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|標題:||Rheological Characterization of Gel Point for Several Complex Biopolymer Solutions
|關鍵字:||gel point;凝膠點;rheology;electrostatic interaction;synergism;sol-gel transition;fractal;流變學;靜電交互作用力;增效交互作用力;溶膠-凝膠相轉變;碎形||出版社:||化學工程學系||摘要:||
The gel point of complex biopolymer solutions was studies by rheometers (SR5 and RFS), and the effect of physical attractive interactions and composition (or stoichiometry) to the network structure of mixed gels was explored. The attractive forces include electrostatic interaction and synergism. Electrostatic interaction takes place between two polymers with opposite charges. Synergistic interaction is often found in mixed solutions of plant galactomannans (locust bean gum or guar gum) and galactans (carrageenan or agar). According to the definition of gel point defined by Winter, when the system reaches the sol-gel transition, tand does not vary with frequency and remains constant (tand = tan(np/2), n is a critical exponent). Fractal dimension, df, can be obtained by theoretical formula of Muthukumar. The larger of the fractal dimension is, the tighter is the network formed by the mixed gels. The mixed gels with larger df generally exhibit better mechanical properties. In food science, the larger df may imply firmer texture.
Among the mixed gel systems in electrostatic interaction that we studied, the gel point can only be found in chitosan-gelatin mixed system. In other systems, insoluble coacervates generally formed so the gel point cannot be detected by a rheometer. In the chitosan-gelatin mixed system, the relationship between mixing ratio and n is similar to that in traditional chemical cross-linking systems. This may imply that the network structure is independent of the forming mechanism (i.e. chemical binding or physical interaction). In the synergistic interaction systems, owing to less solubility of galactomannan and konjac glucomannan, the mixed gel solutions show high degree of heterogeneity, making the exact gel point be screened. In that case, only "pseudo" gel-ponits were found.
本研究以流變儀研究生物性高分子複合溶液凝膠系統的凝膠點，探討系統中物理性吸引力及組成比例對凝膠網狀結構的影響。吸引力機制包含靜電交互作用力及增效作用 (synergism)。靜電吸引交互作用力只發生在帶相反電性的高分子，有時稱為polyanion-polycation interactions。增效交互作用力則常見於plant galactomannans (locust bean gum與guar gum) 與具雙螺旋結構的galactans (carrageenan、agar) 的混合膠系統。利用動態黏彈測試偵測混合膠系統在溶膠-凝膠相轉變之際的凝膠點，系統到達凝膠點時tand值不隨所施的頻率而變，即tand = tan(np/2)，n為動態臨界指數。將n代入Muthukumar的凝膠理論公式即可預測混合系統在凝膠點時的碎形維度, df。由碎形維度對應到一特定的碎形結構，可得知凝膠系統網狀結構密度疏鬆程度。碎形指數越大，表系統的網狀結構越緊密，意味混合膠的機械性質越強；在食品上則可代表混合膠口感較佳。
在靜電交互作用力系統方面，大多數的混合系統呈現高度的非均質狀態，且迅速形成不溶物，僅幾丁聚醣與gelatin混合膠系統呈均質狀態，可用流變儀偵測其凝膠過程。在此混合系統中，組成與n之間的變化關係與化學交聯系統十分相似，此可能代表網狀結構與形成的機制無關 (化學鍵結或物理作用力)。在增效作用系統方面，由於galactomanna與konjac glucomannan的水溶性不佳，造成混合膠溶液呈現高度的混濁狀，導致真正的凝膠點被模糊或遮蔽掉，利用流變儀測量，只能找到假性凝膠點。
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