Rheological behaviour and microstructure of microfibrillated cellulose suspensions/low-methoxyl pectin mixed systems. Effect of calcium ions

Effect of pectin modification on the properties of cellulose nanofibrils prepared from citrus peel

Cellulose nanofibrils (CNFs) from non-woody biomass, including citrus peel (CpCNFs), are promising naturally occurring nanomaterials; however, their properties depend on the composition of non-cellulosic components, including pectin. In this study, the effects of pectin modifications on CpCNFs were examined, including demethylesterification using alkaline treatment and enzymatic degradation of pectin using pectinase. CpCNFs could be redispersed in water with little aggregation after drying; however, the redispersibilities of both alkaline-treated (AT-CpCNFs) and pectinase-treated CpCNFs (PT-CpCNFs) were improved. Both AT-CpCNFs and PT-CpCNFs exhibited higher viscosity than untreated CpCNFs (UT-CpCNFs); redispersion in water after drying further increased the viscosity. A quartz crystal microbalance revealed that interactions between AT-CpCNFs were barely detectable, and interactions between PT-CpCNFs were stronger than those between UT-CpCNFs. The increase in the carboxylate groups of pectin due to demethylesterification in AT-CpCNF may have increased the viscosity and reduced the interactions between AT-CpCNFs, explaining the improved redispersibility. The increase in the viscosity of PT-CpCNFs may be attributed to the increased purity of CNFs, which is assumed to be more viscous than pectin. Our results show that the properties of CpCNFs are affected by the structure, properties, and content of pectin and can be controlled by pectin modification.

Immunocytochemical evaluation of aquaporins and cell wall components and their influence on foliar water uptake in Andean Melastomataceae

Andean ecosystems are characterized by high humidity, mainly from rain and fog events. Because of differences in altitude two Andean ecosystems - sub-Andean forest and Páramo -face different environmental pressures that affect leaf anatomy and cell wall composition and, consequently, species foliar water uptake (FWU) capacity. Here, FWU capacity of eight species in the Melastomataceae was evaluated and found to be related to proportions of cell wall components and aquaporins in the two ecosystems. Cellulose was labelled with Calcofluor white, and aquaporin and pectins were labelled with monoclonal antibodies. There were differences in plant FWU capacity in both ecosystems, with higher FWU capacity in sub-Andean forest species than in Páramo forest species. Cell wall components were positively related to FWU, with increased FWU related to pectin and aquaporin content of the plasma membrane. Differences in water availability in the two analysed environments led to differences in FWU capacity that are associated with leaf anatomical traits and cell wall composition. In these two environments, plants with similar traits are selected to respond to given environmental pressures. Traits that favour FWU in sub-Andean forest species may lead to further advances of these species in this environments.

Mapping 3D Printability of Ionically Cross-Linked Cellulose Nanocrystal Inks: Architecting from Nano- to Macroscale Structures

Engineering the rheological properties of colloidal inks is one of the main challenges in achieving high-fidelity 3D printing. Herein, we provide a comprehensive study on the rheological behavior of inks based on cellulose nanocrystals (CNCs) in the presence of given salts to enable high-quality 3D printing. The rheological properties of the CNC suspensions are tailored by considering the nature of the electrolyte (i.e., 10 types of salts featuring different ion sizes, charge numbers, and inter- and intra-molecular interactions) at various concentrations (25-100 mM). A high printing fidelity is achieved in a narrow CNC and salt concentration range, significantly depending on the salt type. The structure-property relationship is explored in a "3D-printing" space (2D map), introducing a guideline for researchers active in this field. To further unravel the effect of salt type on morphological properties, CNC aerogels are developed by freeze-drying the printed structures. The results illustrate that enhancing viscoelastic properties render a denser structure featuring smaller pores.

Building an extensible cell wall

This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.

A novel environmentally friendly nanocomposite aerogel based on the semi-interpenetrating network of polyacrylic acid into Xanthan gum containing hydroxyapatite for efficient removal of methylene blue from wastewater

Eco-friendly nanocomposite aerogels were prepared as adsorbents for the removal of a model pollutant (methylene blue, MB) from water. These aerogels were comprised of hydroxyapatite (HA) nanoparticles embedded within a polymer matrix consisting of a semi-interpenetrating network of xanthan gum (XG) and polyacrylic acid (PAA). Microscopy and BET analysis showed that the aerogels formed had a nanofibrous porous microstructure with a surface area of 89 m²/g. Rheological analysis showed that the aerogels were viscoelastic materials whose elasticity increased with increasing HA concentration (up to 5 w/w%). The aerogels were effective at removing MB from water, exhibiting an adsorption capacity of 130 mg/g after 200 min. The binding of the MB to the aerogels was mainly attributed to hydrogen bonding and electrostatic attraction. A reusability test showed that the MB removal efficiency of over 86% was preserved after 10 cycles of adsorption-desorption. These results suggest that our nanocomposite aerogels may be useful for the efficient removal of anionic pollutants from wastewater and water supplies due to their ease of synthesis, cost-effectiveness, good mechanical properties, high thermal stability, and good adsorption performance.

Effects of Salts on Structural, Physicochemical and Rheological Properties of Low-Methoxyl Pectin/Sodium Caseinate Complex

The addition of salts is an effective way to improve the properties of polysaccharide/protein complexes for use in foods. However, there is no comparative study on the effects of different ions on the complex system of low methoxyl pectin (LMP)/ sodium caseinate (CAS) complex. The effects of different concentrations of three salt ions (Na⁺, K⁺, Ca²⁺) on the physicochemical and rheological properties of the LMP/CAS complex were determined in this study, and the structure of LMP/CAS complex was characterized. The results showed that the addition of these three salt ions affected zeta potential, particle size, and turbidity of the LMP/CAS complex, and lead the LMP/CAS complex to form a more regular and uniform network structure, which helped improve its stability, solubility, and rheological properties. The particle size and turbidity value of the complex achieved with Ca²⁺ were higher than those obtained using Na⁺ and K⁺. Moreover, the secondary structure of the proteins in the complex changed to adding high concentrations of Ca²⁺. Our study provides valuable information for the application of the LMP/CAS complex in the food industry.

Fundamentals of composites containing fibrous materials and hydrogels: A review on design and development for food applications

The combination of fiber and hydrogel in a system can provide substantial benefits for both components, including the development of three-dimensional structures for the fiber, followed by modifications in the rheological and mechanical properties of the hydrogel. Despite a large increase in the use of fiber-hydrogel composites (FHCs) in various sciences and industries such as biomedicine, tissue engineering, cosmetics, automotive, textile, and agriculture, there is limited information about FHCs in the realm of food application. In this regard, this study reviews the mechanism of FHCs. The force transmission between fiber and hydrogel, which depends on the interactions between them during loading, is the main reason to enhance the mechanical properties of FHCs. Moreover, articles about such FHCs that have the potential for foods or food industries have been described. Additionally, the information gaps about edible FHCs were highlighted for further research. Finally, the methods of fiber formation have been summarized.

Pectin-microfibrillated cellulose microgel: Effects on survival of lactic acid bacteria in a simulated gastrointestinal tract

Using high pressure microfluidization, we prepared micro-fibrillated soybean cellulose (MFSC) and analyzed its morphology and structure. MFSC was then incorporated into low-methoxyl pectin (PC) to coat lactic acid bacteria (LAB) by ionotropic gelation, and the effects of PC-MFSC microgel on LAB survival in a simulated gastrointestinal tract were investigated. Particle size analysis showed that the MFSC particle size decreased significantly with increasing jet pressure. Transmission electron microscopy analysis indicated that many cellulosic microfibers appeared at 150 MPa. Infrared spectroscopy and X-ray diffraction analysis revealed that the crystal structure changed from β-cellulose I type to II type with increasing jet pressure, but excessive pressure (200 MPa) damaged the crystalline structure of MFSC. Scanning microscopy indicated that cellulosic microfibers not only promoted a compact pectin gel morphology but also adhered to and coated the LAB in the pectin gel. MFSC-150 stabilized the pectin gel network, preventing the weakening of the gel under low pH conditions. Compared with other PC-MFSCs, PC-MFSC-150 microgel significantly decreased LAB susceptibility to gastrointestinal juice and increased the viability of LAB.

Mobility of pectin methylesterase in pectin/cellulose gels is enhanced by the presence of cellulose and by its catalytic capacity

The pectin methylesterase action is usually studied in a homogeneous aqueous medium in the presence of a large excess of soluble substrate and water. However in the cell wall, the water content is much lower, the substrate is cross-linked with itself or with other polymers, and the enzyme has to diffuse through the solid matrix before catalysing the linkage breakdown. As plant primary cell walls can be considered as cellulose-reinforced hydrogels, this study investigated the diffusion of a fungal pectin methylesterase in pectin/cellulose gels used as cell wall-mimicking matrix to understand the impact of this matrix and its (micro) structure on the enzyme’s diffusion within it. The enzyme mobility was followed by synchrotron microscopy thanks to its auto-fluorescence after deep-UV excitation. Time-lapse imaging and quantification of intensity signal by image analysis revealed that the diffusion of the enzyme was impacted by at least two criteria: (i) only the active enzyme was able to diffuse, showing that the mobility was related to the catalytic ability, and (ii) the diffusion was improved by the presence of cellulose in the gel.

Composite Gels Containing Whey Protein Fibrils and Bacterial Cellulose Microfibrils

In this study, we investigated the gelation of WPI fibrils in the presence of bacterial cellulose (BC) microfibrils at pH 2 upon prolonged heating. Rheology and microstructure were investigated as a function of BC microfibril concentration. The presence of BC microfibrils did not influence the gelation dynamics and resulting overall structure of the WPI fibrillar gel. The storage modulus and loss modulus of the mixed WPI‐BC microfibril gels increased with increasing BC microfibril concentration, whereas the ratio between loss modulus and storage modulus remained constant. The WPI fibrils and BC microfibrils independently form two coexisting gel networks. Interestingly, near to the BC microfibrils more aligned WPI fibrils seemed to be formed, with individual WPI fibrils clearly distinguishable. The level of alignment of the WPI fibrils seemed to be dependent on the distance between BC microfibrils and WPI fibrils. This also is in line with our observation that with more BC microfibrils present, WPI fibrils are more aligned than in a WPI fibrillar gel without BC microfibrils. The large deformation response of the gels at different BC microfibril concentration and NaCl concentration is mainly influenced by the concentration of NaCl, which affects the WPI fibrillar gel structures, changing form linear fibrillar to a particulate gel. The WPI fibrillar gel yields the dominant contribution to the gel strength.

Mixed gels from whey protein isolate and cellulose microfibrils

Whey proteins can form different gel structures ranging from fine-stranded to particulate when appropriate conditions are applied. By incorporating polysaccharides, the gelation of WPI can be influenced. We investigated the heat-induced gelation of whey protein isolate (WPI) in the presence of bacterial cellulose (BC) microfibrils at pH 7 at different concentrations of NaCl. Our results showed that WPI and BC microfibrils form a homogeneous dispersion at pH 7. Upon heating, the WPI gel was formed independently in the presence of the BC microfibril gel, resulting in the formation of a composite gel. The gel structure and gelation dynamics of WPI was not influenced by the presence of BC microfibrils. However, the presence of BC microfibrils increased the storage modulus of the WPI gel, with an increase being negligible when the strength of the WPI gel is above a certain value. With an increase of NaCl concentration, the WPI gel structure changes from fine-stranded to a particulate gel, while the BC microfibril gel structure remains unchanged. No macroscopic phase separation could be observed in the WPI-BC microfibril gels. Our results showed that the rheological properties and water holding capacity of the WPI-BC microfibril mixed gels are mainly dominated by the WPI.

Characterization of Nanofibrillated Cellulose Produced by Different Methods from Cabbage Outer Leaves

The potential use of cabbage outer leaves as a starting material for the production of nanofibrillated cellulose (NFC) was investigated. Chemical-free pretreatment methods, namely, autoclaving, ultrasonication (US), and high-shear homogenization (HS), were applied to remove noncellulosic components from cabbage cell walls prior to defibrillation, which was conducted by subjecting a pretreated sample to HS and then high-pressure homogenization. The sample undergone autoclaving at 130 °C for 2 hr was noted to contain a significantly higher cellulose content (36.5% dry mass) compared with the fresh leaves (12.48% dry mass). This led to an increase in the crystallinity index (CI) of the autoclaved cabbages from 30.8% to 50.7%. Further increase in the cellulose content (47.0% to 49.2% dry mass) was observed when subjecting the autoclaved sample to either US at 37 kHz for 1 hr, HS at 3800 × g for 15 min or HS followed by US at the aforementioned conditions. After pretreatment and defibrilllation, a suspension of NFC with the diameters of 4 to 50 nm was obtained, with the CI of 59.1% to 66.7%. Such a suspension exhibited a gel-like behavior with tan δ in the range of 0.12 to 0.13; the suspension exhibited a similar behavior to that prepared by the conventional chemical pretreatment method.

PRACTICAL APPLICATION

NFC could be produced from cabbage outer leaves, which are an abundantly available by-product of a vegetable processing plant, via the combined hydrothermal and mechanical pretreatment without the use of any chemicals. This chemical-free preparation process is highly desirable as it leaves no residues in the product and causes no chemical waste that needs to be treated. Cabbage-based NFC also exhibits similar characteristics to that prepared via a chemically treated route.

Pectin and cellulose cell wall composition enables different strategies to leaf water uptake in plants from tropical fog mountain

Leaf water uptake (LWU) has been observed in plants of different ecosystems and this process is distinct among different species. Four plant species from the Brazilian fog mountain fields were evaluated in order to detect if leaf water uptake capacity is related to the cell wall composition of leaf epidermis. LWU measurements and their relation to anatomical and biochemical traits were analyzed. Cell wall composition was verified through immunocytochemistry using monoclonal antibodies recognizing pectin compounds, and histochemistry with calcofluor white to track cellulose. Differences in LWU among the four species were clearly revealed. Two species presented higher maximum leaf water content and the lowest values of water absorption speed. The other two species presented opposite behavior, namely, low leaf water uptake and the highest values of water absorption speed. The anatomical traits associated with the cell wall composition corroborated the data on the different LWU strategies. The species with abundant detection of cellulose in their epidermal cell walls absorbed more water, but more slowly, while those with abundant detection of pectins absorbed water at a higher speed. These results indicate that cell wall composition regarding pectin and cellulose are significant for water uptake by the leaf epidermis. Pectin provides greater porosity and absorption speed, while cellulose provides greater hydrophilicity and greater water uptake capacity. Current data indicate that the composition of epidermal cell walls is a relevant trait for leaf water uptake.

Pectin as a rheology modifier: Origin, structure, commercial production and rheology

Pectins are a diverse family of biopolymers with an anionic polysaccharide backbone of α-1,4-linked d-galacturonic acids in common. They have been widely used as emulsifiers, gelling agents, glazing agents, stabilizers, and/or thickeners in food, pharmaceutical, personal care and polymer products. Commercial pectin is classified as high methoxy pectin (HMP) with a degree of methylation (DM) >50% and low methoxy pectin (LMP) with a DM <50%. Amidated low methoxy pectins (ALMP) can be obtained through aminolysis of HMP. Gelation of HMP occurs by cross-linking through hydrogen bonds and hydrophobic forces between the methyl groups, assisted by a high co-solute concentration and low pH. In contrast, gelation of LMP occurs by the formation of ionic linkages via calcium bridges between two carboxyl groups from two different chains in close proximity, known as the 'egg-box' model. Pectin gels exhibit Newtonian behaviour at low shear rates and shear-thinning behaviour when the shear rate is increased. An overview of pectin from its origin to its physicochemical properties is presented in this review.

Physicochemical properties of modified citrus pectins extracted from orange pomace

Modified pectin is a polysaccharide rich in galacturonic acid altered by pH adjustment and thermal treatment used especially as an anti-cancer agent. The aim of this work was to study the physical and chemical properties of modified pectins extracted from orange pomace with citric and nitric acids. The galacturonic acid content, degree of esterification, Fourier Transform Infrared Spectroscopy profile, molecular weight, intrinsic viscosity, rheological properties and antioxidant activity of the pectins were evaluated. The modification process caused the de-esterification of pectins and a decrease of molecular weight due to removal of neutral sugars, maintaining the linear chain of galacturonic acid. Such changes also caused a significant increase in the in vitro antioxidant activity (p ≤ 0.05) and influenced the rheological properties of pectin, reducing its viscosity. This work showed that the modification of pectin from orange pomace with citric and nitric acids altered its structural and physical characteristics as well as its biological activity toward a free-radical.

Methylesterase behaviour is related to polysaccharide organisation in model systems mimicking cell walls

Pectin gels and pectin-cellulose binary gels were used as cell wall-mimicking systems to investigate the diffusion ability of a fungal pectin methylesterase. Increasing content of cellulose in the gel appears to result: (i) in longer demethylated blocks thus favouring AaPME processivity, and (ii) in accelerated enzyme kinetics. To better understand this unexpected behaviour, a method was set up to investigate the gel porosity as a function of the cellulose content by following the passive diffusion of three pullulans having different hydrodynamic volumes. Like the enzyme, the pullulans diffused more efficiently in the gels containing the highest proportions of cellulose. Altogether, these results suggest that the gel settled differently during formation according to the respective proportions of the two polysaccharides. With cellulose present, a fraction of pectin would form close interactions with the microfibrils resulting in a larger volume accessible to diffusing molecules. This volume would be related to the cellulose concentration.

Pectin/carboxymethyl cellulose/microfibrillated cellulose composite scaffolds for tissue engineering

Highly porous three-dimensional scaffolds made of biopolymers are of great interest in tissue engineering applications. A novel scaffold composed of pectin, carboxymethyl cellulose (CMC) and microfibrillated cellulose (MFC) were synthesised using lyophilisation technique. The optimised scaffold with 0.1% MFC, C(0.1%), showed highest compression modulus (~3.987 MPa) and glass transition temperature (~103 °C). The pore size for the control scaffold, C(0%), was in the range of 30-300 μm while it was significantly reduced to 10-250 μm in case of C(0.1%). Using micro computed tomography, the porosity of C(0.1%) was estimated to be 88%. C(0.1%) showed excellent thermal stability and lower degradation rate compared to C(0%). The prepared samples were also characterised using XRD and FTIR. C(0.1%) showed controlled water uptake ability and in vitro degradation in PBS. It exhibited highest cell viability on NIH3T3 fibroblast cell line. These results suggest that these biocompatible composite scaffolds can be used for tissue engineering applications.