Citric Acid Crosslinking Research Articles

Citric acid cross-linking of heat-set whey protein hydrogel influences its textural attributes and caffeine uptake and release behaviour

Heat-induced whey protein gels were either pre-loaded with caffeine, then cross-linked with citric acid, or vice versa. Based on the results of Fourier transform infrared spectroscopy, the pre-loading then cross-linking procedure caused less cross-linkage formation than the cross-linking then caffeine loading (post-loaded) procedure. The latter also resulted in a firmer gel. The caffeine-to-protein ratio was approximately 8.5 μg mg−1 in the non-cross-linked hydrogel; those of the caffeine-loaded then cross-linked and the cross-linked then caffeine-loaded hydrogels were 4.5 and 2.9 μg mg−1, respectively. Cross-linking decreased swelling of the freeze-dried gels upon subsequent rehydration. It also influenced the extent of caffeine release from the protein hydrogel. Pre-loaded and post-loaded samples retained ∼78% and ∼88%, respectively, of the entrapped caffeine after immersion in water for 4 h; the non-cross-linked gel retained <55% of the initially loaded caffeine. Caffeine loading decreased gel water-holding capacity; subsequent citric acid cross-linking increased water-holding capacity.

Biocomposite proton‐exchange membrane electrolytes for direct methanol fuel cells

ABSTRACTPoly(vinyl alcohol) (PVA)‐amino acid (AA) biocomposite membranes are prepared by blending PVA with AAs such as glycine, lysine (LY), and phenyl alanine followed by in situ crosslinking with citric acid (CA) and explored as a new class of biocomposite membrane electrolytes for direct methanol fuel cells (DMFCs). CA crosslinks with PVA through esterification offers adequate chemical, thermal, and morphological stability thereby produces methanol‐obstructing close‐packed polymeric network. These biocomposite membranes are characterized in terms of mechanical, thermal, sorption, and proton‐conducting properties. Hydrophilic nature of AA zwitterions significantly facilitates proton conduction and CA crosslinking mitigates methanol crossover through establishing appropriate balance between hydrophilic/hydrophobic domains. The rational design of membrane microstructure with proper arrangement of hydrophobic/hydrophilic domains is a key to enhance electrochemical selectivity of PVA‐AA/CA biocomposite membranes. Biocomposite membrane comprising LY exhibits nearly threefold higher electrochemical selectivity in relation to PVA/CA blend membrane. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43514.

Facile polymerization of β-cyclodextrin functionalized graphene or graphene oxide nanosheets using citric acid crosslinker by in situ melt polycondensation for enhanced electrochemical performance

In this study, we report a facile, environmental friendly route to synthesize water-insoluble β-cyclodextrin (β-CD)/graphene oxide (GO) or reduced graphene oxide (rGO) nanocomposite hydrogels.

어성초 추출물을 이용한 면과 견직물의 항미생물성 염색

Cotton and silk fabrics were dyed with Houttuynia cordata extract using aqueous ethanol solution and the dyeing and post-treatment conditions were optimized to impart antimicrobial activity to the fabrics. The dried Houttuynia cordata can be extracted at for 3 hours using an aqueous ethanol solution containing 70%(w/w) ethanol. For the highest color yields. Both cotton and silk fabrics can be dyed at for 60min with 10g/L of NaCl under pH 4. Silk fabrics can be dyed to higher K/S than cotton fabrics. The color fastness properties of the dyed fabrics were good when either citric acid crosslinking or aluminum alum mordanting was carried out as a post treatment. The dyed silk and cotton fabrics with the post treatments showed excellent antimicrobial activity against both Staphylococcus aureus and Klebsiella pneumoniae.

Non-Toxic Crosslinking of Starch Using Polycarboxylic Acids: Kinetic Study and Quantitative Correlation of Mechanical Properties and Crosslinking Degrees

For the first time, kinetics of citric acid crosslinking of starch was studied, and mechanical properties of starch films were quantitatively correlated with crosslinked degrees and structures of polycarboxylic acids. To substitute toxic crosslinkers to improve the properties of bio-derived polymers, non-toxic polycarboxylic acids were widely used to improve the performance properties of starch-derived industrial products, and were proved effective. However, the mechanism of the reaction and relationship between crosslinking extents and performance properties of starch products had not been clarified. In this study, crosslinking of starch by polycarboxylic acids was verified, and logarithmic relationship between crosslinking degree and strength was elucidated. The polycarboxylic acids with more carboxyl groups could more effectively improve the tensile properties of starch films. In general, crosslinking using polycarboxylic acids might improve the mechanical properties of starch products in a controlled manner and thus could facilitate their industrialization.

Low-temperature crosslinking of proteins using non-toxic citric acid in neutral aqueous medium: Mechanism and kinetic study

For the first time, mechanism and kinetics of citric acid crosslinking of protein in neutral aqueous condition under low temperature are reported. Crosslinking was a conventional method to improve properties of protein products to replace petroleum-derived polymers. However, current crosslinking methods were toxic, expensive or required harsh conditions. Non-toxic carboxylic acid could effectively enhance wet performance properties of multiple protein products under mild conditions, which were critical for preserving the activities during immobilization of enzymes. In this study, more than one carboxyl groups in one citric acid molecule were found capable of reacting with protein in aqueous medium at pH 6.8 under 50 and 75°C, verifying occurrence of crosslinking. The pseudo second-order reaction had activation energy as low as 24.956kJmol−1, indicating its easy occurrence. Mild crosslinking using citric acid might provide alternative to accelerate low-cost and efficient enzyme immobilization as well as industrial utilization of protein-based products.

Characteristics of the bulk hydrogels made of the citric acid cross-linked whey protein microgels

A non-toxic, biocompatible and generally recognized as safe (GRAS) chemical, citric acid, was employed for pH adjustment and cross-linking of whey proteins either before or after their microgelification by heat. A conventionally-made microgel sample was also fabricated by using hydrochloric acid for pH adjustment on 5.8 and subsequent heating at 85 °C. Size approximation of microgel particles from atomic force microscopy images indicated that chemical cross-linking resulted in smaller particles (80–130 nm and 100–150 nm for post- and pre-microgelification cross-linked samples, respectively) compared with conventional counterpart (150–300 nm). Based on Fourier transform infrared spectroscopy results, it was concluded that prolonged citric acid cross-linking of whey proteins prior to microgelification caused extensive cross-linking of protein units and preserved proteins α-helical structure. The pre-cross-linked microgels formed firmer (higher fracture stress and complex modulus) calcium-induced cold-set bulk gel with higher water-holding capacity and denser microstructure compared with conventional and post-cross-linked microgels. These superiorities were attributed to existence of higher number of carboxyl residues in the structure of the pre-cross-linked microgels. The alteration in the properties of the final bulk hydrogels via cross-linking of the microgel building-units can be exploited for modulating and controlling the release behavior of loaded cargo within hydrogel, resulting in a more tuned application of gels as edible delivery systems.

Citric Acid Cross-Linked Nanocellulose-Based Paper for Size-Exclusion Nanofiltration.

This article explores the effect of cross-linking of nanocellulose with citric acid for the development of novel paper filters for potential application within nanofiltration, including sterile (virus) filtration. Cladophora cellulose paper sheets were cross-linked by first soaking in 16 wt % citric acid in the presence of 1 wt % sodium hypophosphate overnight and then curing at 160 °C for 10 min in a hot-press. The cross-linked paper filter samples were then characterized with FTIR, AFM, N2 gas adsorption, and tensile strength analysis (dry and wet strength). The particle retention properties were further studied with respect to filtering of 20 nm Au nanoparticles with SEM and comparing the UV absorbance intensity of the starting solution and the filtrate. The wet strength of the paper filter was greatly improved following the cross-linking, although in the dry state, the paper becomes brittle. The improved wet strength of the paper filter enables increasing the pressure gradient applied for filtration without compromising the integrity of the filter. This is the first report in which a fully nature-derived paper filter is capable of removing tracer particles as small as 20 nm. It is concluded that citric acid cross-linking of nanocellulose is beneficial for developing paper based sterile (virus) removal industrial filters.

Effect of citric acid crosslinking cellulose-based hydrogels on osteogenic differentiation.

Understanding the relationships between material surface properties and cellular responses is essential to designing optimal material surfaces for implantation and tissue engineering. In this study, cellulose hydrogels were crosslinked using a non-toxic and natural component namely citric acid. The chemical treatment induces COOH functional groups that improve the hydrophilicity, roughness, and materials rheological properties. The physiochemical, morphological, and mechanical analyses were performed to analyze the material surface before and after crosslinking. This approach would help determine if the effect of chemical treatment on cellulose hydrogel improves the hydrophilicity, roughness, and rheological properties of the scaffold. In this study, it was demonstrated that the biological responses of human mesenchymal stem cell with regard to cell adhesion, proliferation, and differentiation were influenced in vitro by changing the surface chemistry and roughness.

Ultrafine fibrous gelatin scaffolds with deep cell infiltration mimicking 3D ECMs for soft tissue repair.

In this research, ultrafine fibrous scaffolds with deep cell infiltration and sufficient water stability have been developed from gelatin, aiming to mimic the extracellular matrices (ECMs) as three dimensional (3D) stromas for soft tissue repair. The ultrafine fibrous scaffolds produced from the current technologies of electrospinning and phase separation are either lack of 3D oriented fibrous structure or too compact to be penetrated by cells. Whilst electrospun scaffolds are able to emulate two dimensional (2D) ECMs, they cannot mimic the 3D ECM stroma. In this work, ultralow concentration phase separation (ULCPS) has been developed to fabricate gelatin scaffolds with 3D randomly oriented ultrafine fibers and loose structures. Besides, a non-toxic citric acid crosslinking system has been established for the ULCPS method. This system could endow the scaffolds with sufficient water stability, while maintain the fibrous structures of scaffolds. Comparing with electrospun scaffolds, the ULCPS scaffolds showed improved cytocompatibility and more importantly, cell infiltration. This research has proved the possibility of using gelatin ULCPS scaffolds as the substitutes of 3D ECMs.

Citric acid crosslinking of paper products for improved high-humidity performance

Fibre crosslinking with polycarboxylic acids can be used to improve certain properties of paper products, including wet tensile and compressive strength. In the present work it was proposed that citric acid (CA) crosslinks the cellulosic fibres of linerboard by self-catalysed esterification of cellulosic hydroxyl groups, which makes an additional catalyst unnecessary. An increase in CA dose or curing temperature increased linerboard compressive strength. In CA-treated corrugated board most of the applied CA was esterified with fibres while some CA thermolysis products were also present. A significant portion of the applied CA was unaccounted for. The deficit was attributed to thermolysis to give volatile anhydrides of unsaturated acids. Under cyclic humidity and static compressive loading, CA-treated corrugated boxes showed a greater than three-fold increase in resistance to compressive creep, showing that CA treatment can be used to extend the lifetime of corrugated boxes used for horticultural produce storage.

Development of zero formaldehyde easy care finishing system by using nano titanium dioxide with citric acid and its impact on physical properties

The Performance of nano TiO2 with citric acid cross-linker was assessed by using pad-dry-cure method on cotton fabric. Significant increase in crease recovery performance was observed which was previously only associated with the lengthy ultraviolet irradiation process. The optimum amount of only 0.1 % nano TiO2 was needed with the citric acid to exhibit significant increase in easy care performance of the fabric. Typically, application of a cross-linker to cellulosics will impart a deleterious effect on the softness of the fabric; however, incorporation of nano-TiO2 with the citric acid cross-linker significantly improved the softness of the fabric which was reflected in the mean deviation of coefficient of friction (MMD) and interyarn friction (2HG5) KES-F values. In addition, there was improvement in tensile strength retention of the fabric as well.

The effect of pH on hydrolysis, cross-linking and barrier properties of starch barriers containing citric acid

Citric acid cross-linking of starch for e.g. food packaging applications has been intensely studied during the last decade as a method of producing water-insensitive renewable barrier coatings. We managed to improve a starch formulation containing citric acid as cross-linking agent for industrial paper coating applications by adjusting the pH of the starch solution. The described starch formulations exhibited both cross-linking of starch by citric acid as well as satisfactory barrier properties, e.g. fairly low OTR values at 50% RH that are comparable with EVOH. Furthermore, it has been shown that barrier properties of coated papers with different solution pH were correlated to molecular changes in starch showing both hydrolysis and cross-linking of starch molecules in the presence of citric acid. Hydrolysis was shown to be almost completely hindered at solution pH≥4 at curing temperatures≤105°C and at pH≥5 at curing temperatures≤150°C, whereas cross-linking still occurred to some extent at pH≤6.5 and drying temperatures as low as 70°C. Coated papers showed a minimum in water vapor transmission rate at pH 4 of the starch coating solution, corresponding to the point where hydrolysis was effectively hindered but where a significant degree of cross-linking still occurred.

오미자 열매 추출액의 염색을 통한 면과 견직물의 항미생물성 가공

Cotton and silk fabrics were dyed with Schizandra chinensis fruit extract using the optimized extraction and dyeing conditions in order to impart functional properties such as deodorization and antimicrobial activity to the fabrics. The extracts were easily obtained from the dried Schizandra chinensis fruits by treating them with pure ethanol at <TEX>$80^{\circ}C$</TEX> for 60 min. When dyed at <TEX>$130^{\circ}C$</TEX> for 60 min under pH 3 with a 10% aqueous ethanol solution that contains the extract and salt concentrations of 5.5% and 20% owf, silk fabrics could be dyed to obtain a color depth (K/S) of 14.2. Cottons were dyed to a K/S of 13.4 under the same dyeing conditions except for a pH of 9. The poor color-fast properties of the dyed fabrics were improved by a combined treatment of citric acid crosslinking and post-mordanting with aluminum alum. Moreover, the dyed silk and cotton fabrics showed excellent ammonia deodorization performance and antimicrobial activity against both Staphylococcus aureus and Klebsiella pneumoniae.

Performance enhancement of fire retardant finish with environment friendly bio cross-linker for cotton

Abstract N -methylol dimethylphosphonopropionamide, commercially famous as Pyrovatex, is among the most commonly used fire retardant for cotton fabric. Unfortunately, Pyrovatex has the issue of releasing formaldehyde and further requires formaldehyde based cross-linkers to improve the fire retardancy, durability, and easy care performance. Consequently, toxic formaldehyde level is significantly increased in the treated cotton fabric. In our research, one bath, par-dry-cure method was used for the application and fixation of the fire retardant and cross-linker. Citric acid was used as a formaldehyde free cross-linking agent for cotton. Sodium hypophosphite (SHP) was used as catalyst for cross-linking of citric acid with cotton. In the presence of citric acid and catalyst with Pyrovatex, fire retardancy of the fabric was increased due to the enhanced bonding of Pyrovatex with cotton. In addition, phosphorous based catalyst SHP further increased the phosphorous in the system; consequently, fire retardancy is further improved. Therefore, only 200 g/L of Pyrovatex with citric acid and catalyst exhibited much better fire retardancy than using 400 g/L of the Pyrovatex alone. The crease recovery angle of the 200 g/L Pyrovatex treated fabric was raised from 188° to 239° when 80 g/L of citric acid was also incorporated into the recipe. Washing durability and tensile performance of the combination treated samples were better as compared to standard fabric.

Biothermoplastics from hydrolyzed and citric acid Crosslinked chicken feathers

We demonstrate a novel approach of developing thermoplastic films from feathers by alkaline hydrolysis and crosslinking with citric acid. Unlike previous approaches that used toxic chemicals, complicated and/or expensive methods to develop films, in this research, feathers were hydrolyzed using various concentrations of alkali and the hydrolyzed feathers were compression molded into films using glycerol as plasticizer and crosslinked with citric acid to improve water stability. Alkali hydrolyzed feathers could be compression molded into films with tensile strength of 5.9MPa and elongation of 31.7% but had poor wet strength. Feather films crosslinked with citric acid had tensile strength of 1.9MPa and elongation of 24.6% after being in 90% humidity at 21°C for 24hours. Alkaline hydrolysis and citric acid crosslinking provides an opportunity to develop inexpensive and biodegradable thermoplastics from the inexpensive, renewable and sustainable poultry feathers.

Water‐stable electrospun collagen fibers from a non‐toxic solvent and crosslinking system

Cytocompatible and water-stable ultrafine collagen fibers were electrospun by dissolving collagen in a low corrosive ethanol-water solvent and crosslinked by citric acid (CA) with glycerol as the crosslinking extender. Conventional solvents used for electrospinning of collagen either cause denaturation or contain more than 50% salt potentially leading to poor mechanical properties and water stability of the scaffolds. Collagen scaffolds have to be modified by techniques, such as, crosslinking to overcome the limitations in strength and stability. However, the existing crosslinking methods are either cytotoxic or ineffective. In this research, a benign ethanol-water solvent system and an extender-aided CA crosslinking method were developed. The native collagen conformation was retained after electrospinning, and the dry/wet strengths and water stability of fibers were substantially enhanced after crosslinking. The crosslinked electrospun scaffolds could maintain their fibrous structure for up to 30 days in phosphate-buffered saline at 37°C. Cells exhibited better attachment and growth on the CA crosslinked collagen fibers than on the glutaraldehyde crosslinked scaffolds.

Influence of Weight Ratio of Citric Acid Cross Linker on the Structure and Conductivity of the Crosslinked Polymer Electrolytes

A novel kind of efficient solid polymer electrolyte (SPE) based on crosslinked polyester matrix has been prepared by employing low molecular weight PEG (oligo-PEG, Mw = 400 g/mol), followed by crosslinking of the PEG with citric acid (CA). The oligo-PEG, with small coil size, can be easily penetrated into mesopores of TiO2 photoelectrode, while the mechanical stability of the SPE can also be maintained by crosslinking. The factor of weight ratio of CA cross linker in the hybrid plays an important role in determining the intersegmental distance and free volume of the polymer matrix, which sequentially affects the electrochemical activity of the conductive ions and then the ionic conductivity of the polymer electrolyte. By using the 32.4 wt.% CA in the matrix, the SPE with the optimal room temperature conductivity (σ) of 5.43×10-5 S/cm was obtained.

Enhancement of wrinkle recovery angle of cotton fabric using citric acid cross-linking agent with nano-TiO2 as a co-catalyst

In this research, the non-formaldehyde wrinkle-resistant treatment of cotton fabrics has been investigated using the citric acid (CA) as a cross-linking agent and sodium hypophosphite (SHP) as a catalyst together with nano-titanium dioxide (nano-TiO2) as a co-catalyst compound. The effect of the changes in the concentrations of CA, SHP, and nano-TiO2 on the wrinkle recovery angle (WRA), tensile strength, tearing strength, whiteness index, and flexural rigidity of cotton fabrics was studied using Box–Behnken design. It was found that the addition of nano-TiO2 could enhance the wrinkle resistance and decrease the flexural rigidity of the cotton fabric with little effect on the whiteness index, and tearing and tensile strengths of the treated fabric. The developed empirical models are found to be in good correlation with the selected variables ( r2 ≥ 0.8). From this study, it was concluded that 10% CA, 10% sodium dihydrogen phosphate, and 0.1% nano-TiO2 were the optimum concentrations required to enhance the WRA without adversely affecting the other physical properties of the treated cotton fabrics.

Formaldehyde-Free Chemical and Enzymatic Crosslinking of Plant Proteins

This paper discusses various methods of formaldehyde-free crosslinking of plant proteins and the chemistry of the crosslinking reaction based on recent developments. Currently, there are increasing demands for replacing petroleum-derived raw materials with renewable biodegradable materials. The abundant availability of zein, soy protein, wheat gluten and gliadin from industrial byproducts at low prices leads to the increase of use of renewable plant proteins to produce zein, soy protein, wheat gluten and gliadin fibers. However, poor mechanical properties and weak water stability of these protein fibers are not suitable to meet the needs of industry application. It is necessary to strengthen these renewable protein fibers. This paper reviews various methods related to formaldehyde-free crosslinking of plant proteins. A critical analysis is given on the kinetic study of glutaraldehyde crosslinking of proteins. In this paper, we also discuss a green catalyst for citric acid crosslinking of protein fibers at mild temperatures, the quantitative relationship between the reaction parameters and mechanical properties of crosslinked gliadin fibers, and changes in the molecular weight of the proteins.