不同pH下低甲酯苹果果胶凝胶模型建立及基于流变学的凝胶机理分析

doi:10.3864/j.issn.0578-1752.2016.13.016

摘要/Abstract

摘要： 【目的】采用柠檬酸-磷酸氢二钠缓冲溶液制备不同pH的低甲酯苹果果胶（LMP）凝胶，建立凝胶强度及破裂强度的数学模型，并依据流变学测定的储存剪切模量及损耗剪切模量解释不同pH范围的凝胶机理，为低甲酯苹果果胶在不同pH环境中的应用提供参考。【方法】采用质构仪测定LMP凝胶的凝胶强度及破裂强度，采用Mate lab 10.0模拟建立数学模型。利用流变仪在1 Hz条件下测定凝胶的储存剪切模量及损耗剪切模量。结合储存剪切模量与损耗剪切模量在图形中的交点寻找溶胶-凝胶转变点，确定形成凝胶的pH范围；依据储存剪切模量与损耗剪切模量在图形中的变化，并结合果胶的结构特征、形成凝胶的作用力分析不同pH下的凝胶机理。【结果】不同pH的LMP凝胶强度及破裂强度模型均遵循多项式规律，在pH 3.50附近凝胶强度达到最佳。pH在1.78—3.10时，依靠半乳糖醛酸未解离羧基基团或羟基间的氢键及甲氧酯基团之间的疏水相互作用，使得溶胶向凝胶转变。同时，随着果胶溶液pH的逐渐增大至3.10时，连续解离的羧基负离子与钙离子间逐渐形成钙桥，几种作用力的共同作用使得凝胶强度随着pH的增加而增大。储存弹性模量增加较为明显，凝胶强度增加到21.19 g，但由于连续解离的羧基负离子达不到形成稳定蛋壳结构所需要的量，凝胶质地偏软，凝胶形态不稳定；pH为3.10—4.20时，连续解离的羧基负离子的量达到最佳量，满足了与Ca²⁺间形成稳定的钙桥作用的所需量，凝胶趋于稳定，达到22.00 g左右，此状态下的凝胶弹性较好，凝胶稳定；pH为4.20—6.62时，由于pH的升高导致果胶分子间静电排斥逐渐加强，果胶发生解聚，无法形成钙桥，溶胶凝胶转变点消失，逐渐不能形成凝胶。【结论】LMP形成凝胶的pH范围为1.78-6.30。凝胶在pH为1.78—3.10时，果胶可在强酸性条件下形成凝胶，适用于对酸度要求较高的食品及药品中；pH为3.10—4.20时，凝胶强度最佳，凝胶最稳定，可满足对食品胶凝质地要求较高的食品。

关键词: 低甲酯苹果果胶, pH, 凝胶强度, 流变学, 弹性模量, 数学模型, 凝胶机理

Abstract: 【Objective】The mathematical model of LM apple pectin gel strength which were prepared by citric acid - disodium hydrogen phosphate buffer solution were established with pH 1.78-6.62 and the mechanism of gelling was analyzed by data of rheology. The results of this study will provide a theoretical reference for the application of LMP in different pH environments. 【Method】Gel strength, rupture strength and rheological properties were measured by texture analyzer and rheometer, respectively. A model was established with Mate lab 10.0. According to the sol-gel transition point in graphs which was combined with storage shear modulus and loss shear modulus intersections in the graph, the pH range of gels could be determined. The mechanism of gels under different pH was analyzed on the basis of changes in storage shear modulus and loss shear modulus in the graph and the structure characteristics.【Result】The model of gel strength was followed by the polynomial law. In the range of pH 1.78-3.10, hydrophobic interactions between methoxyl ester groups and hydrogen bonds among undissociated carboxyl groups were formed which resulted in the sol to gel transition and gel strength to 21.19 g. When the pH of pectin solution was about 3.10, hydrophobic interactions between methoxyl ester groups and hydrogen bonds between undissociated carboxyl groups were formed. So that the elastic modulus increased significantly, the gel strength was about 21.00 g. In this pH range, although the pectin charge density was low, Ca²⁺ was still promoted the gel. However, the dissociation of carboxyl anion concentration could not reach the optimal amount of electronegativity carboxyl to form a stable “egg box”, gels were not so stable. In the range of pH 3.10-4.20, Calcium Bridge were formed between galacturonic acid residues and Ca^{2 +}, which made gel tend to be stable and gel strength near to 22.00 g. In the range of pH 4.20-6.62, the electrostatic repulsion was predominant between pectin chains, which generated pectin gradually depolymerization because of free dissociated -COO^- reached a maximum.【Conclusion】In the pH range of 1.78-6.30, LMP could form gels. When pH was in the range of 1.78-3.10, the pectin could be used at strongly acidic conditions and suitable for higher acidity of foods and medicines. When pH was in the range of 3.10-4.20, the stable gels could meet the food requirements of higher food gel texture. The results of this study have practical significance for the application of low methyl ester pectin in food with different pH ranges.

Key words: LM-apple pectin, pH, gel strength, rheological, elastic modulus, mathematical model, gel mechanism

韩万友，董桂茹，屈玉玲，孟永宏，郭玉蓉，邓红. 不同pH下低甲酯苹果果胶凝胶模型建立及基于流变学的凝胶机理分析[J]. 中国农业科学, 2016, 49(13): 2603-2611.

HAN Wan-you, DONG Gui-ru, QU Yu-ling MENG Yong-hong, GUO Yu-rong, DENG Hong . Gel Model and Mechanism of Low-Methoxyl Apple Pectin for Various pH[J]. Scientia Agricultura Sinica, 2016, 49(13): 2603-2611.

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