Food Lipids: Chemistry, Nutrition, and Biotechnology

strongly with each other rather than with water; consequently, they often have poor
functional properties. However, if the chains are branched, the molecules may be
prevented from getting close enough together to aggregate, and so they may exist in
solution as individual molecules. Predominantly polar biopolymers containing monomers
that are incompatible with helix formation (e.g., �-casein) tend to form random
coil structures.
In practice, biopolymers may have some regions along their backbone that have
one type of conformation and others that have a different conformation. Biopolymers
may also exist as isolated molecules or as aggregates in solution, depending on the
relative magnitude of the biopolymer–biopolymer, biopolymer–solvent, and solvent–solvent
interactions. Biopolymers are also capable of undergoing transitions
from one type of conformation to another in response to environmental changes such
as alterations in their pH, ionic strength, solvent composition, and temperature. Examples
include helix ⇔ random coil and globular ⇔ random coil. In many food
biopolymers, this type of transition plays an important role in determining the functional
properties (e.g., gelation).
B. Functional Properties
1. Emulsification
Biopolymers that have a high proportion of nonpolar groups tend to be surfaceactive,
i.e., they can accumulate at oil–water interfaces [1–4]. The major driving
force for adsorption is the hydrophobic effect. When the biopolymer is dispersed in
an aqueous phase, some of the nonpolar groups are in contact with water, which is
a thermodynamically unfavorable condition. By adsorbing to an interface, the biopolymer
can adopt a conformation of nonpolar groups in contact with the oil phase
(away from the water) and hydrophilic groups located in the aqueous phase (in
contact with the water). In addition, adsorption reduces the number of contacts between
the oil and water molecules at the interface, thereby reducing the interfacial
tension. The conformation a biopolymer adopts at an oil–water interface, and the
physicochemical properties of the membrane formed, depend on its molecular structure.
Flexible random coil biopolymers adopt an arrangement in which the predominantly
nonpolar segments protrude into the oil phase, the predominantly polar segments
protrude into the aqueous phase, and the neutral regions lie flat against the
interface (Fig. 6, left). The membranes formed by molecules of these types tend to
Figure 6 The conformation and unfolding of biopolymers at oil–water interfaces depends
on their molecular structure.
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