C 16 ,C 18 , and C18:1fatty acids and is present in fresh milk mainly in the
form of fat globules surrounded by a phospholipid rich layer known as
the milk fat globule membrane. Typically these globules have a diameter
of about 5mm and the milk contains about 10^12 fat globules per litre. If
fresh milk is allowed to stand, the fat rises to the surface of the milk to
produce a distinct cream line. The tendency for this to happen is reduced
if the size of the globules is reduced by passing the milk through a small
orifice under pressure; a process known as homogenization.
About 80–85% of the protein in milk is present as caseins. These are
milk-specific proteins which are precipitated from milk by decreasing the
pH to 4.6. This pH corresponds approximately to their isoelectric point
which is relatively low due to the predominance of acidic amino acids
and the presence of phosphorylated serine residues in the molecules.
There are five main classes of caseins (see Table 5.2); these aggregate
together in association with calcium phosphate in milk to form colloidal
particles known as micelles. Milk contains around 10^15 casein micelles
l^1 with an average diameter of around 0.2mm. The stability of the
micelle is maintained by the presence ofk-casein near or on the surface of
the particle. Loss of this stabilizing effect occurs whenk-casein is cleaved
Table 5.2 Composition of fresh cow’s milk
Concentration g litre^1LIPIDS 37
of which % w/w
Triglycerides 95–96
Diglycerides 1.3–1.6
Free fatty acids 0.1–0.5
Total phospholipids 0.8–1.0
PROTEINS 34
Casein 26
aS1 11.1
aS2 1.7
b 8.2
g 1.2
k 3.7
Whey proteins
a-lactalbumin 0.7
b-lactoglobulin 3.0
serum albumin 0.3
immunoglobulins 0.6
NON-PROTEIN NITROGEN 1.9
LACTOSE 48
CITRIC ACID 1.75
ASH 7.0
CALCIUM 1.25
PHOSPHORUS 0.96
122 Microbiology of Primary Food Commodities