Food Lipids: Chemistry, Nutrition, and Biotechnology

have relatively open structures, to be relatively thick, and to have low viscoelasticities.
Globular biopolymers (usually proteins) adsorb to an interface so that the predominantly
nonpolar regions on their surface face the oil phase; thus they tend to
have a definite orientation at an interface (Fig. 6, right). Once they have adsorbed
to an interface, biopolymers often undergo structural rearrangements that permit them
to maximize the number of contacts between nonpolar groups and oil [4].
Random coil biopolymers have flexible conformations and therefore rearrange
their structures rapidly, whereas globular biopolymers are more rigid and therefore
unfold more slowly. The unfolding of a globular protein at an interface often exposes
amino acids that were originally located in the hydrophobic interior of the molecule,
which can lead to enhanced interactions with neighboring protein molecules through
hydrophobic attraction or disulfide bond formation. Consequently, globular proteins
tend to form relatively thin and compact membranes, high in viscoelasticity. Thus,
membranes formed from globular proteins tend to be more resistant to rupture than
those formed from random coil proteins [3].
To be effective emulsifiers, biopolymers must rapidly adsorb to the surface of
the emulsion droplets formed during homogenization and provide a membrane that
prevents the droplets from aggregating. Biopolymer membranes can stabilize emulsion
droplets against aggregation by a number of different physical mechanisms [1].
All biopolymers are capable of providing short-range steric repulsive forces that are
usually strong enough to prevent droplets from getting sufficiently close together to
coalesce. If the membrane is sufficiently thick, it can also prevent droplets from
flocculating. Otherwise, it must be electrically charged so that it can prevent flocculation
by electrostatic repulsion. The properties of emulsions stabilized by charged
biopolymers are particularly sensitive to the pH and ionic strength of aqueous solutions
[1a]. At pH values near the isoelectric point of proteins, or at high ionic
strengths, the electrostatic repulsion between droplets may not be large enought to
prevent the droplets from aggregating (see Sec. VI.A.5).
Proteins are commonly used as emulsifiers in foods because many of them
naturally have a high proportion of nonpolar groups. Most polysaccharides are so
hydrophilic that they are not surface-active. However, a small number of naturally
occurring polysaccharides have some hydrophobic character (e.g., gum arabic) or
have been chemically modified to introduce nonpolar groups (e.g., some hydrophobically
modified starches), and these biopolymers can be used as emulsifiers.
2. Thickening and Stabilization
The second major role of biopolymers in food emulsions is to increase the viscosity
of the aqueous phase [1a]. This modifies the texture and mouthfeel of the food
product (‘‘thickening’’), as well as reducing the rate at which particles sediment or
cream (‘‘stabilization’’). Both proteins and polysaccharides can be used as thickening
agents, but polysaccharides are usually preferred because they can be used at much
lower concentrations. The biopolymers used to increase the viscosity of aqueous
solutions are usually highly hydrated and extended molecules or molecular aggregates.
Their ability to increase the viscosity depends principally on their molecular
weight, degree of branching, conformation, and flexibility. The viscosity of a dilute
solution of particles increases as the concentration of particles increases [3]:
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