Food Lipids: Chemistry, Nutrition, and Biotechnology

iopolymer solutions even have a characteristic yield stress. When a stress is applied
below the yield stress, the solution acts like an elastic solid, but when it exceeds the
yield stress the solution acts like a liquid. Shear thinning tends to occur because the
biopolymer molecules become aligned with the shear field, or because the weak
physical interactions responsible for biopolymer–biopolymer interactions are disrupted.
The characteristic rheological behavior of biopolymer solutions plays an important
role in determining their functional properties in food emulsions. For example,
a salad dressing must be able to flow when it is poured from a container, but
must maintain its shape under its own weight after it has been poured onto a salad.
The amount and type of biopolymer used must therefore be carefully selected to
provide a low viscosity when the salad dressing is poured (high applied stress), but
a high viscosity when the salad dressing is allowed to sit under its own weight (low
applied stress).
The viscosity of biopolymer solutions is also related to the mouthfeel of a food
product. Liquids that do not exhibit extensive shear-thinning behavior at the shear
stresses experienced in the mouth are perceived as being ‘‘slimy.’’ On the other hand,
a certain amount of viscosity is needed to contribute to the ‘‘creaminess’’ of a
product.
The shear-thinning behavior of biopolymer solutions is also important for determining
the stability of food emulsions to creaming [1a]. As oil droplets move
through an emulsion, they exert very small shear stresses on the surrounding liquid.
Consequently, they experience a very large viscosity, which greatly slows down the
rate at which they cream and therefore enhances stability. Many biopolymer solutions
also exhibit thixotropic behavior (i.e., their viscosity decreases with time when they
are sheared at a constant rate) as a result of disruption of the weak physical interactions
that cause biopolymer molecules to aggregate. A food manufacturer must
therefore select an appropriate biopolymer or combination of biopolymers to produce
a final product that has a desirable mouthfeel and texture.
3. Gelation
Biopolymers are used as functional ingredients in many food emulsions (e.g., yogurts,
cheeses, desserts, egg and meat products) because of their ability to cause the
aqueous phase to gel [1a]. Gel formation imparts desirable textural and sensory
attributes, as well as preventing the droplets from creaming. A biopolymer gel consists
of a three-dimensional network of aggregated or entangled biopolymers that
entraps a large volume of water, giving the whole structure some solid-like characteristics.
The appearance, texture, water-holding capacity, reversibility, and gelation
temperature of biopolymer gels depends on the type, structure, and interactions of
the molecules they contain.
Gels may be transparent or opaque, hard or soft, brittle or rubbery, homogeneous
or heterogeneous; they may exhibit syneresis or have good water-holding
capacity. Gelation may be induced by a variety of different methods, including altering
the temperature, pH, ionic strength, or solvent quality; adding enzymes; and
increasing the biopolymer concentration. Biopolymers may be cross-linked by covalent
and/or noncovalent bonds.
It is convenient to distinguish between two types of gel: particulate and filamentous
(Fig. 8). Particulate gels consist of biopolymer aggregates (particles or
clumps) that are assembled together to form a three-dimensional network. This type
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