Dairy Chemistry And Biochemistry

104 DAIRY CHEMISTRY AND BIOCHEMISTRY

views on the structure of the MFGM and note that complete information on

the structure is still not available. Since the MFGM is a dynamic, unstable

structure, it is probably not possible to describe a structure which is

applicable in all situations and conditions.

3.9 Stability of the milk fat emulsion

The stability, or instability, of the milk fat emulsion is very significant with

respect to many physical and chemical characteristics of milk and dairy

products. The stability of the emulsion depends strongly on the integrity of

the MFGM and, as discussed in section 3.8.7, this membrane is quite fragile

and is more or less extensively changed during dairy processing operations.

In the following, some of the principal aspects and problems related to

or arising from the stability of the milk fat emulsion are discussed. Some of

these relate to the inherent instability of emulsions in general, others are

specifically related to the milk system.

3.9.1 Emulsion stability in general

Lipid emulsions are inherently unstable systems due to:

1. The difference in density between the lipid and aqueous phases (c. 0.9 and
   1.036 g cm-3, respectively, for milk), which causes the fat globules to float
   or cream according to Stokes’ equation:

where V is the rate of creaming; Y, the radius of fat globules; p1 and p2,

the densities of the continuous and dispersed phases, respectively; g,

acceleration due to gravity; and rl, viscosity of the system. If creaming is

not accompanied by other changes, it is readily reversible by gentle

agitation.

2. The interfacial tension between the oil and aqueous phases. Although
   interfacial tension is reduced by the use of an emulsifier, the interfacial
   film may be imperfect. When two globules collide, they may adhere
   (flocculate), e.g. by sharing emulsifier, or they may coalesce due to the
   Laplace principle which states that the pressure is greater inside small
   globules than inside large globules and hence there is a tendency for large
   fat globules (or gas bubbles) to grow at the expense of smaller ones.
   Taken to the extreme, this will lead to the formation of a continuous mass
   of fat.
   Destabilization processes in emulsions are summarized schematically in
   Figure 3.19. The rate of destabilization is influenced by the fat content, shear
   rate (motion), liquid: solid fat ratio, inclusion of air and globule size.

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