Biomedical Engineering Reference
In-Depth Information
main stabilizing component.
29
Margarine, and possibly peanut butter, are other
examples where particle stabilization mechanisms are thought to operate, but in both
products the viscosity of the continuous matrix is so high that droplets are immobile
and cannot be destabilized by an external shear field.
EMULSIFIERS
Considerable confusion remains in the technological usage between the terms “emul-
sifier” and “stabilizer”. A useful distinction can be made in terms of stability. An
emulsifier must confer short-term stability, since this is essential for the preparation
of all emulsions. With some emulsions (e.g., cake batter or ice cream mix), a lifetime
of hours or even minutes is all that may be required. Other products (e.g., cream
liquors or mayonnaise) may need to remain stable for several years, and for these
a stabilizer is required. Long-term stability of oil-in-water emulsions may be
achieved by thickening the aqueous phase or adsorbing a film of polymer molecules
on the aqueous side of the oil-water interface. Most polysaccharides act as stabilizers
through their modification of the rheological properties of the aqueous dispersion
medium. Proteins, on the other hand, act primarily through the properties of their
interfacial films, and are therefore both emulsifiers and stabilizers in many instances.
Water-in-oil emulsions such as butter and margarine are mainly stabilized through
a network of fat crystals in the semi-solid continuous phase.
H
YDROPHILE
-L
IPOPHILE
B
ALANCE
(HLB)
OF
E
MULSIFIERS
Emulsifiers are designated as either hydrophilic or lipophilic according to their polar
and nonpolar moieties, which affect their solubility in water or oil. The polar
emulsifiers are water-soluble, and thus promote the formation of O/W emulsions.
Emulsifiers that are less polar tend to be soluble in oil and promote the formation
of W/O emulsions. In food emulsifiers, the lipophilic properties are generally the
most important ones, but the hydrophilic-lipophilic balance (HLB) can vary con-
siderably according to the chemical composition of the emulsifier. Griffin
30,31
intro-
duced the concept of HLB to measure the affinity of a nonionic emulsifier to oil or
water. He suggested the following equation for calculating HLB:
HLB
=
20 1
(
-
S
⁄
A
)
where S = saponification value of the ester
A = acid value of the fatty acid
In certain cases it is difficult to determine the saponification number accurately,
e.g., for esters of tall oil, lanolin, or rosin and for these the following equation based
on composition is used:
HLB
=
(
E
+
P
)
⁄
5
where E = weight percentage of oxyethylene
P = weight percentage of the polyol
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