If several different monomers are involved, the molecules can be far
more complicated.
Branched polymers occur, and various modes of branching and
subbranching have been observed.
Polyelectrolytes contain monomers that are charged. They are
virtually always heteropolymers as well, since only part of the
monomers will have a charge. A polyelectrolyte may be a polyacid, a
polybase, or a polyampholyte (acid and basic groups).Natural polymers come in a far wider range of composition and
properties than synthetic ones. Natural polymers are for the most part of
three main types, all of which occur in foods.
Nucleic acids, i.e., DNA and RNA. These are linear heteropolyelec-
trolytes (four types of monomers) of very large degree of polymerization
(especially DNA). Their biological function is the transfer of information. In
foods, their concentrations mostly are too small to affect physicochemical
properties, and they will not be further discussed.
Proteins, i.e., linear heteropolyelectrolytes, built of amino acids, i.e., 20
different monomers of highly different configuration and reactivity. The
degree of polymerization typically is about 10^3. They may have any of
several biological functions.
Polysaccharides, i.e., linear or branched heteropolymers of sugars and
derived components; several are polyelectrolytes. The degree of polymeriza-
tion is mostly 10^3 to 10^4. The main biological functions are ‘‘nutritional’’
(primarily starch in plants, glycogen in animals) and ‘‘building material’’ (in
plants). The latter are calledstructural polysaccharides, which occur in a
great variety of types and mostly form mixed and highly complex structures,
especially in cell walls.
Several polysaccharides and proteins can be isolated to be applied in
manufactured foods; examples are given in Table 6.1. It should be realized
that such preparations may significantly vary in properties and also in
purity. Moreover, several natural polymers arechemically modifiedto obtain
altered (‘‘improved’’) properties. For instance, cellulose is insoluble in water,
but by carboxymethylation of part of the 22 CH 2 OH groups, the material
becomes well soluble (and charged, since free carboxyl groups are formed).
Cellulose can also be methylated, whereby it becomes soluble, despite the
presence of hydrophobic groups. Starch is modified in various ways, e.g., by
cross-linking.
Natural or modified polymers are often used asthickeningorgelling
agents, especially polysaccharides. Figure 6.1 gives examples of the effect of
polymer concentration on apparent viscosity. Clearly, polysaccharides can
be very effective thickeners. Polymers may be used to obtain a certain