The Role of Hydrogen Bonding in Amylose Gelation
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0038-9056 |
| Online Access: | http://www.viks.sk/chk/star_3_4_04_122_131.doc |
| Abstract: | Conclusions. In this work, the rheological properties of amylose gels have been systematically studied as a function of water content using a binary DMSO/water solvent system. It has been shown that water content is critical in determining the behavior of starch properties through the formation of intramolecular and/or intermolecular hydrogen bonds. It has been proposed [21] that in DMSO, amylose has a tight, rigid helical conformation maintained by intramolecular hydrogen bonding that is stabilized by the DMSO molecules. Our work suggests that, as water is added, the intramolecular hydrogen bonds are replaced with intermolecular water bonds, causing the helical amylose to become more extended and flexible until the amylose adopts an extended interrupted helical conformation. Banks and Greenwood [22] have carried out important pioneering studies in regard to the use of different solvents and the conformation of amylose and have laid the foundation for this type of work. We have shown that the rheological behavior of amylose was highly dependent on the water content through the formation of an intermolecular hydrogen bonded gel structure. Water content was critical in the interaction between amylose molecules, since as the water content was increased, amylose formed strong, elastic gels, confirming that amylose was the dominant gel-forming component in starch. It is proposed that amylose gels are formed by the interaction between adjacent amylose molecules through hydrogen bonded intermolecular water molecules. The role of hydrogen bonding in amylose gelation was further probed and it was found that in the absence of water, amylose was able to form strong, elastic gels by the addition of various polyols. It is proposed that polyols such as 1,3-propanediol are able to form an intermolecular hydrogen bonded network of amylose molecules, similar to that produced by water. It was also found that amylose gel strength could be significantly reduced by the addition of the intermolecular hydrogen bond-breaking agent, urea, which was not so apparent with the intramolecular bond-breaking agent TMU. The results of this work clearly demonstrate the importance of hydrogen bonding in the formation of a starch gel and support the proposal that the mechanism of starch gelation is primarily via non-covalent interactions in amylose that initiates gel formation followed by a phase separation through crystalline aggregation. Further work at different concentrations of amylose could be carried out to determine under which conditions gelation or precipitation occurs. Work with other starches and degraded products such as thin-boiling starch and gelify-ing maltodextrins would also prove interesting. Summary. Studies of the rheological properties of amylose gels in mixtures of water and DMSO have shown that these properties are dependent upon the formation of intramolecular and intermolecular hydrogen bonds. In DMSO, it is proposed that amylose has a tight helical conformation which is maintained by intramolecular hydrogen bonding. When water is added it is proposed that intramolecular hydrogen bonds are replaced with intermolecular hydrogen bonds and thus the rheological properties are changed due to the more extended and flexible amylose molecules adopting an interrupted helical conformation. Because of these changes in conformation, increased water content results in strong elastic gels, commensurate with it being the dominant gel-forming component in starch, due to its ability to form networks with water through intermolecular hydrogen bonding. The addition of various polyols to solutions of amylose in DMSO also resulted in strong elastic gels being formed. It is proposed that polyols such as 1,3-propanediol are also able to form networks in a similar way to water. The use of intermolecular hydrogen bond breaking agents, such as urea, reduced gel strength significantly, but the intra-molecular bond-breaking agent, 1,1,3,3-tetramethyl urea (TMU), was not so effective. The results of this work are further evidence of intermolecular hydrogen bonding networks between amylose and water as being of fundamental importance in the strength of starch gels. |
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| ISSN: | ISSN 0038-9056 |
