Effects Of Plasma On Structure And Properties Of Starch

In recent years, plasma technology （mainly low-temperature plasma technology） hasexhibited great developmental potential and applicable prospect in the fields of chemicalsynthesis, micro-electronics, superconducting technology, materials science, surface science,environmental science, biotechnology and life science, and has drawn increasing concern.Due to the advantages on broadness in frequency range, availability of the high-densitynon-equilibrium plasma in the larger space, simplicity in process, rapidity, efficiency, as wellas other characteristics including energy-saving and environmental friendly, dielectric barrierdischarge （DBD） plasma is considered to be the most likely plasma technology can be appliedin industry, thus becomes a major concern in the research field of powder modification. Atpresent, as plasma technology is still in the initial exploration stage of application to theresearch of modified starch, and the systematic study of the effects of plasma on themulti-scale structures of starch remains limited, it is difficult to regulate the starch propertiesat the molecular level by using plasma technology. Therefore, this study deals with somecritical scientific concerns of plasma technology used in starch modification, with the greatacademic value and practical significance.In this dissertation, a variety of advanced instrumental analysis techniques have beenused to systematically study the effects of plasma on multi-scale structures （such as thegranular structure, the lamellar structure, the crystalline structure and the chain structure） ofpotato and corn starches based on atmospheric dry starch modification using air as the gassource by DBD. The differences in the mechanism of plasma on potato and corn starches havebeen further studied, followed by establishment of the relative physical model to explain theaction mode and effects. In addition, the effects of plasma on the acidity, solubility,rheological property and gelatinization of potato and corn starches were also beeninvestigated.According to the results, the plasma basically did not change the overall granularmorphology of potato and corn starches, but caused new surface cracks on potato starchgranules, and increased channels and channel diameter to corn starch granules. The crystallineform and the thickness of the lamellar structure of the two kinds of starches were substantiallyfree from the influence of plasma. However, the relative crystallinity was reduced, and inparticular, potato starch showed a higher degree of decrease. Significant difference was foundfor the destruction modes of plasma to the polarization cross （crystalline structure） of potatoand corn starches. The former was destructed from outside to inside and the latter was opposite.Plasma could initiate the oxidation reaction of starch chains, which introduced thecarboxyl group into corn starch but the carboxyl group and the carbonyl group into potatostarch. Moreover, the longer duration of treatment led to the higher degree of oxidation. Withthe plasma process, the short-range ordered structure of the surface for the both starchgranules was damaged, and potato starch displayed a higher degree of destruction to thisstructure, while the short-range ordered structure （helical structure） inside granules changedslightly, which indicated that the plasma reduced the consistency of the helix and causedlattice distortion to affect the crystalline structure. Plasma could break starch molecular chains,leading to the decrease of weight-average molecular molar mass. For example, theweight-average molecular weight of potato starch decreased from107g·mol^-1to106g·mol^-1,and this value of corn starch reduced from107g·mol^-1to105g·mol^-1.The significant effect of plasma on the acidity of starch was also been found, whichcaused the result that the pH value of potato and corn starch slurries rapidly decreased to2.23and1.88respectively. The migration of water molecules and the strongly acidic environmentcould not only result in the further change in the structure of modified starch in the watersystem, for example that helix unwinded partly and formed new larger repeating structure, butalso enhance the extent of modification, indicating the changes of starch properties were theresulted from the combined actions of plasma and water. Plasma changed the gelatinizationprocess of potato starch, while it had no significant affect on the corn starch. Plasma alsoeffectively promoted the dissolution process and increased the solubility form1.4%to64.1%for potato starch, and0.8%to40.6%for corn starch. The two kinds of starches still remainedthe form of non-Newtonian pseudoplastic fluid, while the consistency coefficient andapparent viscosity significantly decreased, the thixotropism also decreased, the flow exponentincreased, and the shear thinning effect mitigated. Finally, the paste of potato and cornstarches approached to the form of Newtonian fluid.The differences in the action mode and effect of plasma on potato and corn starches wererevealed. The mechanism to the potato starch was that the plasma initially acted on thegranular surface and then continued to penetrate into the inside of potato starch, while forcorn starch, it was the result of combined actions of acting on the granular surface and actingthrough the pore structure into the inside of the corn starch granule. Thus, the effect of plasmaon the modification is significantly influenced by the starch structure.In conclusion, the above results demonstrated that the variety of effects of plasma, suchas high-energy particle collisions, radiation, energy transfer and accumulation, stress concentration, could impact the starch multi-scale structure in different degrees, therebyregulating the starch properties, especially giving the starch high solubility at roomtemperature, high concentration and low viscosity characteristics. This study offers significantevidences for further investigations on the powder plasma modified theory, and provides aconvenient, rapid, efficient and green new modification technology for starch deepprocessing.