Amylopectin crystallinity

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. 

Increases in amylopectin crystallinity underlies part of the phenomenon known as bread staling (10). However, it has been recently shown that a rise in the glass transition temperature of the gluten, probably due to moisture migration, is more important (14-16). 

This structure for the granule is generally consistent with the x-ray diffraction data as yet available. However, it is difficult to envisage how amylopectin molecules possessing the three-dimensional structure suggested by Meyer (see p. 353) can align themselves to form any proportion of crystalline regions molecules with an essentially two-dimensional structure (as mentioned above) could do so far more readily. 

Although it has been found that the separated amylose component can be readily orientated to yield fiber patterns, amylopectin usually gives poor or amorphous patterns. In the granule, however, amylopectin does exhibit crystallinity, since waxy maize starch gives a diffraction pattern and other waxy starches behave similarly.193 -195 (This suggests that the branch points in the amylopectin molecule may be in the amorphous part of the granule.)... 

The Semicrystalline Structure of Amylopectin. Amylopectin crystallises according to a cluster molecule,25 as shown in Figure 8b. Within this structure there are crystalline and amorphous regions. An... 

Figure 8 The structure of amylopectin (a) organisation of the molecule (b) crystalline and amorphous regions (c) possible double helix structure...

Birefringence (or double refraction) is the decomposition of a light ray into two rays when it passes through certain types of crystalline material. This occurs only when the material is anisotropic, that is, the material has different characteristics in different directions. Amylose and amylopectin polymers are organized into a radially anisotropic, semicrystalline unit in the starch granule. This radial anisotropy is responsible for the distinctive... 

Maltese cross (Blanshard, 1979). The crystallinity of starch is caused essentially by amylopectin pol)Tner interactions (Banks and Greenwood, 1975 Biliaderis, 1998 Donald, 2004 Hizukuri, 1996). An illustration of currently accepted starch granule structure is given in Fig. 5.5. It is believed that the outer branches of amylopectin molecules interact to arrange themselves into "crystallites" forming crystalline lamellae within the granule (Fig. 5.5 Tester et al., 2004). A small number of amylose polymers may also interact with amylopectin crystallites. This hypothetical structure has been derived based on the cluster model of amylopectin (Hizukuri, 1986 Robin et ah, 1974 Fig. 5.1). 

Figure 4.3 The building block structure of potato amylopectin clusters. Branched building blocks (encircled) are mainly found inside amorphous lamellae (A) of semi-crystalline rings in starch granules. Double helices (symbolized as cylinders) extend from the building blocks into the crystalline lamellae (C). Enlargements of a double helix segment, in which the single strands are parallel and left-handed, and a building block are shown to the right.

Hizukuri, S. (1985). Relationship between the distribution of the chain length of amylopectin and the crystalline structure of starch granules. Carbohydr. Res., 141,295-306. 

When starch is heated in excess water, the crystalline structure is disrupted (due to breakage of hydrogen bonds) and water molecules become linked by hydrogen bonding to the exposed hydroxyl groups of amylose and amylopectin. This causes an increase in granule swelling... 

Potato starch exhibits different granular stmcture and composition, as opposed to cereal starches, which are responsible for the variation in functional behavior of these starches. Cereal starches exhibit the t5 ical A type X-ray crystalline pattern, whereas potato starch shows the B-form, andlegumesthe mixed state pattern C . The A, B, and C patterns are the different polymeric forms of starch that differ in the packing of amylopectin double helices. The structure of potato starch is discussed in more detail in Chapter 4. 

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