Starches in Semidilute Aqueous Solution
| Main Authors: | , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0038-9056 |
| Online Access: | http://www.viks.sk/chk/star_6_97_215_224.doc |
| Abstract: | Summaries. Nine starches from different plants were studied in the semidilute concentration regime. The amylose content varied between 0% (waxy starch) and 76%. The applied concentration range of 5 to 30% (w/v) corresponds to typical concentrations used for thickeners in food and non-food industries. With the exception of the amylose rich starches the maximum concentration was at least 4 times larger than the overlap concentration. Up to a certain concentration a common master curve was found for all starches but at high concentrations this master curve was superimposed by association and aggregation phenomena. The aggregation is promoted by the presence of amylose. The onset of aggregation is shifted to lower concentrations when the amylose content in the starches was high. The aggregation proceeds in time. It could be followed by dynamic light scattering and quantitatively be evaluated. The time dependent ageing of the solutions and the finally formed gels gave evidence for a liquid-solid phase separation that is kinetically hampered by the presence of the branched amylopectin. Conclusions. Semidilute solutions comprise a concentration region between 5 to 30% (w/v). This range is typical for industrial application of thickeners in food chemistry and in the non-food area. The region of semidilute solutions of branched macro-molecules has not yet been much studied. This unexplored area of research was recently entered by our group [4,27, 37] and is continued here with native starches. No basic theory has been developed so far. The present study tried to establish a simplification of the very complex dependencies on many parameters. The scaling concepts, successfully introduced by de Gennes [3] for an universal discription of linear and flexible chains is extended here to branched polymers. The concept led to universal behavior for the osmotic modulus as long as the overlap concentration was not much exceeded. From this observation a non inhibited interpenetration of the chains at the outside of the branched macromolecules can be concluded. Beyond this concentration deviations occurred that are characteristic of the various starches. Amylose free starches and amylose containing starches have principally to be distinguished. In pure amylopectin mainly an impeded interpenetration of the various branched macromolecules led to a shrinking of the molecular dimensions and thus to an increase in the overlap concentration. Gelation did not take place but a weak (reversible) association had also to be taken into consideration. Amylose containing starches, on the other hand, developed a pronounced tendency of (irreversible) aggregation. It occurred at lower concentration for amylose rich starches than for starches with low amylose content. No equilibrium state was reached. The aggregation proceeded in time until gelation took place. Even these gels were not in thermodynami-cal equilibrium but showed ageing. A diffusion controlled liquid/solid phase separation is evidently the driving force for the observed phenomena. The time dependent aggregation process could be quantitatively registered by dynamic light scattering. A slowing down of the diffusion coefficient was observed that indicated a growth of aggregated particles. Simultaneously the so called heterodyne fraction increased. It gave evidence for a strong increase also of the mass fraction of the aggregated clusters. |
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| ISSN: | ISSN 0038-9056 |
