per g of starch, are much smaller than those for native starchð12–22 J?g^1 Þ.
The crystallinity obtained after gelatinization and cooling is, however,
reversible: it disappears at high temperature—which is applied to alleviate
the staling of bread—and reforms, albeit slowly, after cooling. The theory
for crystallization of concentrated polymer systems is roughly applicable.
Therate of retrogradationstrongly depends ontemperature: see Figure
6.27a. At a temperature that is further below the melting point, the
‘‘supersaturation’’ is greater. Although supersaturation is not a well-defined
concept in polymer systems, it is clear that the driving force for
crystallization is greater. For a starch/water ratio of unity, the melting
temperature is about 75 8 C. The staling of bread, which is primarily due to
retrogradation, will thus proceed faster at 5 8 C (refrigerator) than at 25 8 C
(ambient). In a freezer staling rate is very much slower; see Section 16.3 for
the explanation.
Retrogradation rate strongly depends onwater content(Figure 6.27b),
and the relation is similar to that with temperature. According to Eq. (6.14),
the ‘‘supersaturation’’ will be greater for a higher volume fraction of starch.
On the other hand, at very low water content the mobility of the polymer
chains will be very small, which will reduce the rate at which crystallites are
formed. For 20%water at room temperature the mobility is effectively zero
and no retrogradation occurs.
Among other variables isstarch type(Figure 6.27c); it may be noticed
that, especially during the first few hours after gelatinization, the
retrogradation rate may be very different. Gelatinization conditions and
FIGURE6.27 Retrogradation of gelatinized starch. Effects of temperature (a),
starch concentration (b), and starch type (c) on melting enthalpyðDHÞ. Starch type
and concentration, and storage temperature and time indicated. (Approximate
results from various sources.)