Physical Chemistry of Foods

6.3.1 Description

A polymer containing electrically charged groups is called a polyelectrolyte
or a macroion. Three types can be distinguished:

Polybasesor cationic polymers; these mostly contain 22 NH 2 or 55 NH

groups that can be protonated at sufficiently low pH (mostly 7–10).

They are unimportant in foods.

Polyacidsor anionic polymers; the most common charged groups are

carboxyl groupsð 22 COO^22 Þ, which are often present in glucuronic

acid or comparable residues, as in gum arabic, pectins, alginates,

and xanthan gum. Most carrageenans contain 22 O 22 SO^223 groups.

Polyampholytes, which contain both positively charged (basic) and

negatively charged (acidic) groups. Well-known are the proteins,

DNA, and RNA.

Thevalence, i.e., the number of charges per moleculeðzÞ, naturally
depends on the degree of polymerization ðnÞ and on the type of
polyelectrolyte. In some, all monomers have a charge, as in poly(acrylic
acid):ð 22 CHðCOO^22 Þ 22 CH 222 Þn. More often, only part of the monomers is
charged. Some polysaccharides, notably pectins, contain methylated
carboxyl groups 22 COOCH 3 ; often, part of these groups are or become
hydrolysed, leading to 22 COO^22 þCH 3 OH.

Ionization. The number of charges depends, of course, on the
degree of ionization, hence on pH. The sulfate groups mentioned are
virtually always ionized. Carboxyl groups, however, have apKavalue of
about 4.7, or about 3.4 for uronic acid groups (primary carboxyl group on a
sugar ring), implying that their ionization can vary greatly over the pH
range occurring in foods. At low pH, where the hydrogen ion activity is
high, they are fully protonated, having no charge; at high pH they are
ionized, having negative charge.
The situation is more complicated than for a simple monovalent acid.
It is useful to recall Eq. (2.24) given in Section 2.3.1 on dissociation of an
acid HAc:

pHpKa¼log½AcŠlog½HAcŠþlogg

wheregis the ion activity coefficient of Ac. Calling the fraction of the
molecules that is dissociateda, the equation can be rewritten as

log

1 a

a

¼pKapHþlogg ð 6 : 8 Þ