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