Chemical, Physical, and Functional Characteristics of Dairy Ingredients 37Milk Proteins
Proteins are made up of amino acids with
specifi c properties that are determined by the
side chains of the amino acids in the poly-
peptide chain. The conformation of the
protein depends on the hydrogen bonds,
hydrophobic interactions, and salt bridges
formed between the peptide chains. Regular
arrangements include β - sheets and α - helices.
Temperature, ionic strength, and pH affect
protein conformation.
The major classes of protein in milk are
casein and whey or serum proteins. Casein
makes up approximately 80% of the milk
protein, and whey or serum proteins make up
approximately 20%. The proteins are synthe-
sized in the mammary gland, and therefore,
are derived genetically. The protein content
of milk remains constant with a concentra-
tion range of 30 to 35 g/kg. However, it is
infl uenced by the lactation stage of the cow.
Table 2.3 shows the typical protein com-
position of bovine milk.Casein Proteins
Four main types of casein have genetic vari-ants: α (^) s1 - casein, α (^) S2 - casein, β - casein, and
κ - casein. They are phosphorylated and
hydrophobic and associate with themselves
and each other. They represent 38%, 10%,
Compound lipids also occur in milk such
as phospholipids and phosphatides that are
situated mainly in the fat globule membranes
but also in the milk plasma, lipoproteins, and
milk microsomes. The total phospholipid
content in milk is approximately 36 mg/100 ml.
Phospholipids and phosphatides are highly
surface active and polar, and dissolve poorly
in both water and oil.
Lipids can be crystallized, which affects
the fat structure, melting range, and rheologi-
cal properties of milk. Furthermore, autoxi-
dation of the double fatty acid bonds or
residues can occur, leading to off fl avors.
Whole milk contains 10 to 20 mg/100 g
cholesterol (3.3% fat). The amount of cho-
lesterol is positively correlated with the
amount of fat in the product. Cholesterol is
located in the fat globule membrane, and
approximately 10% of the cholesterol is
esterifi ed. The amount of cholesterol in milk
is affected by various factors including
species, breed, feed, season, and stage of
lactation; the amount of cholesterol is high-
est near the end of lactation. The average
cholesterol content of various dairy products
and their corresponding fat percentages are
shown in Table 2.2.
Table 2.2. Average cholesterol and fat content
of various dairy products.
Product % Fat Cholesterol
(mg/100 g)
Whole milk 3.3 14
Skim milk 0.25 2
Goat milk 3.92 11
Human milk 4.03 25
Buttermilk 0.51 1.4
Yogurt 3.75 10
Cream 34.87 110
Whipped cream 31.7 84
Sour cream 18 48
Ice cream 10.77 45
Blue cheese 28.74 75
Cottage cheese (creamed) 4.51 15
Swiss cheese 27.45 92
Cheddar cheese 33.14 105
Butter 81.11 219
Adapted from Kailasapathy (2008) and Souci (2008)
Table 2.3. Typical protein composition of bovine
milk.
Protein Component Weight (g/kg)
Total protein (%) 35.1
Total casein 28.6
Whey protein 6.1
α (^) s1 - casein 11.5
α (^) s2 - casein 3.0
β - casein 9.5
κ - casein 3.4
γ - casein 1.2
α - lactoglobulin 1.2
β - lactoalbumin 3.1
Serum albumin 0.4
Immunoglobulin 0.8
Proteose peptone 1.0
Adapted from Kailasapathy (2008)