Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 308

the single-component system. From the large-deformation results, stress strain curves

for all of the gels studied were quite similar, and the constancy of, for example, failure
strain was taken as support for their molecular IPN description.

-

10.4.4.4 Starch: a mixture of amylose and amylopectin

Starch lies behind only cellulose and chitin as the most common occurring polysacchar-
ide, and whole volumes have been written about its properties and structure. Within the
scope of this chapter, we will consider it only in terms of its properties as a mixed gel of
its two components, the linear polyglucan amylose and the highly branched amylopectin.
The plant source of the starch governs the relative amounts of

amylose and amylopectin and the amount of crystallinity in the starch granule. When
such a mixture or starch dispersion is heated at a given temperature, amylose is leached
out of the granules. The viscosity begins to increase steeply, a phenomenon called
gelatinization, and the result is that the partially crystalline portion is destroyed. When
the gelatinized starch is kept at low temperature, the crystalline structure is partially
recovered, a process called retrogradation.
It is well established that retrogradation is triggered by gelation of the amylose compo-
nent (Miles et al., 1985 ). Kalichevsky and Ring ( 1987 ) obtained a phase diagram for pea
amylose (extracted, or leached, at 70°C) and waxy-maize amylopectin leached at both 70°
C and 90°C ( Figure 10.11 ), and the binodal was drawn through the experimental points. As
the same binodal could be drawn through all of these, within experimental error, the results
show no observable leaching temperature effect on the phase diagram. The concentration
for the onset of coil overlap, obtained by viscometry, was 0.9% for amylopectin and 1.2%
for pea amylose. From this they concluded that phase separation occurs well within the
entangled regime of both polymers. However, it was not possible to investigate a broader
temperature range because of the high gelling temperature of amylose.

- e.g. pea, rice or maize -

Water

10% 5 10%

Amylose Amylopectin

Figure 10.11 Phase diagram for the amylose - amylopectin - water system (using pea amylose leached at 70°C),
obtained at 70°C ( ○ ) and at 90°C ( Δ ). Reprinted with permission from Kalichevsky and Ring
( 1987 ) © 1987 Elsevier.