Crystalline starch

Figure 5.11 Representation ofthe double helix of crystalline starch after modeling a branching point between two strands. Schematic cluster model of amylopectin molecule incorporating the double helical fragments. (Reproduced with permission from reference 45)...

What roles do the starch synthase isoforms play in the formation of the crystalline starch granule and amylopectin structures How is amylose formed Why are starch granules from different species different in size and in the number per cell New methodology and much effort have resulted in major advances in the understanding of starch biosynthesis, but many questions remain unanswered. Here we discuss some of these open questions and possible answers. 

In nature, starch is based on crystalline beads of about 15-100 microns in diameter. Crystalline starch beads in plastics can be used as fillers or can be transformed into thermoplastic starch, which can either be processed alone or in combination with specific synthetic polymers. To make starch thermoplastic, its crystalline structure has to be destroyed by pressure, heat, mechanical work or use of plasticisers. Three main families of starch polymer can be used pure starch, modified starch and fermented starch polymers. 

Fig. 1.—Representation of double-helix packing and unit cells in A and B crystalline starches (a and b, respectively). Dashed lines represent hydrogen bonds. Water molecules have been omitted. (Reprinted with permission from A. Imberty, A. Buleon, Vinh Tran, and S. Perez, Staerke, 43 (1991) 375-384.)...

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. 

Amylopectin is responsible for the crystalline character of the starch granule and its structure can be modelled as a hyperbranched molecule [10,11, 28,29]. The model for the amylopectin molecule proposed by Robin [14] is illustrated in Figure 4.1(c). The A-chains, which are short segments of 15 D-glucopyranosyl residues, are the portion responsible for the crystalline structure of amylopectin. Starch crystallites are thus formed by compact areas made up of A-chains with DP 15. The crystallinity index of regular corn starch with 73% amylose is equivalent to that of waxy maize starch which is 100% amylose this confirms that amylose content has little effect on granule crystallinity. Starch crystallites formed in compact areas are made up of vicinal A-chains in a compact double helix conformation with two extended helices possessing 6 residues per turn that repeat every 21 A [1]. 

Rundle RE, French D. Configuration of starch and the starch-iodine complex. II. Optical properties of crystalline starch fractions. J Am Chem Soc 1943 65 558-561. 

The structure of starch granules has not yet been completely sorted out, but they are believed to contain both crystalline and amorphous regions. At least three types of crystalline starch structure have been recognised and these have been termed A, B and V. 

Crystalline starch beads can be used as a natural filler in traditional plastics [3]. They have been particularly used in polyolefins. When blended with starch beads, polyethylene films biodeteriorate upon exposure to a soil environment. The microbial consumption of the starch component leads to increased porosity, void formation, and loss of integrity of the plastic matrix. Generally, starch is added at fairly low concentrations (6-15 wt%). The total disintegration of these materials is obtained using transition metal compounds, soluble in the thermoplastic matrix, used as pro-oxidant additives to catalyze the photo and thermo-oxidative processes [4]. These products belong to the first generation of degradable polymers that biodeteriorate more than mineralize to CO2 and H2O in a time... 

Starch in foods has been analyzed mainly by enzymatic hydrolysis to free the glucose followed by its specific determination. From a nutritional standpoint, starch is present in foods as digestible and RS. RS is that which is not absorbed in the small intestine of healthy humans. From an analytical standpoint, the distinction between digestible and resistant is unclear (Bjbrk, 1996). Nevertheless, RS is virtually insoluble in water and techniques for analyzing total starch in foods have employed either an alkaline (KOH) or organic solvent (DMSO) treatment to successfully disperse the crystalline starch fractions and keep them solubilized. Quantitative analysis of RS in foods has utilized enzymes for their determination (Bjork, 1996). Nevertheless, techniques for analyzing RS in foods need to be established and tested in formal collaborative studies. 

The accessibility of the polymer to water-borne enzyme systems is vitally important because the first step in the biodegradation of plastics usually involves the action of extracellular enzymes which break down the polymer into products small enough to be assimilated. Therefore, the physical state of the plastic and the surface offered for attack, are important factors. Biodegradability is usually also affected by the hydrophilic nature (wettability) and the crystallinity of the polymer. A semicrystalline nature tends to limit the accessibility, effectively confining the degradation to the amorphous regions of the polymer. However, contradictory results have been reported. For example, highly crystalline starch materials and bacterial polyesters are rapidly hydrolysed. 

Physically inaccessible starch Raw starch granules Nongranular, retrograded, or crystalline starch Chemically modified starches... 

To obtain thermoplastic starch (Figure 4.1), thermal and mechanical processing should disrupt semi-crystalline starch granules. As the melting temperature of pure starch is substantially higher than its decomposition temperature there is a necessity to use plasticizers, such as water. Under the influence of temperature and shear forces, disruption of the natural crystalline sfructure of starch granules and polysaccharides to form a continuous polymer phase has been reported [10, 12-22]. 

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