Only a limited number are used in foods. These include polar lipids,
especially monoacylglycerides and phospholipids.
Macromolecules, generally linear polymers of molar mass>10 kDa.
The chemical composition varies widely. They are discussed in
Section 10.3.2. Proteins make up by far the most important category
used in foods.
These two types differ widely in most properties. Moreover, they can
interact with each other in various ways; this is discussed in Section 10.3.3.
10.3.1 Small-Molecule Surfactants
These surfactants generally consist of an aliphatic chain (‘‘tail’’) to which a
polar ‘‘head’’ group is attached. The aliphatic part would readily dissolve in
oil (if separated from the head group); the head group would readily dissolve
in water. This is why these substances preferentially go to an O/W interface:
the total free energy then is smallest. They also adsorb onto an A/W
interface, because then the aliphatic chain is not surrounded by water
molecules, which also causes a decrease in free energy, though less than for
adsorption on an O/W interface. According to the nature of the head group,
the surfactants are classified as nonionic (neutral), anionic (negatively
charged head group in water, unless the pH is quite low) andcationic
(positively charged). Examples are given in Table 10.2. Phospholipids are
special: they have two aliphatic chains, and the head group of most types is
zwitterionic, which means that it contains a positive as well as a negative
charge.
An important characteristic of small-molecule water-soluble surfac-
tants is their tendency to formassociation colloidsin water. Some examples
are given in Figure 10.9. Phospholipids readily formbilayers, the basic
structure of all cell membranes. Avesicleis a closed bilayer. The primary
driving force for association is the hydrophobic effect (see Section 3.2).
Close packing of the hydrophobic tails greatly diminishes their contact with
water. This lowers free energy, despite the resultant decrease in mixing
entropy. The presence of the polar heads counteracts the association, since
they repel each other by electrostatic repulsion or by hydration.
Consequently, high surfactant concentrations are needed for most types of
association colloids to form, except for micelles and some vesicles. A
bewildering variety ofmesomorphic phases can be formed, according to
chemical structure of the surfactant, temperature, water content, and other
variables. One fairly common type, alamellar phase, is depicted in Figure
10.9b; it is typically formed at a water content of, say, 50%.