Recrystallization amylopectin

Another laboratory invoked the role of water in bread staling involving a zipper mechanism that results in the formation of interchain crosslinks 492 Moisture redistribution from crumb to crust played a significant role in crumb firming and amylopectin recrystallization for breads stored for more than seven days 493 According to Piazza and Masi494 to inhibit staling it is more important to slow the dehydration phenomena than it is to increase the initial moisture content in the bread. 

Amorphous amylopectin Recrystallized amylopectin Free polar lipid... 

Besides plasticizer features, properties of the starch itself such as the amylose/ amylopectin ratio, also have an influence on the final material. As mentioned above, amylose and amylopectin recrystallize in different ways. Amylose will form the more crystaUine part of a thermoplastic and amylopectin the amorphous component the opposite behavior is found in the native starch granule, in which amylopectin is predominantly the crystalHne part. The source of the starch determines the ratio and the molar mass distribution of amylose and amylopectin, and consequently the properties of the thermoplastics made out of it [7]. In this chapter the effects of and relations between the plasticizer, the amylose/amylopectin ratio, and the mechanical properties of four starches from different sources-potato, pea, wheat, and waxy maize-are described and discussed. Besides an overview of the properties, this should finally lead to better insight into the behavior of starch materials during processing. 

The most widely used complexing agents are alcohols (butanol, n-propyl alcohol and n-pentyl alcohol1). Schoch33 now recommends the use of Pen-tasol, a commercial mixture of pentyl alcohols, for the first precipitation, and 1-butanol for recrystallizations. For com (maize) starch, this avoids contamination of the amylopectin with an intermediate fraction which is sufficiently linear to be precipitated with Pentasol and yet has a degree of branching which prevents complex formation with butanol. 

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... 

Starch is a polysaccharide formed by two polymers of glucose linear amylose and branched amylopectin. The gelatinization process can be regarded as a fusion of fhe crystalline starch regions in the presence of enough wafer and heafing. Starch retrogradation is a recrystallization process that is controlled by diffusion and depends on solute mobility in the system. 

Recrystallization of amylopectin is a reversible process that can be analyzed by DSC below 100°C (Miles et al., 1985). Amylopectin retrogradation was measured in starch systems containing sucrose from DSC runs immediately after freezing and after 91 days of storage at —80 and — 18°C. Aliquots of gelatinized pastes were run between 20 and 120°C at a heating rate of 10°C min in order to detect the presence of the amylopectin retrogradation peak. 

Waxy corn (WCN30G30W) materials are amorphous after moulding at a temperature above 120°C. Potato starch materials (PN30G30W) show B-type crystallinity. The relative crystallinity (compared to native potato starch) decreases from 68% (at 110°C) to 36% (at 140°C). Above 140°C the relative crystallinity increases to a value of 70% at 190°C. These values are in agreement with values previously reported.14 It has been shown that the total B-type crystallinity of potato starch plasticized with glycerol and water can be considered as a summation of residual amylopectin crystallinity and recrystallization of both amylose and amylopectin depending on processing conditions. 

One practical effect of starch retrogradation is on bakery products recrystallization of amylopectin has a great role in staling of baked products during storage. As amylopectin... 

On cooling back to room temperature, the whole material is transformed from a viscoelastic dough to a soft solid as the amylopectin begins to recrystallize. Of course, the yeast-caused foam bubbles play a critical role in the final texture. 

The reason for the nanocomposites increased thermal properties is associated to the relocation of the plasticizer(s), water inclusive, from the starch matrix to the cellulose nanocrystals surfaces, which decreases the plasticization effect on the amorphous regions (Angles and Dufresne 2000, 2001). They proposed the formation of the transcrystaUine zone, around the cellulose nanofillers in order to explain the results obtained in plasticized waxy maize starch reinforced with tunicin nanowhiskers. This effect is caused by the recrystallization of amylopectin chains assisted by the plasticizer accumulation and the nucleating effect of C-NW. 

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