Chemistry Reference
In-Depth Information
1000
100
10
1
0.1
0.01
0
1
2
Time (hours)
3
4
Fig. 6.6
Time dependence of G
(filled symbols) and G
(open symbols) for the formation
of alginic acid gels made from 1% high G alginate (Squares) and 1% intermediate G
alginate (Triangles) of similar molecular weights. (Data from Draget et al., 2003).
which may cause instability in the system or, conversely, have a
favourable interaction. With respect to processing, alginate is an at-
tractive gelling agent to use. The gel-forming mechanism is generally
considered as independent of temperature, which is unlike most other
polysaccharides, and once formed, the gels cannot be melted by heat-
ing. The gelation temperature, however, and indeed the pH and ionic
strength used during gelation do affect the mechanical properties.
6.3
ALGINIC ACID
Alginic acid is the water-insoluble protonated form of alginate (Draget
et al.
, 1994). The pKa of guluronic acid and mannuronic acid in 0.1
M NaCl are known to be 3.65 and 3.38, respectively (Haug, 1961). A
sudden decrease in pH of an alginate solution to below the pKa of the
monomers causes the precipitation of alginic acid (Haug and Larsen,
1963). It is also well known that if the reduction in pH is performed
in a controlled manner, it is possible to form alginic acid gels (Draget
et al.
, 1994, 1996, 2003). An excellent example of a product that utilises
these properties is the anti-heartburn medication Gaviscon
R
. This orally
administered liquid contains sodium alginate as the active ingredient.
When the medicine reaches the stomach, the acidic gastric fluid causes
the alginate solution to form an alginic acid gel raft on the surface,
subsequently preventing gastric reflux. This
in situ
gelation has also
been explored in food systems as an appetite suppressor, which will be
discussed later in this chapter.
The rheological properties of alginic acid, similar to alginate salts,
vary with molecular weight and G:M ratio. This is illustrated in Fig. 6.6,
