Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 204

in densities between the fat and plasma phases of milk. However, in cold raw milk,
creaming takes place faster than is predicted from this fact alone.
IgM, an immunoglobulin in milk, forms a complex with lipoproteins. This com-
plex, known as cryoglobulin, precipitates onto the fat globules and causes floccula-
tion. The process is known as cold agglutination. As fat globules cluster, the speed
of rising increases and sweeps up the smaller globules with them. The cream layer
forms very rapidly, within 20 to 30 min, in cold milk.
Homogenization of milk prevents this creaming by decreasing the diameter and
size distribution of the fat globules, causing the speed of rise to be similar for the
majority of the globules. Also, homogenization causes the formation of a recombined
membrane that is much similar in density to the continuous phase. Recombined
membranes are very different from native FGM. Processing steps such as homogeni-
zation decreases the average diameter of the fat globule and significantly increases
the surface area. Some of the native FGM will remain adsorbed, but there is no lon-
ger enough of it to cover all of the newly created surface area. Immediately after dis-
ruption of the fat globule, the surface tension raises to a high level of 15 mN/m, and
amphiphilic molecules in the plasma quickly adsorb to the lipid droplet to lower this
value. The adsorbed layers consist mainly of serum proteins and casein micelles.
While homogenization is the principal method for achieving stabilization of the
fat emulsion in milk, fat destabilization is necessary for structure formation in but-
ter, whipped cream, and ice cream. Fat destabilization refers to the process of clus-
tering and clumping (partial coalescence) of the fat globules, which leads to the
development of a continuous internal fat network or matrix structure in the product.
Fat destabilization (sometimes “fat agglomeration”) is a general term that describes
the summation of several different phenomena. These include the following: coales-
cence is an irreversible increase in the size of fat globules and a loss of identity
of the coalescing globules. Flocculation is a reversible (with minor energy input)
agglomeration/clustering of fat globules with no loss of identity of the globules in the
flocculation; the fat globules that flocculate can be easily redispersed if they are held
together by weak forces, or they might be harder to redisperse if they share part of
their interfacial layers. Partial coalescence is an irreversible agglomeration/cluster-
ing of fat globules held together by a combination of fat crystals and liquid fat, and
a retention of identity of individual globules as long as the crystal structure is main-
tained (i.e., temperature dependent; once the crystals melt, the cluster coalesces).
They usually come together in a shear field, as in whipping, and it is envisioned
that the crystals at the surface of the droplets are responsible for causing colliding
globules to stick together while the liquid fat partially flows between them and acts
as the “cement.” Partial coalescence dominates structure formation in whipped, aer-
ated dairy emulsions, and it should be emphasized that crystals within the emulsion
droplets are responsible for its occurrence.
9.5.3.3 milk lipids—Functional Properties
Like all fats, milk fat provides lubrication. They impart a creamy-mouth feel as
opposed to a dry texture. Fat globules produce a “shortening” effect in cheese by
keeping the protein matrix extended to give a soft texture. Milk proteins are one
of the most important constituents. The primary structure of proteins consists of