Dextrinization structural changes

Figure 24. Dimensional representation of phosphorylase limit dextrin structure and changes produced by action of the debranching enzyme...

The structural changes that take place in the starch molecule during dextrinization are not well established. The hydrolysis of the long starch molecules into shorter "chain lengths" is understood however, and this change becomes evident when the dextrin is cooked in water. The viscosity of the dextrin paste will be thinner than that of the starch paste at the same solids level. 

Structural changes of starch and hydrogen starch granules on heating have been discussed. These changes facilitate the next stage of dextrinization, which is chemical in character. 

Structural changes in the course of dextrinization find only a minor response in the infiared-spectral characteristics of dextrins (see Fig. 21). Excellent articles have been published on the infrared absorption (including the far-i.r. region) " and Raman-scattering spectra of mono-, oligo-, and poly-saccharides. E>etailed band-assignments for these spectra commend attention on these spectroscopic techniques as useful tools in recognizing... 

While nature uses coenzyme-dependent enzymes to influence the inherent reactivity of the coenzyme, in principle, any chemical microenvironment could modulate the chemical properties of coenzymes to achieve novel functional properties. In some cases even simple changes in solvent, pH, and ionic strength can alter the coenzyme reactivity. Early attempts to present coenzymes with a more complex chemical environment focused on incorporating coenzymes into small molecule scaffolds or synthetic host molecules such as cyclophanes and cyclo-dextrins [1,2]. While some notable successes have been reported, these strategies have been less successful for constructing more complex coenzyme microenvironments and have suffered from difficulties in readily manipulating the structure of the coenzyme microenvironment. 

Incomplete coverage of the surface of such a fifth-generation POPAM dendrimer exposes hydrophobic areas of adamantyl units remaining uncomplexed by cyclodextrins on the dendritic outer shell [38]. Pyrenes were used as neutral fluorescence probes to examine whether this might lead to aggregation in water driven by the hydrophobic effect [39a]. Their inclusion in the dendrimer/cyclo-dextrin aggregate leads to changes in fluorescence intensity and in the vibrational fine structure. Formation of excimers was also observed. 

Further chemical changes that occur during dextrinization should be discussed separately for the case of formation of British gum, and for variously catalyzed thermal processes. Thus, when starch (1) is heated in the presence of moisture, random hydrolytic scission at the (1 - 6), and more readily at the (1 — 4), linkages has been proposed to occur in the branched chains of amylopectin, to give more-linear structures, leading to the observed decrease in viscosity. o-Glucosyl oxocarbonium ions (2) were postulated as intermediates, but free radicals (3) formed by homolytic fission at the (1 4)-glycosidic bond are also feasible. 

Although the essential features of the structural modifications undergone by amylose on pyrolysis are now established, full investigations of the dextrinization of the more complicated, amylopectin component have yet to be attempted. The most useful approach in this aspect of the field is to correlate the changes in such physical properties as molecular size with alterations in chemical structure. 

As used in this chapter, a starch modification refers to starch molecules which have a general change in the polyglucan structure without the addition of a chemical sustituent. Examples include depolymerized starches and dextrins (including pyrodextrins and cycloamylases). Derivatized starches have had the addition of chemical groups at the hydroxyls. These include the starch ethers and esters. Oxidized starches can be both modified (when depolymerized) and derivatized (with carboxyl and carbonyl groups). 

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