gave G′values similar to a normal m

gave G′values similar to a normal maize starch, whereas the unmodified waxy
maize starch gave very low values [201].
10.6.3.3 Cold Storage and Freeze–Thaw Stability
A critical test for the modified starches is the improvement of freeze–thaw
stability. Also, the behavior during cold storage is of importance. It has been
observed that storage of a starch gel for 7 days at 4°C results in more syneresis
than one freeze–thaw cycle [205]. That the starch gel is not stable enough for
cold storage or freezing can be observed by the liquid exudates (syneresis),
by changes in the rheological properties of the gel, and by the presence of a
DSC endotherm due to retrogradation of amylopectin [295,299]. The degree
of syneresis has been observed to decrease with increasing degree of substitution for several modified starches (e.g., hydroxypropyl potato starch, acetylated smooth pea starch, acetylated distarch phosphate smooth pea starch gels)
[205,207]. In relation to the syneresis, it is of interest to notice that the waterbinding capacity (measured as unfreezable water using DSC) decreases with
most modifications, the only exception being pregelatinized starch [300]. Very
small, or no, endotherms were observed after storage for 7 days at 4°C for
hydroxypropyl distarch phosphates [295]. Similar DSC results were obtained
after 10 freeze–thaw cycles. Also, for hydroxypropyl potato starch, the DSC
endotherm due to retrogradation was found to decrease with increasing MS
[208].
10.6.3.4 Interactions with Other Components
Because the starch molecules (especially amylose) are modified due to the
chemical modification, it is to be expected that molecular interactions such as
the formation of the amylose–lipid complex change with modifications. When
the interactions between monomyristin and hydroxypropyl potato starch were
investigated, it was found that the DSC endotherm due to the amylose–lipid
complex could be observed only at the lowest MS value (0.045). At higher MS
values (0.125 or 0.170), no endotherm was observed [301]. For acetylated highamylose maize starch, it was found that interaction with the surfactant (CTAB)
gave lower transition temperature and enthalpy values than the modified highamylose maize starch [201].
10.7 CONCLUSIONS
During the last decade or so, important achievements have been made in the
starch research area. These include the arrival of new techniques such as the
DSC 13
C-CP/MAS NMR, and an increasing use of x-ray diffraction techniques
and fundamental rheological methods. Another important development is the
polymeric approach to interpreting starch behavior which encompasses the
© 2006 by Taylor & Francis Group, LLC
Starch: Physicochemical and Functional Aspects 451
glass transition concept and the description of the starch gel as a composite
material. For the future, several challenges still must be met. The structure of
the starch granule is still not known in detail. Interactions between starch and
other components, especially macromolecules, have only just begun to be
explored. Perhaps the biggest challenge is to relate our knowledge about the
physiocochemical properties of starch with our chemical knowledge about
amylose and amylopectin in order to create tailor-made starches with the
desired properties, both functional and nutritional.
REFERENCES