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amylose content to retrogradation [91,145,160]. For DSC measurements, inparticular, the extent of retrogradation should be greater for waxy starchesbecause the DSC measures the recrystallization of amylopectin [91,142,160].It is thus safe to say that the relation between the amylose/amylopectin ratioand retrogradation is not straightforward, as other factors are involved.If retrogradation is to take place, the chains must first aggregate, but aminimum requirement for the aggregation of chains to occur has been shownto be 8 to 10 glucose units [47,176]. The short chains of amylopectin have beenshown to be responsible for crystallization in the amylopectin molecule [177],and chains with less than 15 glucose units do not take part in the crystallization[158,177]. If the external chains of the amylopectin molecule are removed byβ-amylolysis, no retrogradation at all occurs [178a,b]. Amylopectin frompotato, tapioca, and kuzu starches, which are B-starches, retrograde to a different extent, which has been related to differences in average chain length[178]. Amylopectin from cereals has also been shown to retrograde to a lesserextent than pea, potato, and canna amylopectin, which has been attributed toshorter average chain length in the cereal amylopectin [167,173]. The lengthof the external chains in the amylopectin molecule is also of relevance [178a,b].The transition temperature, Tc, at which the melting of the recrystallizedstarch occurs is nearly the same for all the cereal starches (with the exceptionof amylomaize), despite differences in gelatinization temperatures of up to24°C [171]. The gelatinized starches are fully hydrated, and the recrystallized(retrograded) starch melts according to its melting temperature at the existingwater content. The similar Tcvalues of cereal starches indicate that theircrystallites have similar stability and are therefore of similar short chainlengths. A higher melting temperature Tchas been reported for B-starches withlonger short chains [173]. The structural differences in cereal amylopectinsrelated to retrogradation can be related to differences in the amorphous regionsor differences in the ratio of short chains to long chains and the ratio of Achains to B chains. A greater amount of short chains over 15 glucose unitsand an increased ratio of A chains to B chains probably promote retrogradation.It has also been reported that very short chains (6 to 9 glucose units) caninhibit or retard retrogradation of starch gels [179,180].10.5 RHEOLOGICAL BEHAVIOR OF THE STARCH GELThe events occurring during gelatinization of a starch suspension, especiallythe swelling and leaking of amylose/amylopectin, will dramatically change therheological properties of the starch suspension. The subsequent retrogradationwill then further modify the rheological properties. Perhaps the most commonmethod for studying starch properties is to study the changes in viscosity duringa programmed heating–cooking–cooling cycle. The measurements are carried
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