Starch granules, crystalline

The qua si-crystalline stmcture of natural starch granules causes them to be insoluble in water at normal room temperature and gives them relative resistance to carbohydrases other than a-amylase and glucoamylase unless the granules become swollen. Three-dimensional arrangements of crystalline and amorphous zones in starch granules have been suggested (2). 

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). 

Gelatinization, as we understand now, is not only associated with crystalline order, but is also influenced by structural changes in the amorphous region. XRD does not detect or account for the structural changes that occur in the amorphous regions of the starch granules. 

Starch crystallinity is caused by paraUely arranged starch polymers which act as crystals. No evidence has been presented in published literature to describe the actual physical nature of "starch crystallites." It should not be considered that starch granules contain "crystals" or "crystallite particles" formed by starch polymers. Starch crystallinity represents the relative arrangement of starch polymers in granules, not the presence of physical "crystals."... 

Nakazawa et al. (1984) argued that when starch-water mixtures (30-50% starch) are held at a certain temperature (55-80 °C), for a certain period (0-45 h), and depending on the time-temperature combination, starch granules increase their amorphous portion and decrease their crystalline portion. These amorphous and crystalline phases melted sequentially during DSC phase transition experiments. Their experiments... 

Cheetham, N. W.H. and Tao, L. (1998). Variation in crystalline type with amylose content in maize starch granules X-ray powder diffraction study. Carbohydrate Polymers. 36, 277-284. 

Blends of starch and a hydrophobic polymer make it possible to overcome the disadvantages described above. Starch compounds with Ecoflex are used to enhance hydrophobicity as well as the mechanical and thermal properties of compounded products. To obtain high quality film products, the starch has to be treated before being blended with Ecoflex . The crystalline structure of starch granules has to be destroyed because starch granules are as large as the film thickness of typical film applications and would therefore reduce the mechanical properties of the films. 

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.

Buleon, A., Pontoire, B., Riekel, C., Chanzy, H., Helbert, W., Vuong, R. (1997). Crystalline ultrastructure of starch granules revealed by synchrotron radiation microdiffraction mapping. Macromolecules, 30, 3952-3954. 

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. 

Oostergetel, G. T., Bruggen, E. F. J. v. (1993). The crystalline domains in potato starch granules are arranged in a helical fashion. Carbohydr. Polym., 21, 7-12. 

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