Structural and rheological properties of diluted alkali soluble pectin from apple and carrot.

Pectin, a complex polysaccharide found in plant cell walls, plays a crucial role in various industries due to its functional properties. The diluted alkali-soluble pectin (DASP) fractions that result from the stepwise extraction of apples and carrots were studied to evaluate their structural and rheological properties. Homogalacturonan and rhamnogalacturonan I, in different proportions, were the main pectin domains that composed DASP from both materials. Atomic force microscopy revealed that the molecules of apple DASP were longer and more branched. A persistence length greater than 40 nm indicated that the pectin molecules deposited on mica behaved as stiff molecules. The weight-averaged molar mass was similar for both samples. Intrinsic viscosity values of 194.91 mL·g-1 and 186.79 mL·g-1 were obtained for apple and carrot DASP, respectively. Rheological measurements showed greater structural strength for apple-extracted pectin, whereas carrot pectin was characterized by a higher linear viscoelasticity limit. This comparison showed that the pectin fractions extracted by diluted alkali are structurally different and have different rheological properties depending on their botanical origin. The acquired insights can enhance the customized use of pectin residue and support further investigations in industries relying on pectin applications.

Influence of pectin and hemicelluloses on physical properties of bacterial cellulose.

The properties of bacterial cellulose (BC)-based films produced by in situ biosynthesis with various polysaccharides (water-soluble pectin, arabinan, rhamnogalacturonan I, arabinoxylan, xyloglucan, glucomannan) were investigated. The addition of the polysaccharides to the bacterial growth environment changed the composition of the films by incorporating characteristic monosaccharides. BC-based films contained up to 26.7 % of non-cellulosic polysaccharides. The applied modification had a clear impact on water sorption and caused a decrease in the thermal stability of most BC films, which was connected with the depletion of geometrical dimensions of cellulose nanofibers observed with AFM. The FT-IR and Raman spectra demonstrated a decrease in % Iα of cellulose films, most notably for xyloglucan and glucomannan, as well as a change in their degree of crystallinity and the length of cellulose chains. The addition of xyloglucan had the most pronounced effect on film hardening; the other additives had a similar but lesser effect.

A mini-review on the plant sources and methods for extraction of rhamnogalacturonan I.

Rhamnogalacturonan type I (RG-I) is one of the pectin family member abundant in plant cell walls. Process of RG-I extraction from cell walls, either as a one-step or several-stage process, conditions the structure and properties of obtained polysaccharides. In this paper, we provide comprehensive overview of the factors related to the source and extraction techniques that determine the yield and chemical composition of pectin belonging to RG-I. The role of the source material, solvent, pH, temperature, time and additional factors related to applied techniques, such as microwaves, ultrasounds, high and low pressure or enzymatic treatments are discussed.

Structure and functionality of Rhamnogalacturonan I in the cell wall and in solution: A review.

Rhamnogalacturonan I (RG-I) belongs to the pectin family and is found in many plant cell wall types at different growth stages. It plays a significant role in cell wall and plant biomechanics and shows a gelling ability in solution. However, it has a significantly more complicated structure than smooth homogalacturonan (HG) and its variability due to plant source and physiological state contributes to the fact that RG-I's structure and function is still not so well known. Since functionality is a product of structure, we present a comprehensive review concerning the chemical structure and conformation of RG-I, its functions in plants and properties in solutions.

The primary, secondary, and structures of higher levels of pectin polysaccharides.

Pectin is a heteropolysaccharide abundant in the cell wall of plants and is obtained mainly from fruit (citrus and apple), thus its properties are particularly prone to changes occurring during ripening process. Properties of pectin depend on the string-like structure (conformation, stiffness) of the molecules that determines their mutual interaction and with the surrounding environment. Therefore, in this review the primary, secondary, and structures of higher levels of pectin chains are discussed in relation to external factors including crosslinking mechanisms. The review shows that the primary structure of pectin is relatively well known, however, we still know little about the conformation and properties of the more realistic systems of higher orders involving side chains, functional groups, and complexes of pectin domains. In particular, there is lack of knowledge on the influence of postharvest changes and extraction method on the primary and secondary structure of pectin that would affect conformation in a given environment and assembly to higher structural levels. Exploring the above-mentioned issues will allow to improve our understanding of pectin functionality and will help to tailor new functionalities for the food industry based on natural but often biologically variable source. The review also demonstrates that atomic force microscopy is a very convenient and adequate tool for the evaluation of pectin conformation since it allows for the relatively straightforward stretching of the pectin molecule in order to measure the force-extension curve which is directly related to its stiffness or flexibility.

Changes of pectin nanostructure and cell wall stiffness induced in vitro by pectinase.

Structural modifications of fruit cell-wall pectins are controlled by various enzymes. In this in vitro study, the cell wall material (CWM) from pear fruit (Pyrus communis L.) was treated using pectinases in two concentrations. Water soluble (WSP), chelator soluble (CSP) and sodium carbonate soluble (DASP) pectin fractions were extracted from CWM. By visualization of enzymatic-induced changes of structure and CWM stiffness using an atomic force microscopy (AFM), the role of pectins in the mechanical properties of cell walls was shown. Galacturonic acid (GalA) content in pectin fractions was assayed as well. This experiment unveiled evidence of the structural degradation of molecules in pectin fractions extracted from CWM caused by in vitro pectinase action and softening of CWM due to pectin removal that might be related to the creation of empty spaces in the cellulose-hemicellulose network.

Raman imaging of changes in the polysaccharides distribution in the cell wall during apple fruit development and senescence.

MAIN CONCLUSION: Du ring on-tree ripening, the pectin distribution changed from polydispersed in cell wall to cumulated in cell wall corners. During apple storage, the pectin distribution returned to evenly dispersed along the cell wall. The plant cell wall influences the texture properties of fruit tissue for example apples become softer during ripening and postharvest storage. This softening process is believed to be mainly connected with changes in the cell wall composition due to polysaccharides undergoing an enzymatic degradation. These changes in polysaccharides are currently mainly investigated via chemical analysis or monoclonal labeling. Here, we propose the application of Raman microscopy for evaluating the changes in the polysaccharide distribution in the cell wall of apples during both ripening and postharvest storage. The apples were harvested 1 month and 2 weeks before optimal harvest date as well as at the optimal harvest date. The apples harvested at optimal harvest date were stored for 3 months. The Raman maps, as well as the chemical analysis were obtained for each harvest date and after 1, 2 and 3 months of storage, respectively. The analysis of the Raman maps showed that the pectins in the middle lamella and primary cell wall undergo a degradation. The changes in cellulose and hemicellulose were less pronounced. These findings were confirmed by the chemical analysis results. During development changes of pectins from a polydispersed form in the cell walls to a cumulated form in cell wall corners could be observed. In contrast after 3 months of apple storage we could observe an substantial pectin decrease. The obtained results demonstrate that Raman chemical imaging might be a very useful tool for a first identification of compositional changes in plant tissue during their development. The great advantage Raman microspectroscopy offers is the simultaneous localization and identification of polysaccharides within the cell wall and plant tissue.

The stiffening of the cell walls observed during physiological softening of pears.

MAIN CONCLUSION: The Young's modulus of the primary cell walls of pears decreases linearly during the pre-harvest on-tree maturation and increases during postharvest storage, and does not correlate with firmness of fruit. The determination of mechanical properties of cell walls is indispensable for understanding the mechanism of physiological softening and deterioration of quality of fruits during postharvest storage. The Young's modulus of the primary cell walls from pear fruit (Pyrus communis L., cultivars 'Conference' and 'Xenia') during pre-harvest maturation and postharvest storage in an ambient atmosphere at 2 °C followed by shelf life was studied using atomic force microscopy (AFM). The results were related to the firmness of fruits, galacturonic acid content in water, chelator, sodium carbonate and insoluble pectin fractions, polygalacturonase and pectin methylesterase activities. The Young's modulus of the primary cell walls decreased linearly during the last month of pre-harvest maturation from 3.2 ± 1.8 to 1.1 ± 0.7 MPa for 'Conference' and from 1.9 ± 1.2 to 0.2 ± 0.1 MPa for 'Xenia' which correlated with linear firmness decrease. During postharvest storage the cell wall Young's modulus increased while firmness continued to decrease. Correlation analysis for the entire period of the experiment showed a lack of straightforward relation between the Young's modulus of primary cell walls and fruit firmness. The Young's modulus of cell walls correlated negatively either with galacturonic acid content in sodium carbonate soluble pectin ('Conference') or with insoluble pectin fractions ('Xenia') and positively with polygalacturonase activity. It was therefore evidenced that covalently linked pectins play the key role for the stiffness of fruit cell walls. Based on the obtained results, the model explaining the fruit transition from firm and crispy to soft and mealy was proposed.