Impact of sodium ions on material properties, gelation and storage stability of citrus pectin

Abstract

Material properties, gelation and storage stability of demethoxylated pectin samples strongly varied in dependence on the applied modification method. It was assumed that the content of sodium ions and their resulting electrostatic interactions with free carboxyl groups were crucial for these differences. Sodium ions were widely removed by acidic modification but added during alkaline and enzymatic modification using NaOH in a pH-stat method. It was the aim of the present study to investigate the individual impact of sodium ions on pectin properties using samples with similar molecular parameters but different sodium ion content.Sodium enrichment of pectin increased the pectin particle surface and, as a consequence, the pectinwater-interactions. Differences in molecular structure and material properties were reflected in simultaneous thermal analysis; an exothermic starting peak in DSC vanished and pectin pyrolysis was accelerated after sodium ion enrichment. Gel formation was affected by sodium ions. It was delayed in a sugar-acid system by reducing the number of hydrogen bonds and accelerated in a sugar-calcium system by reducing electrostatic repulsion. Sodium ions increased the storage stability of pectin. They were bound to free carboxyl groups (-COONa) and restricted degradation reactions during storage which required these groups, in particular depolymerisation by decarboxylation.

Thermal degradation of citrus pectin in low-moisture environment – Investigation of backbone depolymerisation

“…The two types of samples differ in their content of neutral sugars and sodium ions, which are higher in AL-than in A-pectins (see 3.1). Both factors reduce depolymerisation, as described above for degradation at 40% rh, as recently discussed in detail for storage at 80% rh and as shown for sodium ions in a parallel paper (Kastner et al, 2020). Another crucial factor is the number of methoxyl groups and free carboxyl groups in the pectin samples.…”

“…They are available in A-pectins, but they are mostly associated with sodium ions (forming carboxylate groups -COONa) in AL-pectins, since the content of sodium ions was < 0.01% in the A-pectins but 0.69% in HMP-AL and 1.99% in LMP-AL. Eliminative decarboxylation and backbone hydrolysis of the AL-modified samples were restricted by the presence of sodium ions, as was proved in a parallel paper (Kastner, Einhorn-Stoll, Fatouros, & Drusch, 2020). The two reactions were additionally limited for the AL-pectins by a lower formation of free -COOH by less intensive demethoxylation during storage in comparison to A-pectins.…”

Production and molecular characterization of tailored citrus pectin-derived compounds

“…Specifically, the hydrolyzed sample was transferred into Spectra/Por® dialysis tubing (0.1 -0.5 kDa, molecular weight cut-off (MWCO)) and thoroughly dialyzed against demineralized water for 72 h. Dialyzed samples were then lyophilized (Christ Freeze Dryers, Osterode, Germany) and stored in a desiccator at room temperature until use. Removal of ions, particularly the high amounts of Na + added to maintain conditions for EPG action, was crucial since these ions have been reported to impact different functional properties of pectin, such as cation-binding capacity and gelling potential (Kastner et al, 2020). A summary of the pectin-derived samples generated, and their denotations are presented in Table 1.…”

“…However, considering that high amounts of this cation were present in the sodium acetate buffer (40 mM and 150 mM) and NaOH (0.1 M) used during the production of the samples (to maintain an optimal pH during the enzymatic hydrolysis), it can be observed that a substantial amount of this cation was removed from the samples during dialysis. Assessing the concentration of Na + ions present in the samples was crucial, since the presence of monovalent ions, specifically Na + , have been reported to influence some functional properties of pectin such as its gelling and cation-binding capacity (Kastner et al, 2020). For instance, Kastner et al (2020) reported that the presence of Na + -ions in a low methylesterified pectin material, at a concentration of 1.2%, had a substantial impact on the formation of a gel network.…”

“…Assessing the concentration of Na + ions present in the samples was crucial, since the presence of monovalent ions, specifically Na + , have been reported to influence some functional properties of pectin such as its gelling and cation-binding capacity (Kastner et al, 2020). For instance, Kastner et al (2020) reported that the presence of Na + -ions in a low methylesterified pectin material, at a concentration of 1.2%, had a substantial impact on the formation of a gel network.…”