Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 203

a product of early lipolysis or simply incomplete synthesis. Other classes of lipids
include phospholipids (0.8%), which are mainly associated with the fat globule mem-
brane, and cholesterol (0.3%), which is mostly located in the fat globule core.
The physical properties of milk fat can be summarized as follows:
Density at 20°C is 915 kg/m(−3).
Refractive index (589 nm) is 1.462, which decreases with increasing temperature.
Solubility of water in fat is 0.14% (w/w) at 20°C, and increases with increas-
ing temperature.
Thermal conductivity is about 0.17 J m(−1) s(−1) K(−1) at 20°C.
Specific heat at 40°C is about 2.1 kJ kg −1 K −1 .
Electrical conductivity is <10(exp −12) ohm(exp −1) cm(exp −1).
Dielectric constant is about 3.1.
At room temperature, the lipids are solid, and therefore, are correctly referred to as
“fat” as opposed to “oil,” which is in liquid form at room temperature. The melt-
ing points of individual triglycerides ranges from −75°C for tributyric glycerol to
72°C for tristearin. However, the final melting point of milk fat is at 37°C because
higher-melting triglycerides dissolve in the liquid fat. This temperature is significant
because 37°C is the body temperature of the cow, and the milk would need to be
liquid at this temperature. The melting curves of milk fat are complicated by the
diverse lipid composition:
Trans unsaturation increases melting points.
Odd-numbered and branched chains decrease melting points.
Crystallization of milk fat largely determines the physical stability of the fat
globule and the consistency of high-fat dairy products, but crystal behavior
is also complicated by the wide range of different triglycerides.
9.5.3.2 milk Fat Structure—Fat Globules
More than 95% of the total milk lipid is in the form of a globule ranging in size from
0.1 to 15 μm in diameter. These liquid fat droplets are covered by a thin membrane, 8
to 10 nm in thickness, whose properties are completely different from those of both
milk fat and plasma. The native fat globule membrane (FGM) is composed of apical
plasma membrane of the secretory cell, which continually envelops the lipid droplets
as they pass into the lumen. The major components of the native FGM, therefore, are
protein and phospholipids. The phospholipids are involved in the oxidation of milk.
There may be some rearrangement of the membrane after release into the lumen, as
amphiphilic substances from the plasma adsorb onto the fat globule and parts of the
membrane dissolve into either the globule core or the serum. The FGM decreases
the lipid-serum interface to very low values, 1 to 2.5 mN/m, preventing the glob-
ules from immediate flocculation and coalescence, as well as protecting them from
enzymatic action. It is well known that, if raw milk or cream is left to stand, it will
separate. Stokes' Law predicts that fat globules will cream due to the differences