The starch pastes are unstable at room temperature. Amylose and amylopectin are two incompatible polymers in solution, a phase separation occurs during cooling to a temperature below 90°C (Kalichevsky and Ring, 1987). In the absence of complexing agent, and when the concentration is sufficient (1.5%-2%), amylose and amylopectin reorganize separately, forming an opaque white gel. This is a mixed gel consisting of a matrix of amylose enclosing ghost grains rich in amylopectin (Miles et al., 1985). However, the nature of the dispersed and dispersing phases in models gels is based on the amylose-to-amylopectin ratio (Leloup, 1989), when this ratio is less than 0.43, amylopectin constitutes the continuous phase and amylose the dispersed phase (Leloup et al., 1991). At the time of the establishment of three-dimensional network of the gel, the amylose and amylopectin chains can be reorganized in the helix form (partial recrystallization in type B). It is retrogradation. The retrogradation speed is even faster when the final temperature is low and the difference between the heating temperature and the temperature after cooling is important. Pregelatinized starches or the high-amylose starches retrograde faster than other starches (Fechner et al., 2005).
Amylose Gelation
Gelation of amylose is a quick process (1-2 h), requiring a concentration above 1.5% (m/m). The gels formed are very stable with melting temperature near 120°C (Miles et al., 1985; Doublier and Choplin, 1989) and have usually a type B structure (such as amylopectin gels).
Amylopectin Gelation
Gelation of amylopectin is a much slower process that requires high polymer
concentrations of about 10%. The amylopectin gels are unstable and have a
melting temperature close to 45°C (Miles et al., 1985). This stability is low
because of the low DP (15 maximum) of short chains of amylopectin, determined by chromatography of hydrolysis residues (Ring et al., 1987). The
retrogradation would begin by intramolecular interactions between the short
chains of amylopectin, thereby forming double helices, which then stack to
form the B-type crystals (model Putaux et al., 2000).