Starch, crystallinity

The marked advances made in the study of bacterial polysaccharides serve only to emphasize the vast amount of work still to be carried out in this field. The recent remarkable achievements in enzymic chemical synthesis (starch, crystalline sucrose, etc.) indicate the growing necessity for greater cooperation11 between enzymologist and chemist in the solution of many biological phenomena. 

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."... 

Figure 5.16 Schematic model for the arrangement of amylopectin in potato starch. Crystalline layers containing double helical linear segments in amylopectin molecules form a continuous network consisting of left-handed helices packed in tetragonal arrays. Neighboring molecules are shifted relative to each other by half the helical pitch. (Adapted with permission from reference 3)...

Acid conversions when starch is treated with an acid such as HCI or H2SO4 at 40-60°C (commonly a starch slurry with 35-40% solids is treated with 0.2-0.5 N HCI for a few hours). Following the acid conversion, the acid is neutralized. In this process, the DP value decreases (and several physical properties are modified). Although chemical studies show that the a-D-(1- 4) links are more sensitive to hydrolysis than a-D-(1->6) links, it was determined that due to starch crystallinity more a-D-(1->6) links are hydrolyzed [55]. This happens because the a-D-(1 6) links are in the amorphous regions and they are more accessible to the reagent (less hydrogen bonds). This explains why the DP of amylose is reduced less than that of the amylopectin component. 

Underlying these visible changes there are a number of changes to the physical properties of dough components, such as a decrease in water absorbtion capacity, decrease in soluble starch and increase in starch crystallinity. 

Increase in starch crystallinity is often considered as the most straight forward way of understanding the changes that take place in bread during storage. The consumer associates the softness of bread with freshness. Keeping bread softer for longer will increase the pre- and post-sale shelf-life of the product. 

Lopez-Rubio, A., Flanagan, B., Gilbert, E., and Gidley, M. (2004). A novel approach for calculating starch crystallinity and its correlation with double helix content A combined XRD and NMR study. Biopolymers 89(9), 761-768. 

Amylose. A component (20-30%) of starch surrounded by amylopectin. A. is a linear a-l,4-glucan, Mr 50000-200000 (see figure at starch). Crystalline A. occurs in various polymorphic forms (A, B, C, and V-A.), that differ in conformation and crystal packing. A. is soluble in water and gives the characteristic blue color with iodine-potassium iodide solution (Lugol s solution) (formation of inclusion compounds, traces of iodide ions are necessary for occurrence of the blue color, formation of I5 ions I -1 I I -1). Because of its predominately unbranched structure, A. can be degraded to oligosaccharides both by a- and by /S-amylase. The screw-like (helical) conformation also allows the formation of inclusion compounds with alcohols. 

The main technique used to study starch crystallinity is X-ray diffraction, from which starch can he classified to A, B and C crystallites or polymorph forms[17, 32-34]. Starches with these polymorphisms are called A-, B- and C-type, each type presenting its characteristic diffraction patterns. The most commonly ohserved structures in native starch are A and B, the former being associated mainly with cereal starches, while the latter dominates generally... 

The different starch crystalline polymorphic forms exhibit different signal multiplicities in the C-1 regions, which reflect the symmetry of the heUx packing... 

Figure 17. Temperature dependence of heat capacity of potato starch-water mixture sample containing 58.5 mass% of H2O [4,73, 125] AB - vitreous saturated solution of water in starch + partially crystalline and vitreous water surplus phase BC - highly elastic saturated solution of water in starch + partially crystalline and vitreous water surplus phase AD - vitreous saturated solution of water in starch + entirely crystalline water surplus phase DE - highly elastic saturated solution of water in starch + crystalline water surplus phase EF - melting of water surplus phase GH - highly elastic saturated solution of water in starch liquid water surplus phase.

Shape holding agent Starch Crystalline cellulose... 

Considering the linear chains and chain segments as short-chain (low molecular) -> amylose, similar interactions like those of amylose are possible. The crystalline regions of the - starch granules are formed by elementary cells of double helices, which are joined together by antiparallel unwinding. The main share of this starch crystallinity is due to a. 

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