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Thermoanalytical characterisation of processing-dependent structural changes and state transitions of citrus pectin

Einhorn-Stoll, Ulrike, Kunzek, Herbert
Food hydrocolloids 2009 v.23 no.1 pp. 40-52
phase transition, thermal stability, food processing quality, citrus fruits, pectins, carbohydrate structure, thermal degradation, solubility, alkali treatment, acid treatment, thermal analysis
Pectins are stabilised by inter- and intramolecular attraction forces, such as hydrophobic interactions and hydrogen bonds, and destabilised for example by electrostatic repulsion forces and steric hindrances. These interactions are changed during the processing in dependence on the methods and conditions. Two original high methoxylated citrus pectins were modified and the derivatives were tested for their molecular parameters (degree of methoxylation, DM; galacturonan content, GC; intrinsic viscosity, [η]; ash content, cations) and for their thermal degradation and dissolution behaviour. It was found that pectins were degraded later after removing side chains, low-molecular sugars and cations by washing with acidified ethanol as well as after amidation with ethanolic ammonia. The materials were degraded earlier after re-precipitation and acidic (HCl) or alkaline (K₂CO₃) demethoxylation. Additionally, for all pectins the pre-drying treatment and drying procedure were of considerable importance with respect to state and state transitions. The characteristic degradation temperatures and other parameters of the thermal analysis gave important information on the stability and homogeneity of the materials as well as on the molecular interactions and the preferred conformation. Conformations and conformational changes are assumed to be detectible to a certain extent by thermal analysis. Altogether, the thermal analysis proved to be a quick and reproducible method for the detection of changes of several material properties of dry pectins that result from modifications and processing.