No abstract

The aim of the study was the application of a recently published method, using structuring parameters calculated from dG'/dt, for the characterisation of the pectin sugar acid gelation process. The influence of cooling rate and pH on structure formation of HM pectin gels containing 65 wt.% sucrose were investigated. The results show that the structure formation process as well as the properties of the final gels strongly depended on both parameters. With increasing cooling rates from 0.5 to 1.0 K/min the initial structuring temperature slightly decreased and the maximum structuring velocity increased. The lower the cooling rates, the firmer and more elastic were the final gels. With increasing acid content (decreasing pH from 2.5-2.0) the initial structuring temperatures were nearly constant. The final gel properties varied visibly but not systematically. Gels with the lowest and highest pH were less elastic and weaker compared to those with medium acid concentrations.

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.

Pectin powder is degraded during storage and transport by demethoxylation and depolymerisation. The degradation mechanisms and especially the influence of pre-treatments on the degradation reactions are not completely understood. In this study, commercial citrus pectin was modified by either acidic or alkaline demethoxylation. The modified pectins, as well as the commercial pectin, were thermally degraded during four weeks of storage at 60 °C and 80% relative humidity. Demethoxylation and depolymerisation as well as colour alterations were examined during degradation, and the course of the reactions was monitored. It was found that the type of pre-treatment during modification determined the material properties and, thus, the water uptake of the modified pectin powders. The resulting water availability in the samples was crucial to the extent of demethoxylation and to the type and intensity of depolymerisation since some of these reactions competed for the water in the climate chamber. The pre-treatment also determined the content of neutral sugars and sodium ions of the modified pectins. High contents of these components limited the extent of degradation in different ways. A previously assumed third depolymerisation mechanism of pectins, beside backbone hydrolysis and ß-elimination, was confirmed.

No abstract

No 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.

Aqueous D-galacturonic acid (D-GalA) model systems treated at 130 °C at different pH values show an intense color formation, whereas other reducing sugars, such as D-galactose (D-Gal), scarcely react. GC-MS measurements revealed the presence of several phenolic compounds: e.g., 3,8-dihydroxy-2-methyl-4H-chromen-4-one (chromone) and 2,3-dihydroxybenzaldehyde (2,3-DHBA). These phenolic compounds, especially 2,3-DHBA, possess an intense browning potential and cannot be found within heated model solutions of reducing sugars. Investigations regarding the formation of these substances show that α-ketoglutaraldehyde plays an important role as an intermediate product. In addition, MS analysis of model systems of norfuraneol in combination with 2,3-DHBA showed the formation of oligomers that could also be detected in D-GalA model systems, leading to the assumption that, in addition to reductic acid, these compounds are jointly responsible for the strong color formation during the heat treatment of D-GalA.