Role of β-conglycinin and glycinin subunits in the pH-shifting-induced structural and physicochemical changes of soy protein isolate

Soy β-conglycinin (7S) and glycinin (11S) were incubated up to 4 h in acidic (pH 1.5 to 3.5) or alkaline (pH 10 to 12) solutions to induce protein structural unfolding followed by refolding 1 h at pH 7.0, a process known as pH-shifting. The pH-shifting markedly increased (P < 0.05) emulsifying activity of 11S and to a lesser extent 7S; the former also produced more uniform oil droplets. The emulsifying activity improvements were accompanied by a significant rise in protein surface hydrophobicity, slight loss of the secondary structure (circular dichroism), and substantial dissociation of disulfide-linked basic and acidic 11S subunits. The findings suggested that 11S globulins of soy protein isolate (SPI) were more responsive to pH-shifting treatments than were 7S globulins, and the resulting emulsifying activity enhancements of 11S, in parallel with that of SPI, were indicative of its determinant role in the overall emulsifying properties of pH-shifting-treated SPI. PRACTICAL APPLICATION: Extreme alkaline (pH 12) and acidic (pH 1.5) medium treatments can significantly modify the structure and enhance the emulsifying properties of both β-conglycinin and glycinin components of SPI. The functionality improvement by the pH processes is more remarkable for the glycinin protein fraction. Therefore, SPI enriched with glycinin seems to be particularly suitable for extreme acidic or alkaline processes to produce surface-active functional ingredients for food applications.     

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Characterization of supercritical fluid extrusion processed rice–soy crisps fortified with micronutrients and soy protein

Protein and micro-nutrients enriched rice–soy crisps (RSC) were prepared using supercritical fluid extrusion and their impact on quality attributes was determined. A low-shear, twin screw, co-rotating extruder was used to produce puffed RSC using supercritical CO₂ (SC-CO₂), which served as an expansion agent during the process carried out at lower temperatures (∼100 °C) compared to conventional steam based extrusion (∼130–180 °C). The fortified RSC contained 25–40 g/100 g soy protein and four micronutrients (iron, zinc, vitamin A and C) at the recommended daily values in 100 g product. The RSC were analyzed for physical characteristics and nutrient composition. The increasing soy protein fortification from 25 to 40 g/100 g reduced the crisps expansion ratio (4.27–2.95), crispiness (15.0–9.5), and increased piece density (0.21–0.27 g/cm³), bulk density (0.17–0.22 g/cm³) and hardness (76.39–129.05 N). The nutrient fortification improved protein (334–568%) and dietary fiber (571–901%) and the extrusion process retained all of the added minerals and about 50% retention of vitamin A and C in the final products. The SC-CO₂ assisted extrusion is an effective process-based approach to produce low-moisture, fortified crispy products. These products are appropriate for consumption as nutribars especially for school lunch programs in developing countries to reduce malnutrition through process based nutrient fortification approaches.     

Aggregation kinetics and ��-potential of soy protein during fractionation

The mechanism of soy protein fractionation was explored. A Focused Beam Reflectance Measurement (FBRM) technique was used to study the effects of Ca²⁺ and Mg²⁺ concentrations on the aggregation/precipitation processes of soy globulins. An electrophoretic technique was used to provide information about the electrical charge of the resulting individual protein precipitate. FBRM measurements demonstrated the presence of a two-step process for glycinin (around 90% purity) aggregation. First, in the absence of Ca²⁺ and Mg²⁺, about 9730 counts of the primary particles (~ 3.33 ??m) per second were formed immediately, with the pH being adjusted to 5.8 for glycinin precipitation. Second, more than 90% of the primary particles aggregated slowly within approximately 15 min into larger secondary particles (~ 54.72 ??m). The addition of Ca²⁺ and Mg²⁺ significantly increased the amount of the primary aggregate of soy globulins and altered their aggregation process. However, the number of primary particles induced by Ca²⁺ was smaller than that by Mg²⁺. The aggregation kinetic pattern for soy globulins in the second precipitation step (mainly ??-conglycinin) significantly differed from that of the first precipitation step (mainly glycinin) in the stability of secondary particles. Electrophoretic measurements showed that, as Ca²⁺ and Mg²⁺ concentrations were increased, the net ??-potential of the glycinin-rich fractions decreased, and the reduced effect by using Ca²⁺ was greater than that by using Mg²⁺. The different influences on the ??-potential for soy protein between Ca²⁺ and Mg²⁺ were probably due to their different influences on phytate precipitation, respectively. The combined effects of Ca²⁺ and Mg²⁺ and phytate led to the reduction in the ??-potential and thus to colloidal stability for soy globulins, resulting in more protein precipitation.     

Study of the retention and release of n-hexanal incorporated into soy protein isolate–lipid composite films

This work deals with the study of the kinetic of aroma release, which had been previously incorporated into soy protein isolate (SPI)–lipid composite films. The aim was to determine the influence of type and amount of lipidic material on aroma (n-hexanal) release and retention, as well as the apparent diffusion coefficients. To carry out this study it have been employed SPI-based films containing two SPI:LIPID ratios (1:0.25 and 1:0.5), and two types of lipids, oleic acid (OA) and beeswax (BW), in OA:BW ratios 100:0, 70:30, 50:50, 30:70 and 0:100. The measurements were performed by a gas chromatography technique. The films that showed more retention were SPI:LIPID 1:0.5 100% BW and control film. Concerning release rate, films containing BW as unique lipid material gave the lowest aroma release rate. Apparent diffusion coefficients (D_app) of all films are in the same order of magnitude (10⁻¹⁵ m² s⁻¹). D_app decreases when BW increases in the film matrix and when oleic acid amount decreases. In conclusion, for encapsulating n-hexanal, SPI-BW films demonstrated the best performances, followed by control film (without lipids).     

Determination and quantification of the kokumi peptide, γ-glutamyl-valyl-glycine,in commercial soy sauces

Recent studies have demonstrated that kokumi substances, such as glutathione, are perceived through the calcium-sensing receptor (CaSR), and screening by CaSR assay and sensory evaluation has shown that γ-glutamyl-valyl-glycine (γ-Glu-Val-Gly) is a potent kokumi peptide. In this study, the contents of γ-Glu-Val-Gly in six commercial brands of dark-coloured soy sauces, two brands of light-coloured soy sauce, and one brand of white soy sauce, were investigated by high performance liquid chromatography–tandem mass spectrometry (LC/MS/MS), followed by derivatization with 6-aminoquinoyl-N-hydroxysuccinimidyl-carbamate (AQC). The analyses indicated that γ-Glu-Val-Gly was present in all investigated soy sauces at concentrations ranging from 0.15 to 0.61 mg/dl, demonstrating that it is widely distributed in soy sauces.     

Influence of sugars on the characteristics of glucono-δ-lactone-induced soy protein isolate gels

The effects of the reducing sugars (glucose and lactose) and the non-reducing sugar (sucrose), heated in combination with soy protein isolate (SPI) at neutral pH, on the physicochemical and rheological properties of SPI were determined. After formation of gels induced by glucono-δ-lactone (GDL), the textural profile and physicochemical bonds of the non-heated and heated SPI gels were investigated. The gelation of SPI was induced in three stages of processing that is similar to some tofu-making procedures. First, SPI was heated in the presence of sugars at neutral pH above the denaturation temperature of SPI; then gelation was induced by GDL at iso-electric pH and finally the acidic gels were heat treated again. Heat treatment with glucose at neutral pH resulted in SPI with higher glycation degree than with lactose, whereas SPI heat treated in the presence of sucrose was not glycated. GDL-induced gels of SPI glycated with glucose was more soluble in water than gels of SPI reacted with lactose, which in turn was more soluble than the control and gels of SPI heated in the presence of sucrose. This indicates a change in the net charge of proteins caused by the glycation reaction. Glucose and lactose had a protective effect on protein denaturation at neutral pH, albeit less than sucrose, resulting in GDL-induced gels with increased water holding capacity and reduced gel hardness than sucrose. Chemical analysis indicated that disulphide bonds were involved in maintaining the structure of the gels, and solubility profiles of gels in different buffers indicate that other types of covalent bonds besides disulphide bonds were formed in gels of glycated SPI, resulting in reduced gel elasticity.     

Effect of additives on mesophilic α-amylase and its application in the desizing of cotton fabrics

An investigation was carried out on thermal stability of α-amylase. The influence of various additives (calcium acetate, sodium lactate, L-histidine, and water-soluble chitosan) on the stability of α-amylase was studied. Results showed the inactivation behavior of α-amylase with or without additives all followed the first-order kinetics. All additives (Ca2+, sodium lactate, L-histidine, and water-soluble chitosan) displayed good stabilizing effect on α-amylase lower than 80 °C, and only water-soluble chitosan had an efficient stabilizing effect on α-amylase when the treatment temperature exceeds 80 °C. All additives improved the catalytic activity of α-amylase at 70–90 °C, and the appearance of water-soluble chitosan increased the catalytic activity of α-amylase at 90 °C sharply. A desizing ratio of 68.42% was obtained by treating the cotton fabrics in the buffer solution at 100 °C without α-amylase. To obtain a desizing ratio exceed 95% when fabrics were treated at 100 °C for 10 min, the addition of water-soluble chitosan saves 2/3 α-amylase dosage. Moreover, water-soluble chitosan showed a further improvement in desizing effect than the additive of calcium acetate.     

Effect of calcium and sodium caseinates on physical characteristics of soy protein isolate–lipid films

The effect of addition of caseinates to soy protein isolated (SPI) based films containing lipids (33% of oleic acid or 85:15 oleic acid (OA)–beeswax blend (BW)) on water vapour permeability (WVP), mechanical and optical properties was evaluated. SPI–lipids was combined with caseinates (sodium or calcium) in different SPI:caseinate ratios with the aim of improving water vapour barrier, mechanical and optical properties of SPI films containing lipids. Caseinate incorporation to SPI based films provoked an increase of elastic modulus and tensile strength at break, mainly for calcium caseinate. Both caseinates contributed to increase the water vapour barrier properties of soy protein-based films. Caseinates also provoked an increase of transparency of SPI based films and colour softening. The most effective combination was 1:1 sodium caseinate:SPI ratio, when film contains 85:15 oleic acid:beeswax ratio.     

Carboxymethyl chitosan–soy protein complex nanoparticles for the encapsulation and controlled release of vitamin D3

In this study, complex nanoparticles were developed from carboxymethyl chitosan (CMCS) and soy protein isolate (SPI) by a simple ionic gelation method. The effect of Ca²⁺ concentration, pH and CMCS/SPI mass ratio on the formation of nanoparticles was systematically investigated. Vitamin D₃ (VD), a hydrophobic micronutrient, was successfully incorporated into the polymeric complex, forming particles with sizes between 162 and 243 nm and zeta potentials ranging from −10 to −20 mV. In comparison with CMCS, the CMCS/SPI complex required a lower concentration of Ca²⁺, and it showed better particle forming capability over a broad range of pH. These features resulted in an increased loading efficiency to 6.06%. In addition, the complex nanoparticles achieved significantly higher encapsulation efficiency (up to 96.8%), possibly due to their compact structure and high capability of hydrogen bonding evidenced by Fourier transform infrared spectroscopy (FTIR). In contrast to the ones prepared with SPI, the complex nanoparticles exhibited a reduced (42.3% compared to 86.1%) release of VD in simulated gastric fluid and an increased (36.0% compared to 8.2%) release under simulated intestinal condition. These characteristics made the CMCS/SPI complex nanoparticles an attractive candidate for the encapsulation and controlled release of hydrophobic nutraceuticals and bioactives.     

Effects of heat treatment and glucono-δ-lactone-induced acidification on characteristics of soy protein isolate

The storage modulus (G′) and gel hardness of non-heated and heat-treated SPI at neutral pH and those of subsequently formed GDL-induced gels were measured. The values obtained for acid-induced gels formed by heat-treated SPI were significantly increased compared to those formed by non-heated SPI. The physicochemical properties of non-heated SPI and heat-treated SPI at pH 6.9 (denaturation degree, dispersibility, water-holding capacity, sulfhydryl groups and surface hydrophobicity) were measured to correlate their effects to gel strength and gel hardness, sulfhydryl groups and water-holding capacity of GDL-induced gels.     

Effects of engineered methionine in the 8Sα globulin of mungbean on its physicochemical and functional properties and potential nutritional quality

The major storage protein of mungbean, 8Sα globulin or vicilin, was engineered using site-directed mutagenesis to increase the number of methionine (Met) residues in the molecule for improvement of functional and nutritional qualities. Eight Met-rich proteins were designed and prepared to have 2 to 10 Met residues introduced in disordered regions II and IV. The designed proteins were highly expressed as soluble form in Escherichia coli. Their production level of the modified proteins was estimated to be about 30%, and was almost the same as that of 8Sα globulin wild type (WT). The modified proteins formed stable native conformation similar to WT as shown by gel filtration chromatography. They demonstrated greater stability in terms of thermal denaturation temperature and greater emulsifying ability and emulsion stability, especially the 10-Met protein, compared to the wild type. Met-rich proteins with 3, 5, and 10 Met residues had 74, 96, and 145% of nutritional requirement for Met compared with 41% for WT. Based on allergenicity prediction programs, WT and all the modified proteins had no allergenic potential.     

Fermentation of commercial soy beverages with lactobacilli and bifidobacteria strains featuring high β-glucosidase activity

An increase of isoflavone aglycone content in soy foods can be attained through fermentation with food-grade bacteria. In this study, two commercial soy beverages with distinctive chemical composition (AS and VS) were fermented by strains of lactobacilli (eight strains) and bifidobacteria (two strains) with a high β-glucosidase activity. Along fermentation, growth of the strains and isoflavone deglycosylation in the soy beverages were monitored. Large differences in growth, aglycone content and chemical parameters in AS and VS beverages fermented by different species and strains were observed. Isoflavone glycosides were completely transformed into their corresponding aglycones by most strains during fermentation of AS beverage, whereas large amounts of undeglycosilated isoflavones were still present in fermented VS. Four strains showing strong deglycosylation activity and appropriate technological properties (Lactobacillus casei LP71, Lactobacillus plantarum E112, Lactobacillus rhamnosus E41 and Bifidobacterium pseudocatenulatum C35) were proposed as industrial starters to improve functionality in soy-based fermented foods.     

Understanding the crispy–crunchy texture of raw red pepper and its change with storage time

Red sweet peppers held in cold storage were periodically sampled at 1-week intervals over a 3-weeks period using three-point bending, puncture, cutting, and Volodkevich (coupled with acoustic emission) tests, confocal laser scanning microscopy (CLSM) and other physicochemical measurements. At each sampling, tissue specimens were soaked in mannitol solutions (0.0–0.9M) and puncture test, dimension changes and CLSM were used to identify degrees of turgidity present in osmotically manipulated pepper tissue. Pepper texture became crumbly with increased storage time due to softening and wilting processes. The Young's modulus, derived from the bending test using the single-edge notched bend geometry without notches decreased progressively during cold storage and resulted as the best mechanical parameter for measuring the loss of whole-tissue stiffness by both decreased cell wall stiffness and turgor pressure. Osmotic adjustment indicated that the pepper structure is extremely anisotropic, with the specimen's “average” relative thickness (RT) being the dimension change more affected. Incipient plasmolysis was evident in the highest mannitol concentration (0.9M), therefore, the turgor pressure of nonsoaked tissue could not be inferred. However, significant correlations were found between RT and puncture parameters such as initial slope, initial and final distances, and the number of flesh and skin force peaks, which depended on the dilation or shrinkage caused by the osmotic adjustment. During storage, soaked tissues had lower crunchy texture than nonsoaked, reflecting that cell wall stiffness plays a more significant role in determining pepper crunchiness than cell turgor pressure.     

Rheological properties and permeability of soy protein-stabilised emulsion gels made by acidification with glucono-δ-lactone

BACKGROUND: Soy protein, an important efficient emulsifier, is widely used by the food industry for incorporation into milk, yogurts, ice cream, salad dressings, dessert products, etc. The objective of this study was to investigate the rheological and physical properties of soy protein‐stabilised emulsion gels as affected by protein concentration and gelation temperature. RESULTS: The rheological properties and permeability were determined using oscillatory rheometry, permeability and whey separation. The modulus (G′ and G″), fracture stress and fracture strain of acid‐induced emulsion gels after 20 h of glucono‐δ‐lactone addition depended strongly on soy protein concentration and gelation temperature. At increasing soy protein concentrations, acid‐induced emulsion gels had shorter gelation times but higher storage moduli (G′), fracture stresses and strains. Increasing gelation temperature decreased the gelation time, G′, fracture stresses and strains. Permeability and whey separation were significantly affected by the protein concentration and the gelation temperature. A significant positive correlation was observed between whey separation and permeability coefficient in emulsion gels formed at different temperatures. CONCLUSION: The rheological properties and permeability of soy protein‐stabilised emulsion gels were significantly influenced by protein concentration and gelation temperature. Copyright © 2011 Society of Chemical Industry     

The effect of protein concentration and heat treatment temperature on micellar casein–soy protein mixtures

The objective of this study was to investigate the effect of concentration and temperature on the rheological properties of soy proteins (SP) and micellar casein (MCN) systems. Individual and mixed (1:1) protein systems of 2–15% concentration were prepared and heat treated for 5 min at 40–90 °C. After cooling to 20 °C, their rheological properties were determined using steady-shear rheology. Zeta potential and particle size measurements were also conducted. Both proteins were negatively charged under all experimental conditions, but the absolute values of zeta potential and thus the stability of the protein solutions decreased with temperature and concentration. For SP solutions, viscosity and apparent yield stress increased with concentration. Shear thinning behavior was prevalent, becoming more pronounced with increasing concentration. Heat treatments at T ≥ 80 °C induced glycinin denaturation, followed by aggregation and network formation when C ≥ 7.5%. Heat treatment did not significantly affect viscosity of MCN systems, while increasing concentration resulted in a significant increase in apparent viscosity and apparent yield stress. Most MCN systems exhibited Newtonian flow behavior, with the exception of systems with C ≥ 12.5% treated at T ≥ 80 °C, which became slightly shear thickening. Mixed SP–MCN systems mimicked the behavior of SP, with most values of rheological parameters intermediate between SP and MCN-only systems. Mixtures of 7.5–12.5% concentration treated at 90 °C displayed local phase separation, low viscosity and apparent yield stress, while 15% mixtures treated at 90 °C showed protein aggregation and incipient network formation. The data generated in this study can be used to develop a range of protein based products with unique flow characteristics and storage stability.     

High pressure processing assisted enzymatic hydrolysis – An innovative approach for the reduction of soy immunoreactivity

Soybean (Glycine max (L.) MERR.) is recognized as a potent food allergen causing one of the most frequent food allergies worldwide. The effect of high pressure processing (HPP) prior to and during enzymatic hydrolysis using the enzyme preparation Flavourzyme® on the degree of hydrolysis (DH), molecular weight distribution (SDS-PAGE) and β-conglycinin (Gly m5) immunoreactivity of soy protein isolate (SPI) was studied. Enzymatic hydrolysis was carried out at atmospheric pressure (0.1 MPa) and HPP (100–600 MPa) at 50 °C for 15 min. Pressures higher than 300 MPa enhanced the degradation of Gly m5, which was confirmed by SDS-PAGE and LC-MS/MS analyses. The immunoreactivity of the samples was assessed by in vitro sandwich ELISA using mouse monoclonal anti-Gly m5 antibodies. Depending on the antibody tested, the residual immunoreactivity was completely inhibited or significantly impaired up to 99.5% applying HPP during hydrolysis at 400 and 500 MPa. By means of principal component analysis, the beany and green off-flavors characteristic for unprocessed SPI could be reduced by pressure enhanced hydrolysis at 400–500 MPa. The resulting hydrolysates possessed improved protein solubility, foaming activities and oil-binding capacities, which were improved by 45%, 66%, and 210%, respectively. HPP prior to and during enzymatic hydrolysis at 400–500 MPa constitutes an innovative approach for the production of low-allergen food ingredients that combine good taste and enhanced functional properties.Industrial relevanceFood allergy has emerged in the last years as the incidence and prevalence are rising dramatically. Up to now, enzymatic hydrolysis is the only feasible method to mitigate soy allergy. However, the major drawback associated with enzymatic hydrolysis is the incomplete destruction of allergenic epitopes and the formation of a strong bitter taste. This research activity demonstrates that high pressure assisted enzymatic hydrolysis using the enzyme preparation Flavourzyme effectively reduces the immunoreactivity of soy proteins. Degree of hydrolysis analysis, SDS-PAGE, mass spectrometry as well as sandwich ELISA with mouse monoclonal anti-Gly m5 antibodies have been applied to analyze the destruction of allergenic proteins as well as to determine the residual immunoreactivity. This study provides preliminary evidence that this innovative combination process of high pressure and enzymatic hydrolysis has great potential to produce tasty low-allergen soy-based food ingredients with good physicochemical properties, i.e. protein solubility and foamability.