reduced, and at starch concentrations of 2–4%(depending on starch type),
the chain overlap concentration for amylose will be reached. Above this
concentration, cooling will lead to the formation of an amylose gel,
enclosing swollen granules.
Gelatinized starch systems thus show retrogradation, as can be
measured by calorimetry. However, the change inDHproceeds for a far
longer time than in an amylose gel—compare Figures 6.26 and 6.27—
suggesting that also amylopectine exhibits retrogradation. The latter is
corroborated by studying amylose-free systems. Starch crystallites are
observed, albeit in far smaller proportion than in pure amylose. The
crystallites are about 10 nm in size, and their structure is to some extent
comparable to the B type crystallites in native starch.
Reversibility. Nevertheless, the supermolecular structure present in
native starch granules, like the one depicted in Figure 6.23, does not
reappear upon cooling after the starch has been gelatinized. This is borne
out by the observation that the melting enthalpies involved, about 2 to 10 J
FIGURE6.26 Retrogradation of a 7%amylose solution. The solution was cooled
from 90 to 26 8 C and then kept for the time indicated. Stiffness (as shear modulus)
and melting enthalpy (DH) were monitored; arbitrary scales. (After results by M. J.
Miles et al.Carbohydr. Res.135 (1985) 271.