Molecular amylose

The crumb structure also changes, although at a lower rate. The crumb becomes firm, its elasticity and juiciness are lost, and it crumbles more easily. The so-called staling defect of the crumb is basically a starch retrogradation phenomenon (cf. 4.4.4.14.2) which proceeds at different rates with amylose and amylopectin. On cooling bread, the high-molecular amylose very rapidly forms a three-dimensional network and the crystalline states of order of amylose/lipid complexes increase. These processes stabilize the crumb. 

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. 

Com and rice starches have been oxidized and subsequently cyanoethylated (97). As molecular size decreases due to degradation during oxidation, the degree of cyanoethylation increases. The derivatized starch shows pseudoplastic flow in water dispersion at higher levels of cyanoethylation the flow is thixotropic. Com and rice starches have been oxidized and subsequently carboxymethylated (98). Such derivatives are superior in the production of textile sizes. Potato starch has been oxidized with neutral aqueous bromine and fully chemically (99) and physically (100) characterized. Amylose is more sensitive to bromine oxidation than amylopectin and oxidation causes a decrease in both gelatinization temperature range and gelatinization enthalpy. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

With regard to molecular morphology, whether it is A- or B-amylose, there is no room for a water or similar molecule to enter the cavity in the interior of the... 

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

Molecular Structure. Most starches consist of a mixture of two polysaccharide types amylose, an essentially linear polymer, and amylopectin, a highly branched polymer. The relative amounts of these starch fractions in a particular starch are a major factor in determining the properties of that starch. 

Fig. 2. From left to right, Corey-Pauling-Koltun molecular models of cyclohexa-amylose, cycloheptaamylose, and cyclooctaamylose viewed from the secondary hydroxyl side of the torus. 

Cellulose is a high molecular weight polymer of D-glucose with fi( 1 -4)-glycosidic bonds, found in plant fibres it is the major component of most plant tissues. Starch is another common polysaccharide, containing two polymers of glucose, amylose and amylopectin. It was used in some paint preparations and in the production of paper. Acid treatment of starch produces dextrins, which are used as adhesives and additives in water colour paintings. 

Addition of an aqueous solution of PEG to a saturated aqueous solution of a-CD at room temperature did not lead to complex formation unless the average molecular weight of PEG exceeded 200 [46]. Moreover, carbohydrate polymers such as dextran and pullulan failed to precipitate complexes with PEG, and the same was true for amylose, glucose, methyl glucose, maltose, maltotriose, cyclodextrin derivatives, such as glucosyl-a-CD and maltosyl-a-CD, and water-soluble polymers of a-CD crosslinked by epichlorohydrin. These facts suggested to Harada et al. the direction for further research. 

The labile nature of the components necessitates that, for fundamental investigations, the starch should preferably be extracted from its botanical source, in the laboratory, under the mildest possible conditions.26 Industrial samples of unknown origin and treatment should not be used. The characterization of the starch would appear to entail (1) dissolution of the granule without degradation, (2) fractionation without degradation, (3) complete analysis of the finer details of structure of the separated components (including the possibilities of intermediate structures between the extremes of amylose and amylopectin), and (4) the estimation of the size, shape, and molecular-weight distribution of these fractions. 

Methods which can be used to determine the size and shape of polysaccharides have been reviewed.107 (A critical survey of these has recently been given by Sadron108 and by Ogston.109) Special problems exist in the case of the undegraded starch components. In view of the branched nature of amylopectin and the large size of the amylose molecule, chemical methods of estimating size are inadequate, and it is questionable whether results are valid.38 The free components may also aggregate in aqueous solution. Study of derivatives is therefore more convenient, and the preparation of these is an essential preliminary to estimations of molecular size. 

Comparison of Chemical End-group Assay and Corresponding Molecular-Weight Determinations for Amylose... 

Several investigations of the molecular weight of subfractions of amyloses... 

The Results of Molecular-Weight Determinations on Acetylated Amyloses... 

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