Food Lipids: Chemistry, Nutrition, and Biotechnology

� = � (1 � 2.5�) (3)
0
where � is the viscosity of the solution, � 0 is the viscosity of the pure solvent, and
� is the volume fraction of particles in solution.
Biopolymers are able to enhance the viscosity of aqueous solutions at low
concentrations because they have an effective volume fraction that is much greater
than their actual volume fraction [1a]. A biopolymer rapidly rotates in solution because
of its thermal energy, and so it sweeps out a spherical volume of water that
has a diameter approximately equal to the end-to-end length of the molecule (Fig.
7). The volume of the biopolymer molecule is only a small fraction of the total
volume of the sphere swept out, and so the effective volume fraction of a biopolymer
is much greater than its actual volume fraction. Consequently, small concentrations
of biopolymer can dramatically increase the viscosity of a solution [Eq. (3)]. The
effectiveness of a biopolymer at increasing the viscosity increases as the volume
fraction it occupies within the sphere it sweeps out decreases. Thus large, highly
extended linear biopolymers increase the viscosity more effectively than small compact
or branched biopolymers.
In a dilute biopolymer solution the individual molecules (or aggregates) do not
interact with each other. When the concentration of biopolymer increases above some
critical value c*, the viscosity increases rapidly because the spheres swept out by
the biopolymers overlap with each another. This type of solution is known as a
semidilute solution, because even though the molecules are interacting with one
another, each individual biopolymer is still largely surrounded by solvent molecules.
At still higher polymer concentrations, the molecules pack so close together that they
become entangled, and the system has more gel-like characteristics. Biopolymers
that are used to thicken the aqueous phase of emulsions are often used in the semidilute
concentration range [3].
Solutions containing extended biopolymers often exhibit strong shear-thinning
behavior; that is, their apparent viscosity decreases with increasing shear stress. Some
Figure 7 Extended biopolymers in aqueous solutions sweep out a large volume of water
as they rotate, which increases their effective volume fraction and therefore their viscosity.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.