Crystallization of amylopectin

Both amylose and amylopectin take part in the retrogradation process and network structure formation and up to four different steps of ordering have been postulated [30]. The first one, which takes place in a few hours, has been ascribed to gelation of the solubilized amylose in the intergranular space forming double helices structures. The second and the third one, for which days are needed, have been ascribed to chain reordering and crystallization of amylopectin. 

Using results of these kinds of studies, the characteristic structure of amylose can be differentiated from that of amylopectin. Amylose has a small number of branches and crystallizes and precipitates when complexed with 1 -butanol. The iodine affinity of amylose is much greater (i.a. 18.5 to 21.1) than that of amylopectin (i.a. 0.0 to 6.6),79,152-158,163,169-174 and the iodine affinity of amylose (3-limit dextrin is similar to that of the parent amylose. The average chain length of amylose (3-limit dextrins is much larger than that of the amylopectin (3-limit dextrin.160... 

Amyloses of five commercially important starches (wheat, corn, rice, tapioca and potato) were purified by fractional crystallization of their amylose-alcohol complexes and shown to be free of amylopectin.111,205 All five amyloses showed comparable iodine affinity, blue value and max of their polyiodide complexes (Table 10.5), which is consistent with their being (1 —4)-linked a-glucans with an average DPn of —500 or above.206 The average size of amylose molecules in cereal starches is smaller than... 

It should be noted that the proamylopectin in this still hypothetical pathway would be larger than the phytoglycogen found in the mutants lacking debranching activity. This is because proamylopectin would have a size comparable to amylopectin, while phytoglycogen, much smaller, may be the product of degradation of a proamylopectin unable to crystallize into amylopectin and may be so unprotected that it would be subject to the action of amylases. 

As discussed above, linear alkyl chains such as the fatty acid moiety of monoglycerides can form a complex with helical segments of the solubilized starch molecules. The recrystallization of amylose is apparently little affected by complexation with mono glycerides. Differential scanning calorimetry (DSC) studies of day-old bread crumb show that all the amylose seems to be present in a crystalline state. Complex formation with monoglyceride by the short side chains of amylopectin, however, markedly slow the rate of retrogradation. The half-life for reformation of amylopectin crystals (measured by DSC) is increased twofold to threefold when... 

From their property of crystallizing spontaneously, it may be concluded that the amylose complexes possess a highly ordered structure. The outer branches of the complexed form of amylopectin molecules are, however, bound to have the same ordered structure therefore, these structures are isomorphous with the amylose complex, and hence will take part in the crystallization reactions of the latter. As, however, only the complexed outer branches of the amylopectin molecules will fit the requirements of the lattice of the amylose complex, they evidently will act as so many dislocations... 

From Fig. 3, it may be seen that lowering of the temperature to 60-70 will cause separation of amylopectin. In general, this phase separation takes the same route as that for the amylose (except for the peculiar, morphological phenomena of the latter). As crystallization is much slower for the branched fraction of starch, the critical temperature of phase separation is sufficiently high to permit the existence of a coherent, liquid phase for short periods of time. The fact that freshly obtained amylopectin precipitate is soluble in cold water, whereas, after several hours, it is completely insoluble in cold water can only be interpreted as being the result of crystallization. In accordance with this conclusion, it is to be noted that this phenomenon is perfectly reversible. 

Interpretation of the WAXS patterns of native starch is often difficult because of the low crystallinity, small size, defects and the multiple orientations of the amylopectin crystallites (Waigh et al, 1997). Two main types of X-ray scattering patterns have been commonly observed (A and B). Potato starch has been shown to crystallize in a hexagonal unit cell in which the amylopectin molecules twist in a double helix (the B structure) (Lin Jana Shen, 1993). Between adjacent helices a channel is formed in which 36 water molecules can be located within the crystal unit cell. By means of heat treatment this structure can be transformed into a more compact monoclinic unit cell (the A structure) (Shogren, 1992). Amylose (the linear and minor component of starch) can be crystallized from solution in the A and B structures (Buledn etal, 1984), yielding X-ray diffraction patterns similar to those of amylopectin but with higher orientation. 

Compression moulding of the starch granules leads to sintered relatively brittle materials. Here the amylopectin crystals of the native powder largely remain... 

Schardinger " himself started a trend in giving the early names crystallized amylose to a-dextrin and crystallized amylodextrin to /3-dextrin. Pringsheim reported relatively higher yields of /3-dextrin from glycogen and crude preparations of amylopectin, and postulated that amylose is polymerized a-diamylose amylopectin and glycogen are polymerized /3-triamylose. 

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