant fatty acids in bovine milk being myristic acid, palmitic acid, stearic acid, and oleic acid.
- Fat contains sterol cholesterol and phospholipids, primarily triglyceride (95%) with saturated, polyunsaturated, and monounsaturated fatty acid components.
- Contains many short-chain fatty acids, such as butyric acid, caproic, caprylic, and capric acids.
Milk Fat
- Low density, easily skimmed-off of the milk, yielding low-fat or skim milk.
- Exists at levels of 3–4% in whole milk, lesser levels in reduced-fat milks, and significantly higher percentages in cream.
Milk Proteins
- 3-4% of the milk composition
- Extracted through centrifugation
- The primary protein is casein – which is 80% of the milk protein
Casein Proteins
- Phosphoproteins containing phosphate groups esterified to the amino acid serine.
- The four fractions α s1-, β -, k, and α s2-casein occur in the weight ratio 3:3:1:0.8.
- α s- and β-casein fractions contain several phosphate groups and are “calcium-sensitive” and may be coagulated by addition of calcium.
- Kappa-casein contains only one phosphate group and is not calcium sensitive.
Learn about the properties of casein fractions, including α s-, β-, and k-casein, and how they form stable spherical structures called casein micelles. Understand the hydrophobic and hydrophilic characteristics of each fraction.
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