Unmodified Soy Protein Isolate Research Articles

Laccase-catalyzed soy protein and gallic acid complexation: Effects on conformational structures and antioxidant activity

The ability of laccase to oxidize polyphenols arouses our interest that laccase can be applied for protein-polyphenol cross-linking. In this study, laccase promoted the cross-linking of gallic acid (GA) and soy protein isolate (SPI) under neutral pH. SPI-GA complexes changed the secondary structures with a decrease in β-fold and an increase in α-helix and β-turn. The free-radical scavenging activity and reducing power determination results suggested that GA elevated the SPI antioxidant activity significantly. Specifically, DPPH free radical scavenging rate and ABTS free radical scavenging ability increased almost 5- and 1.5-fold compared with unmodified SPI, respectively. Moreover, the reducing power had more than 3-fold compared to the SPI control. This study provided a novel enzyme-induced approach to modulate the physicochemical properties of SPI binding polyphenol.

Self-healable and biodegradable soy protein-based protective functional film with low cytotoxicity and high mechanical strength

Soy protein isolate (SPI) is a natural organic polymer, being used to prepare protective functional materials due to its sustainability, abundance, and easy processing. However, the protective effect of unmodified SPI films is significantly limited due to its poor mechanical properties, irreversible damage, and monotone functionality. Herein, high-flowability polyethyleneimine (PEI) was used to improve the dynamic properties of the protein matrix and cross-linking, and the phenolic hydrogen bonding of polydopamine (PDA) was used to develop a facile strategy for fabricating SPI-based films with satisfactory self-healing properties, high strength, excellent UV barrier performance, and remarkable fluorescence for the heavy metal detection. PEI disrupted the strong hydrogen bonds between protein molecules, causing them to be aggregated due to the PDA cross-linking. Thus, the phenols of PDA formed hydrogen bonds and constructed a reversible network, which endowed the SPI-PEI-PDA films with excellent self-healing properties. Because the hyperbranched PEI imparted the films with more cavities, the microcracks caused by external forces were blunted. Due to the further PDA crosslinking, the SPI-PEI-PDA films exhibited high mechanical strength. The SPI-PEI-PDA films also showed outstanding UV barrier performance and fluorescence properties for the heavy metal detection due to the bionic melanin-like characteristic of PDA andthe Michael addition reaction between the reactants. Additionally, SPI-PEI-PDA films demonstrated the low toxicity in vitro and in vivo, as well as the favorable biodegradability. These films improved the adaptability and safety of protective materials and could greatly promote functionalapplications of other biopolymer-based materials.

Covalent modification of soy protein isolate by (-)-epigallocatechin-3-gallate: effects on structural and emulsifying properties.

Soy protein isolate (SPI) has promising applications in various food products because of its excellent functional properties and nutritional quality. The structural and emulsifying properties of covalently modified SPI by (-)-epigallocatechin-3-gallate (EGCG) were investigated. SPI was covalently modified by EGCG under alkaline conditions. SDS-PAGE analysis revealed that EGCG modification caused crosslinking of SPI proteins. Circular dichroism spectra demonstrated that the secondary structure of SPI proteins was changed by EGCG modification. In addition, the modifications resulted in the perturbation of the tertiary structure of SPI as evidenced by intrinsic fluorescence spectra and surface hydrophobicity measurements. Oil-in-water emulsions of modified SPI had smaller droplet sizes and better creaming stability compared to those from unmodified SPI. The covalent modification by EGCG improved the emulsifying property of SPI. This study therefore provided an innovative approach for improving the emulsifying properties of proteins. © 2018 Society of Chemical Industry.

Improvement in Functional Properties of Soy Protein Isolate-Based Film by Cellulose Nanocrystal⁻Graphene Artificial Nacre Nanocomposite.

A facile, inexpensive, and green approach for the production of stable graphene dispersion was proposed in this study. We fabricated soy protein isolate (SPI)-based nanocomposite films with the combination of 2D negative charged graphene and 1D positive charged polyethyleneimine (PEI)-modified cellulose nanocrystals (CNC) via a layer-by-layer assembly method. The morphologies and surface charges of graphene sheets and CNC segments were characterized by atomic force microscopy and Zeta potential measurements. The hydrogen bonds and multiple interface interactions between the filler and SPI matrix were analyzed by Attenuated Total Reflectance–Fourier Transform Infrared spectra and X-ray diffraction patterns. Scanning electron microscopy demonstrated the cross-linked and laminated structures in the fracture surface of the films. In comparison with the unmodified SPI film, the tensile strength and surface contact angles of the SPI/graphene/PEI-CNC film were significantly improved, by 99.73% and 37.13% respectively. The UV–visible light barrier ability, water resistance, and thermal stability were also obviously enhanced. With these improved functional properties, this novel bio-nanocomposite film showed considerable potential for application for food packaging materials.

Improvement of interfacial interactions using natural polyphenol-inspired tannic acid-coated nanoclay enhancement of soy protein isolate biofilms

In this study, a novel and economic surface modification technique for montmorillonite (MMT) nanosheets, a biocompatible coupling cross-linking agent, was developed on an attempt at improving the interfacial adhesion with soy protein isolate (SPI) matrix. Inspired by natural polyphenol, the “green dip-coating” method using tannic acid (TA) to surface-modify MMT (TA@MMT). SPI nanocomposite films modified with MMT or TA@MMT, as well as the control ones, were prepared via the casting method. The TA layer was successfully coated on the MMT surface through the (FeIII) ions coordination chemistry and the synthetic samples were characterized by the Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The compatibility and interfacial interactions between modified MMT and SPI matrix were greatly enhanced by the TA-FeIII coating on the MMT surface. The mechanical properties, water resistance, and thermal stability of the resultant biofilm were increased accordingly. Compared with that of the unmodified SPI film, the tensile strength of the nanocomposite films modified by the green dip-coating was increased by 113.3%. These SPI-based nanocomposite films showed the favorable potential in terms of food packing applications due to their efficient barriers to water vapor and UV and/or visible light.

Physico-chemical properties improvement of soy protein isolate films through caffeic acid incorporation and tri-functional aziridine hybridization

In this study, tri-functional trimethylolpropane-tris-(2-methyl-1-aziridine) propionate (TMPTAP) and plant-derived caffeic acid (CA) were used as a cross-linking system to simultaneously enhance the strength and retain elongation at break (EB) of soy protein isolate (SPI) films. The cross-linking reactions in the film by CA/TMPTAP system were examined with attenuated total reflectance-Fourier transform infrared spectroscopy, 13C Nuclear Magnetic Resonance, UV–vis spectroscopy, X-ray diffraction, and thermogravimetric analysis. The film morphologies were observed with the scanning electron microscopy and atomic force microscopy. The results indicated that the catechol groups and oxidized quinone in CA readily reacted with the amine/sulfhydryl groups on SPI, while the tri-functional TMPTAP might react with carboxyl groups through aziridine ring-opening reactions. As expected, the modified SPI films by cross-linking with the CA/TMPTAP system simultaneously enhanced the strength and EB of SPI films. The tensile strength and EB of SPI/TMPTAP/CA film, in comparison to untreated SPI film, were improved by 103.0% and 17.1%, respectively, as a result of the relatively ordered three-dimensional cross-linked network among TMPTAP, CA, and SPI. The water absorption of SPI/TMPTAP/CA film also showed a 49.2% reduction compared to that of the unmodified SPI film.

Soy protein isolate-based films reinforced by surface modified cellulose nanocrystal

In this study, biodegradable films were prepared from soy protein isolate (SPI), glycerol, and ethylene glycol diglycidyl ether (EGDE). To enhance the mechanical properties of the films, cellulose nanocrystal (CNC) was employed as reinforcement. CNC was also modified with a silane coupling agent (MCNC) for introducing superficial amino groups. As a result, the interaction between the MCNC and SPI matrix was enhanced by forming the chemical-crosslinking network through the reaction between amino and epoxy groups, thus enhancing physical properties. The mechanical properties, hydrophobic properties, water resistance, and thermal stabilities of the obtained films were then fully analyzed. Results showed that, in a comparison of the un-modified and CNC modified films, the mechanical properties and water resistance of the modified SPI films were improved considerably after the incorporation of MCNC. The tensile modulus and tensile strength of the CNC modified films increased by 128.3% and 53.0%, respectively, compared to the un-modified SPI films, resulted from the formation of hydrogen bonding and physical crosslinking structures between CNC and matrix. The tensile modulus and tensile strength of the MCNC modified films increased by 212.7% and 90.1%, respectively, due to the chemical-crosslinking network achieved among SPI, MCNC, and EGDE. It was also accounted for the improvement of the thermal stabilities and water resistance of the MCNC-modified SPI films.

Preparation and characterization of poly(vinyl alcohol) and 1,2,3‐propanetriol diglycidyl ether incorporated soy protein isolate‐based films

ABSTRACTNovel biodegradable films were prepared from soy protein isolate (SPI), poly(vinyl alcohol) (PVA), glycerol, and 1,2,3‐propanetriol diglycidyl ether (PTGE). The mechanical, hydrophilic, and compatible properties of the films were investigated. The influence of PTGE as a crosslinker on the properties of the SPI/PVA/PTGE films was examined with Fourier transform infrared spectroscopy, X‐ray diffraction (XRD), thermogravimetric analysis, mechanical analysis, contact angle measurements, and scanning electron microscopy. XRD and contact angle examination confirmed that the addition of PTGE altered the film microstructure to a crystalline one. The mechanical properties and water resistance of the SPI/PVA/PTGE films increased notably compared with those of the unmodified SPI films. All results indicate that the networks were formed between SPI and PTGE and played an important role in forming a homogeneous structure in the obtained films. The novel biodegradable films provide a convenient and promising way for preparing environmentally friendly film materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42578.

Thermal modification of soy proteins in the vacuum chamber and wood adhesion

Adhesives based on renewable resources have inferior efficiency compared to the synthetic ones. To improve the adhesive properties of soy protein isolate (SPI) adhesives, a new method of thermally modifying SPI was developed. The commercial SPI powder was thermally modified in a vacuum chamber at different temperatures (50, 100, 150, and 200 °C). Thermally modified and unmodified SPI powders were dispersed in distilled water and stirred at 24, 50 or 90 °C. Dispersions were prepared without and with pH adjustment to 10.0. The viscosity of the resulting adhesives was measured. Effective penetration and bond strength in longitudinal tensile shear were tested with bonded beech specimens. Regardless of the pH value and dispersion preparation temperature, SPI thermally modified at 150 and 200 °C exhibited no adhesive properties, while adhesive bonding was achieved with SPI modified at 50 and 100 °C. Adhesives without pH adjustment had viscosity below 28 mPas and exhibited no adhesive penetration. pH adjustment increased the viscosity and adhesive penetration, which improved the adhesive bond strength. Thermal modification at 50 °C resulted in improved wet shear strength, while the temperature of 100 °C deteriorated it. Increased dispersion preparation temperature to 50 °C increased wet shear strength, while temperature of 90 °C decreased it. The combination of thermal modification at 50 °C, dispersion preparation at 50 °C, and pH increase resulted in an adhesive with the highest shear strength.

Preparation of Denatured Alcohol-Modified SPI-Based Adhesive

An environment-friendly soy protein isolate (SPI)-based adhesive for wood was prepared using denatured alcohol-modified SPI, calcium hydroxide and sodium silicate. On the basis of the single factor test results, an orthogonal test of 4 elements and 3 levels was carried out to optimize formula ingredients. The modified SPI adhesive contained more calcium hydroxide than unmodified SPI adhesive and exhibited good performance. The shear strength of the specimen bonded with the modified adhesive was 82.3 MPa; after three cycles of water-soaking and air-drying, the decrease in shear strength was as low as 7.9%.

Aggregate structure and effect of phthalic anhydride‐modified soy protein on the mechanical properties of styrene‐butadiene copolymer

AbstractThe aggregate structure of phthalic anhydride (PA) modified soy protein isolate (SPI) was investigated by estimating its fractal dimension from the equilibrated dynamic strain sweep experiments. The estimated fractal dimensions of the filler aggregates were less than 2, indicating that these particle aggregates have a distorted or broken two‐dimensional sheet‐like structure. The results also indicated that the aggregate structure has a greater effect on the composite reinforcement than the overall aggregate size. Tensile strength, elongation, Young's modulus, and toughness of hydrolyzed/modified soy composites are comparable with those of carbon black reinforced composites at 10–15% filler fraction. The moduli of PA‐modified SPI composites were less sensitive to the pH of the composite preparation compared to the unmodified SPI. The composites prepared at acidic pH, with lower filler fraction, or filled with hydrolyzed/modified SPI are more elastic and less fatigue. The composites of PA‐modified SPI had better recovery properties when prepared at acidic instead of alkali pH. PA‐modified hydrolyzed SPI composites prepared at acidic pH showed a similar recovery property to that of carbon black reinforced composites, but with greater shear elastic moduli. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Improving Foaming Properties of Yolk‐Contaminated Egg Albumen by Basic Soy Protein

Yolk contamination of egg white is a common problem in the egg breaking industry. Foaming properties of egg white protein are affected by such contamination, but proteins of basic nature may restore the foaming properties of the yolk-contaminated egg white protein. The purpose of this study was to chemically modify a soy protein, that is, to esterify the acidic groups on the protein and to study the potential of such modified protein in improving foaming. We showed that the modification changed the isoelectric point of soy protein isolate (SPI) from 4.5 to about 10. Sonication was proven to be a very effective means to redisperse the methanol-denatured soy protein during reaction, as shown by the improved solubility profile. Such modified basic protein, that is, the sonicated-modified SPI (SMSPI), when added to the yolk-contaminated (at 0.4% level, as-is basis) egg white, gave significantly improved foaming properties. We have shown that the slight change in pH due to the addition of SMSPI was not the reason for improved foaming performance; instead, the modified protein itself was the main reason for such improvement. Addition of SMSPI increased the foaming performance of both pure egg white and yolk-contaminated egg white. SMSPI consistently performed better than the unmodified SPI for improving foaming. Addition of SMSPI (16%, based on dry egg white, and 1.6% based on liquid egg white) fully restored foam expansion and foam liquid stability of 0.4% yolk-contaminated egg white, and it even out-performed the foaming of pure white protein. Therefore, a feasible solution to restore the foaming properties of yolk-contaminated egg white has been identified. It is expected that such modified SPI can be used as an additive or ingredient in foaming formulation, especially when the egg white protein is suspected of lipid contamination.

Effects of Resorcinol On the Mechanical Properties of Soy Protein Isolate Films

Soy protein isolate (SPI) films were formed by cross linking with resorcinol. The resorcinol content was varied and its effect on mechanical properties was measured. The variation of tensile creep modulus with time for different loadings of resorcinol indicated a higher value when the resorcinol content was 10 and 20% of the weight of SPI. The addition of 20% resorcinol led to an overall increase in the tensile strength from 4.9 to 24.7 MPa and modulus increases from 801 to 1358 MPa than SPI films. The percent elongation was also increased from 3.2 to 8.4 when compared to unmodified SPI film. The impact strength was at a maximum and moisture content at a minimum for 20% resorcinol content.

Effects of dietary soy protein on iliac and carotid artery atherosclerosis and gene expression in male monkeys

Male cynomolgus macaques (n=91) consumed an isoflavone (IF)-free, atherogenic control diet containing casein/lactalbumin for 5 months, then were randomized to three groups: control (n=30) continued on the control diet; low IF (n=30) received a mixture of unmodified and IF-depleted soy protein isolate (SPI) (0.94mg IF/g protein, approximating a human intake of 75mg/day); high IF (n=31) received unmodified SPI (1.88mg IF/g protein, approximating a human intake of 150mg/day) for 31 months. Iliac and carotid artery atherosclerosis, and arterial and hepatic mRNA transcripts related to inflammation and estrogen receptors (ER) were measured. Trend analysis identified a significant inverse relationship between dietary IF content and plaque area in the iliac (p<0.05) but not carotid arteries (p>0.13). No significant effect of diet on inflammatory gene or estrogen receptor expression was observed. Plaque area was positively correlated with the mRNA transcript levels for arterial MCP-1, ICAM-1, and the macrophage marker CD68 (all r>0.25, p<0.03), and negatively correlated with ERα and ERβ (all r<−0.23, p<0.03). Coronary artery plaque area appeared to be more closely associated with gene expression patterns of the iliac arteries than the carotid arteries. The data suggests benefits of dietary soy on atherosclerotic plaque development in males may be mediated through inflammation-independent pathways. The negative associations of arterial ERα expression with atherosclerosis lend support to a mechanistic role for estrogen receptors in atherosclerosis susceptibility which merits further study.

Soy protein adhesion enhanced by glutaraldehyde crosslink

AbstractSoy protein has been considered as an alternative to partly replace petroleum‐based polymers for adhesive applications. The weakness of protein‐based adhesive is poor water resistance, which limits its outdoor applications. The objective of this research was to improve the water resistance of soy protein adhesive by introducing crosslinkage between amino groups of amino acid residue. Laboratory prepared soy protein isolate (SPI) was used in this study. Glutaraldehyde at concentrations of 4, 20, 40, and 80 μM was used as the crosslinking reagent for SPI modification. Adhesive properties of soy protein modified by glutaraldehyde, as well as thermal and morphological properties, were investigated. Crosslinking‐induced protein conformation and structure changes through decrease of amino groups and adding of hydrophobic groups, subsequently affect adhesive performance of SPI. At optimum glutaraldehyde concentration (20 μM), dry, wet, and soak strengths of modified SPI increased to 31.5, 115, and 29.7%, respectively, compared with unmodified SPI. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 130–136, 2007

Thermal properties and adhesiveness of soy protein modified with cationic detergent

AbstractThis research studied the effects of cationic detergents on the adhesiveness and thermal properties of soy protein isolate (SPI). Three cationic detergents, hexadecyltrimethyl ammonium bromide, ethylhexadecyldimethyl ammonium bromide (EDAB), and benzyldimethylhexadecyl ammonium chloride, each at concentrations of 1.3, 2.6, 5.2, and 7.8 mM, were used to modify SPI. The effect of pH at selected EDAB concentrations was also studied. Results showed that both detergent concentration and pH had significant effects on the adhesiveness of modified SPI. SPI modified with detergent at a concentration of 2.6 mM yielded the greatest dry tensile strength and water resistance, which indicated that a moderate protein denaturation might be favorable to the adhesion of SPI. Both modified and unmodified SPI showed greater adhesive strength at their optimal pH values. Modified SPI showed greatest adhesive strength at pH 7, whereas unmodified SPI showed greatest adhesive strength at pH 4.5; the tensile strength of modified SPI was greater than that of unmodified SPI. The protein‐denaturation temperature and the enthalpy of modified SPI adhesives were also analyzed by using DSC. Denaturation of the native structure of SPI increased as detergent concentration increased.

Characterization of Phytagel® modified soy protein isolate resin and unidirectional flax yarn reinforced “green” composites

The mechanical properties and moisture resistance of the soy protein isolate (SPI) resin and flax yarn reinforced composites were improved significantly by incorporation of a poly-carboxylic acid based modifier, Phytagel®. SPI and Phytagel® were blended to form an interpenetrating network-like cross-linked complex. This complex showed significantly improved tensile and moisture properties as well as higher thermal stability as compared to the unmodified SPI resin. The incorporation of Phytagel® (40% w/w of SPI powder) in SPI resin and subsequent lowering of the amount of glycerol (from 30% to 12.5%) led to an overall 10-fold increase in the tensile fracture stress and a nine-fold increase in the Young's moduli of the SPI resin along with a seven-fold decrease in fracture strain. The dynamic mechanical properties such as storage and loss modulus of the modified resin increased and the glass transition temperature also increased by about 56°C. The unidirectional flax yarn reinforced composites were made using two modified resins with 20% and 40% Phytagel® contents. Both resins contained 12.5% glycerol. The composites fabricated using resin containing 20% Phytagel® showed significantly higher tensile and flexural moduli as well as fracture stress in the axial direction than the composites with resin containing 40% Phytagel®, which was higher than the SPI resin based composites. POLYM. COMPOS., 26:647–659, 2005. © 2005 Society of Plastics Engineers

Enzymatically-modified soy protein part 2: adhesion behaviour

Adhesive properties of soy protein isolated from soy protein concentrate (SPC) and its modification by proteolytic enzymes (such as papain, trypsin, chymotrypsin and pepsin) and urease were investigated on different types of woods (such as rubberwood, Bhutan pine, teakwood and plywood). Adhesive strength and viscosity of native and enzyme modified soy protein were compared with commercially available adhesives using the same types of woods. Papain and urease modified soy protein isolate (SPI) showed better adhesive strength compared to unmodified SPI adhesive on rubberwood, while chymotrypsin modified SPI showed zero adhesive strength. All enzymatically-modified adhesives showed lower viscosity in comparison to unmodified SPI adhesive. Hydrophobicity of the native and modified SPIs was determined by the sodium dodecyl sulfate (SDS) binding method. Chymotrypsin-modified SPI (CSPI) showed maximum value of hydrophobicity followed by papain and trypsin modified SPI. Urease modified SPI showed only slight increase in hydrophobicity with respect to unmodified SPI. Structural changes after hydrolysis of SPI by different enzymes were investigated by FT-IR spectroscopy. The ratio of the intensity of νc=o of carboxyl and amide I or ratio of νc=o of carboxyl and amide II showed maximum value in case of pepsin modified SPI followed by chymotrypsin modified SPI. Papain, trypsin and urease modified SPIs showed only slight increase in the ratio of carboxyl/amide I or amide II.

Modification of the nitrogen solubility properties of soy protein isolate following proteolysis and transglutaminase cross-linking

The effect of (a) limited hydrolysis [0.5–2.0% degree of hydrolysis (DH)] with Alcalase™, (b) cross-linking with transglutaminase (TGase) and (c) combinations of these modifications on the nitrogen solubility (pH 3–8) of soy protein isolate (SPI) was investigated. Between pH 3.0 and 5.0, SPI hydrolysates, hydrolysates of cross-linked SPI and the cross-linked products of SPI hydrolysates displayed significant ( P<0.05) increases in solubility compared to unmodified SPI. Cross-linking pre- or post hydrolysis did not alter the overall trend of increased ( P<0.05) solubility relative to the unmodified control at low pH. At 2% DH, cross-linking pre- or post-hydrolysis resulted in greater solubility ( P<0.05) than that observed in hydrolysates per se at low pH. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS–PAGE) indicated that the 22 kDa 11S basic polypeptide was relatively resistant to Alcalase hydrolysis and that the 18 and 22 kDa 11S basic polypeptides were not susceptible to TGase cross-linking. The results demonstrate that a combination of enzymatic treatments and the order in which they are applied may have potential for creating novel food ingredients with improved functional properties, especially those properties that are dependant on high solubility at low pH.

Alkali‐modified soy protein with improved adhesive and hydrophobic properties

AbstractAdhesive and hydrophobic properties of alkalimodified soy protein (AMSP) and trypsin‐modified soy protein (TMSP) on wood were investigated. Modifying soy protein (i) under moderate alkaline conditions (pH 10.0 at 50°C) and (ii) with trypsin, enhanced adhesive strengths (730 and 743 N, respectively) compared with unmodified soy protein (340 N). Hydrophobicities of AMSP, TMSP, and unmodified soy protein isolate by sodium dodecyl sulfate binding and 1‐anilino‐8‐naphthalene sulfonate methods were 7.6, 6.4, 5.0 and 39, 27, 13, respectively. Modified soy protein adhesives with higher hydrophobicities (AMSP and TMSP) had enhanced water‐resistance properties.