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

Abstract

Thermal degradation of modified pectin samples with varying molecular structure during storage was recently studied at 60 °C and 80% relative humidity (rh) for 28 days. Demethoxylation and depolymerisation were identified as main degradation reactions. The present paper aims on improving the understanding of the different depolymerisation reactions and their interplay with demethoxylation during storage. Therefore, thermal degradaton of acidic and alkaline demethoxylated pectins was studied at a further reduced rh of 40%. The alterations were examined in detail via molecular parameters and were reflected by Differential scanning calorimetry and attenuated total reflectance Fourier-transformation infrared spectroscopy. The impact of thermal degradation on pectin particle structure was studied via particle surface area and microscopy.At low relative humidity (rh) demethoxylation and depolymerisation were reduced, and the formation of brown reaction products, resulting from further decomposition of intermediate uronides and neutral sugars, was restricted. By comparing thermal degradation at different humidity, eliminative decarboxylation was identified as the main depolymerisation reaction.Reduction of rh affected also the alteration of pectin material properties, particle surface reduction was less pronounced. Molecular alterations were stronger in case of acidic demethoxylated samples, and alterations of material properties were higher in case of alkaline demethoxylated samples.

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

“…Comparable values for this signal are described also by other references (Chylinska, Szymanska-Chargot, & Zdunek, 2016, Coimbra et al, 1999, the signal maximum may partly depend on residual water in the sample. Similar differences with respect to the carboxylate signal were also found in a corresponding paper (Einhorn-Stoll et al, 2020), comparing acidic and alkaline demethoxylated pectin (alkaline demethoxylation was performed using 0.5 M NaOH). The signal at 1730 cm -1 was nearly unchanged after sodium ion loading, probably due to the overlay of the carboxyl groups and the methoxyl group signals and the nearly constant DM (LMP+Na: 41.4 ± 0.8% and LMP: 42.8 ± 0.1%).…”

“…Major alterations in the ATR-FTIR signals (Fig. 2b, c) due to depolymerisation of the pectin samples were similar to those discussed more detailed in a corresponding paper (Einhorn-Stoll et al, 2020).…”

Pectin from Mangifera indica L. cv. Haden residues: Kinetic, thermodynamic, physicochemical, and economical characterization

“…With this value, and the constants for Mango pectin used (K = 1.40x10 -6 dL g -1 ;  = 1.34) [34] was calculated the MWmean = 55909.76 g mol -1 or 55.91 kDa. The molecular weight obtained was within the previously reported average range for pectin , closer to the lower end of the range [10] which could be an indication of the degradation suffered by pectin during its extraction due to temperature and its de-polymerization to monosaccharides or oligosaccharides [43,44]. The molecular weight is usually directly associated to the strength of the gel formed by the pectin, due to the number of junction zones that can be formed per molecule affects the extent of cross-linking, however, low molecular weights do not affect pectin emulsifying properties or its ability to form hydrogels [15].…”

“…According to Table 1, it was observed that yield increased with respect to extraction time and temperature, nevertheless, statistical analysis showed that temperature is the main factor who affected the pectin yield extraction (Online Resource 1) since no significant differences between time and temperature vs time interaction were detected. This behavior may be explained by the fact that thermal degradation of pectin occurred, as a consequence of a prolonged time of exposure to high temperatures causing a de-polymerization to monosaccharides or oligosaccharides that could be reflected in low molecular weights [43,44]. It is also interesting to note that the yield increased as the temperature increased, possibly due to an increment in the solubility of the pectin before of being degraded [24].…”

“…A relative weak acid extraction is also supported by the near to zero free acidity value (0.27  0.00 mEq carboxyl free/g) found in the pectin, which is associated with the residual acid [11] and is usually inversely proportional to the equivalent weight, furthermore, the obtained value is lower than the reported for commercial pectin of 0.733 mEq carboxyl free/g [33]. According to the specific characteristics, that is, high DE (81.81%) and %MeO (13.35%), galacturonic acid content in the range of acceptance for industrial applications (71.57%) and high equivalent weight (3657.55 mg), Haden mango peel pectin could be used for different purposes, like food, environmental or pharmaceutical applications where its gelling, emulsifier and biodegradable characteristics could be appreciated [44,55] for example as a gelling agents [43], delivery systems [5] or as emulsifier [20]. In addition, higher DE can be associated to a better microcapsule formation ability in the meaning of thermal degradation stability and encapsulated retention [56].…”