Food Lipids: Chemistry, Nutrition, and Biotechnology

mogenization determines the minimum droplet size that can be produced: the faster
the adsorption rate, the smaller the size. The magnitude and range of the repulsive
forces generated by a membrane, and its viscoelasticity, determine the stability of
the droplets to aggregation.
IV. BIOPOLYMERS
Proteins and polysaccharides are the two most important biopolymers used as functional
ingredients in food emulsions. These biopolymers are used principally for their
ability to stabilize emulsions, enhance viscosity, and form gels.
A. Molecular Characteristics
Molecular characteristics of biopolymers, such as molecular weight, conformation,
flexibility, and polarity, ultimately determine the properties of biopolymer solutions.
These characteristics are determined by the type, number, and sequence of monomers
that make up the polymer. Proteins are polymers of amino acids [19], whereas polysaccharides
are polymers of monosaccharides [20]. The three-dimensional structures
of biopolymers in aqueous solution can be categorized as globular, fibrous, or random
coil (Fig. 5). Globular biopolymers have fairly rigid compact structures; fibrous biopolymers
have fairly rigid, rodlike structures; and random coil biopolymers have
highly dynamic and flexible structures. Biopolymers can also be classified according
to the degree of branching of the chain. Most proteins have linear chains, whereas
polysaccharides can have either linear (e.g., amylose) or branched (e.g., amylopectin)
chains.
The conformation of a biopolymer in solution depends on the relative magnitude
of the various types of attractive and repulsive interaction that occur within and
between molecules, as well as the configurational entropy of the molecule. Biopolymers
that have substantial proportions of nonpolar groups tend to fold into globular
structures in which the nonpolar groups are located in the interior (away from the
water) and the polar groups are located at the exterior (in contact with the water)
because this arrangement minimizes the number of unfavorable contacts between
hydrophobic regions and water. However, since stereochemical constraints and the
influence of other types of molecular interaction usually make it impossible for all
the nonpolar groups to be located in the interior, the surfaces of globular biopolymers
have some hydrophobic character. Many kinds of food protein have compact globular
structures, including �-lactoglobulin, �-lactalbumin, and bovine serum albumin [6].
Biopolymers that contain a high proportion of polar monomers, distributed fairly
evenly along their backbone, often have rodlike conformations with substantial
amounts of helical structure stabilized by hydrogen bonding. Such biopolymers (e.g.,
collagen, cellulose) usually have low water solubilities because they tend to associate
Figure 5 Typical molecular conformations adopted by biopolymers in aqueous solution.
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