Chemistry Reference
In-Depth Information
Plasma = milk − fat (skim milk)
Serum = plasma − casein micelles (whey)
Solids-not-fat (SNF) = proteins, lactose, minerals, acids, enzymes, vitamins
Total milk solids = fat + SNF
It has been found that both the composition and physical structure are important in
determining the properties (colloidal or emulsion) of milk. As found in its role in
nature, milk is in a liquid form (at the body temperature). We may find this observa-
tion curious if we take into consideration the fact that milk has less water than most
fruits and vegetables. It is useful to analyze milk as follows:
An oil-in-water emulsion with the fat globules dispersed in the continuous
serum phase
A colloid suspension of casein micelles, globular proteins, and lipoprotein
particles
A solution of lactose, soluble proteins, minerals, vitamins, and other components
On image analyses of milk under a microscope at low magnification (5×), a uniform
but turbid liquid is observed. At 500× magnification, spherical droplets of fat, known
as fat globules, can be seen. At even higher magnification (50,000×), casein micelles
can be observed. The main structural components of milk—fat globules and casein
micelles—will be examined in more detail later. Milk fatty acids originate either
from microbial activity in the rumen and are transported to the secretory cells via
the blood and lymph, or from synthesis in the secretory cells. The main milk lipids
are a class called
triglycerides,
which are composed of a glycerol backbone binding
up to three different fatty acids. The fatty acids are composed of a hydrocarbon chain
and a carboxyl group. The major fatty acids found in milk are
Long chain
C
14
myristic 11%
C
16
palmitic 26%
C
18
stearic 10%
C
18:1
oleic 20%
Short chain (11%)
C4 butyric
*
C6 caproic
C8 caprylic
C10 capric
Saturated fatty acids (no double bonds), such as myristic, palmitic, and stearic, make
up two-thirds of milk fatty acids. Oleic acid is the most abundant unsaturated fatty
acid in milk, with one double bond. Triglycerides account for 98% of milk fat. The
small amounts of mono-, diglycerides, and free fatty acids in fresh milk may be
* Butyric fatty acid is specific for milk fat of ruminant animals and is responsible for the rancid flavor
when it is cleaved from glycerol by lipase action.
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