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

Characterization of structural and functional properties of soybean 11S globulin during renaturation after denaturation induced by changes in pH

BACKGROUND

This study explored the denaturation of 11S globulin, a protein known for its diverse functional properties in soy protein applications, at pH 3.0 and pH 10.0, followed by a gradual return to pH 7.0 to facilitate renaturation. It investigated the structural and functional changes during renaturation induced by a change in pH, revealing the stabilization mechanism of 11S globulin.

RESULTS

The findings revealed that during pH adjustment to neutral, the denatured soybean 11S globulin - resulting from alkaline (pH 10.0) or acidic (pH 3.0) treatments - experienced a refolding of its extended tertiary structure to varying extents. The particle size and the proportions of α-helix and β-sheet in the secondary structure aligned progressively with those of the natural-state protein. However, for the alkali-denatured 11S, the β-sheet content decreased upon adjustment to neutral, whereas an increase was observed for the acid-denatured 11S. In terms of functional properties, after alkaline denaturation, the foaming capacity (FC) and emulsifying activity index (EAI) of 11S increased by 1.4 and 1.2 times, respectively, in comparison with its native state. The solubility, foamability, and emulsifiability of the alkali-denatured 11S gradually diminished during renaturation but remained superior to those of the native state. Conversely, these properties showed an initial decline, followed by an increase during renaturation triggered by pH neutralization.

CONCLUSIONS

This research contributes to the enhancement of protein functionality, offering a theoretical foundation for the development of functional soy protein products and expanding their potential applications. © 2024 Society of Chemical Industry.

Structure and functionality of whey protein, pea protein, and mixed whey and pea proteins treated by pH shift or high-intensity ultrasound

Three modifications (pH shift, ultrasound, combined pH shift and ultrasound) induced alterations in pure whey protein isolate (WPI), pea protein isolate (PPI), and mixed whey and pea protein (WPI-PPI) were investigated. The processing effect was related to the protein type and technique used. Solubility of WPI remained unchanged by various treatments. Particle size was enlarged by pH shift while reduced by ultrasound and combined approach. All methods exposed more surface hydrophobic groups on WPI, while pH shift and joint processing was detrimental to its emulsifying activity. The PPI and mixture exhibited similar responses toward the modifications. Solubility of PPI and the blend enhanced in the sequence of pH shift and ultrasound > ultrasound > pH shift. Individual approach expanded while co-handling diminished the particle diameter. Treatments also caused more disclosure of hydrophobic regions in PPI and WPI-PPI and emulsifying activity was ameliorated in the order of pH shift and ultrasound > ultrasound > pH shift.

Ameliorating the stability of native/thermally denatured chicken-derived myofibrillar proteins particles in an aqueous system: The synergistic effect of acidification combined with inulin and inulin/sodium alginate

The effect of acidification through hydrochloric acid combined with inulin (In), and inulin/sodium alginate (In/SA) on the stability of native/thermally denatured myofibrillar proteins (MPs/TMPs) particles in an aqueous system was investigated. At the same pH, MPs-In and TMPs-In particles were smaller and had higher absolute potentials than MPs-In/SA and TMPs-In/SA particles. Additionally, the size of MPs-In particles reached 1 μm, and the solubility increased from 21.73 ± 0.57 % to 76.26 ± 1.27 % when the pH was reduced from 5.0 to 3.0. The absolute potential of TMPs 3-In particles increased from 15.77 ± 0.72 to 28.20 ± 0.30 mV, and the solubility increased from 18.65 ± 0.72 % to 74.53 ± 0.74 %. Confocal laser microscopy revealed that, compared with pH 5.0 or 4.0, MPs-In/TMPs-In particles dispersed more evenly at pH 3.0 compared with pH 5.0 or 4.0. This further confirmed that electrostatic repulsion between particles maximally contributed to particle stability. Furthermore, the α-helix content in TMPs-In particles at pH 3.0 decreased from 41.51 ± 1.09 % (TMPs control) to 16.61 ± 1.87 %. This decrement of an up to 60 % led to decreased intramolecular hydrogen bonds and improved surface hydrophobicity. Therefore, a single polysaccharide (In) combined with MPs/TMPs particles exhibited higher dispersion and stability at pH 3.0. These findings could provide new insights into chicken-derived protein beverage processing.

L-lysine moderates thermal aggregation of coconut proteins induced by thermal treatment

This work attempts to investigate the inhibitory effect of L-lysine (Lys) on the thermal aggregation of coconut protein (CP). The results showed that under neutral conditions (pH = 7), temperature reduced the solubility and enhanced the thermally induced gel formation of CP. In addition, Lys reduced the fluorescence properties, particle size and increased the turbidity of CP, which had an inhibitory effect on heat induced gels. The results indicate that Lys plays an important role in inhibiting protein thermal aggregation by interacting with CP to create steric hindrance and increase protein electrostatic repulsion.

Effects of environmental pH on protein properties and flavor factors of hairtail (Trichiurus haumela) in thermal processing

The flavor and texture of hairtail (Trichiurus haumela) products easily change depending on the processing conditions including the programed temperature, environmental pH, and so on. In the present study, we aimed to explore the differences in the overall texture and flavor of hairtail under heat treatment with varied environmental pH. The results indicated that the secondary structure of the myofibrillar protein in thermal processed hairtail meat presented a transformation from α-helix to β-sheet structure with the decrease of solution pH. Moreover, heat treatment in an acidic solution environment effectively improved the sensory and flavor properties of hairtail. In addition, pH-mediated changes on protein characteristics of cooked hairtail meat showed significant correlation with the texture properties, while weakly correlated with the flavor.

Relationship between flexibility and interfacial functional properties of soy protein isolate: succinylation modification

BACKGROUND

In this paper, the effects of different succinic anhydride (SA) additions on the flexibility of soy protein isolate (SPI) were investigated, and changes in protein conformation and interfacial functional properties were measured. The structure-effect relationship between conformation, flexibility, and interfacial functional properties was established.

RESULTS

SPI was bound to SA through disulfide bonds, and the zeta potential was reduced. The β-sheet content decreased, the disordered structure increased, and there were changes in tertiary structure and microstructure. The surface hydrophobicity, disulfide bond content, and solution turbidity were reduced to 5063, 1.0967 μmol g-1 , and 0.0036 μmol g-1 respectively. The best flexibility of SPI (0.3977) and interfacial functional properties were obtained when the mass ratio of SA/SPI was 15%. Correlation analysis showed a highly significant positive correlation (P < 0.01) between flexibility and emulsification and foaming properties, with correlation coefficients of 0.960 and 0.942 for flexibility with emulsifying activity and emulsion stability respectively, and 0.972 and 0.929 for flexibility with foaming capacity and foaming stability respectively.

CONCLUSION

The results suggest that succinylation-induced conformational changes of SPI improved its interfacial functional properties by changing its flexibility. These results provide theoretical guidelines for the development and application of highly emulsifiable and stable soy protein products utilizing succinylation. © 2022 Society of Chemical Industry.

Molecular structural modification of β-conglycinin using pH-shifting with ultrasound to improve emulsifying properties and stability

This present work underlines the effect of pH-shifting at pH 2 and pH 12 individually or combined with ultrasound treatment to modify the molecular structure of β-conglycinin (7S) on its emulsifying properties and stability. Fourier transform infrared (FTIR) spectroscopy and intrinsic fluorescence spectroscopy showed that pH-shifting improves the molecular structure of 7S, while ultrasound further promotes structural changes. In particular, the pH-shifting at pH 12 combined with ultrasound treatment (U-7S-12) resulted in more significant changes than the pH-shifting at pH 2 combined with ultrasound (U-7S-2). U-7S-12 showed a significant reduction in protein particle size from 152 to 34.77 nm and a relatively smooth protein surface compared to 7S. The protein had the highest surface hydrophobicity and flexibility at 81,560.0 and 0.45, respectively, and the free sulfhydryl content from 1.57 to 2.02 μmol/g. In addition, we characterized the emulsions prepared after 7S treatment. The single or combined treatment increased the interfacial protein adsorption of the samples, which showed lower viscosity and shear stress compared to 7S. The U-7S-12 emulsion exhibited the highest emulsifying properties and was more stable than other emulsions under creaming, heating, and freeze-thaw conditions. In summary, the concerted action of pH-shifting and ultrasound can modify the structure, and combined alkaline pH-shifting and ultrasound treatment can further improve the emulsifying properties and stability of 7S.

Effects of proteins on emulsion stability: The role of proteins at the oil-water interface

To obtain a stable protein-added emulsion system, researchers have focused on the design of the oil-water interface. This review discussed the updated details of protein adsorption behavior at the oil-water interface. We evaluated methods of monitoring interfacial proteins as well as their strengths and limitations. Based on the effects of structure on protein adsorption, we summarized the contribution of pre-changing methods to adsorption. In addition, the interaction of proteins and other surface-active molecules at the interface had been emphasized. Results showed that protein adsorption is affected by conformation, oil polarity and aqueous environments. The monitoring of interfacial proteins through spectroscopic properties in actual emulsion systems is an emerging trend. Pre-changing could improve the protein adsorption and the purpose of pre-changing of proteins is similar. In the interaction with other surface-active molecules, co-adsorption is desirable. By co-adsorption, the respective advantages can be exploited to obtain a more stable emulsion system.

Comparison of the interfacial properties of native and refolded myofibrillar proteins subjected to pH-shifting

The emulsion abilities of pale, soft, exudative (PSE)-like chicken breast protein are unsatisfied, which are urgently needed to be ameliorated. This study evaluated the improvement of pH-shifting (11.0-, 11.5- and 12.0-7.0) on emulsion properties of the PSE-like chicken breast myofibrillar proteins (MPs) and the underlined structure-driven interfacial mechanism. It was found pH-shifting promoted the exposure of buried hydrophobic groups and free sulfhydryl groups, and changed secondary structures. Emulsions stabilized by refolded MPs exhibited more uniform and dispersed distributions with more adsorbed proteins at the interface. Electrophorogram showed both disulfide and non-disulfide covalent bonds were involved during interfacial protein-protein interaction. The results from circular dichroism and front-surface fluorescence spectroscopy revealed interfacial MPs were exposed to a more hydrophobic environment and increased β-sheets enhanced their molecular interactions. In addition, interfacial proteins after pH-shifting was less likely to be replaced by Tween 20.

An insight into the changes in conformation and emulsifying properties of soy β-conglycinin and glycinin as affected by EGCG: Multi-spectral analysis

The binding mechanisms between soy β-conglycinin/glycinin and (-)-epigallocatechin-3-gallate (EGCG) were evaluated using multi-spectral techniques and molecular modeling. Additionally, the emulsifying properties of β-conglycinin/glycinin were investigated in their interactions with EGCG. Fluorescence analysis revealed that the quenching of β-conglycinin/glycinin by EGCG was static quenching. Specifically, EGCG to β-conglycinin/glycinin resulted in the conformation changes of the Trp and Tyr residues, around which the polarity toward more hydrophilic. The dominated binding between β-conglycinin and EGCG was hydrogen bonding, whereas was mainly hydrophobic force between glycinin and EGCG. Such affinity induced a more organized protein confirmation with decreased random coil and increased α-helix and β-structures. The docking data indicated the better affinity between glycinin and EGCG, compared to β-conglycinin. The emulsifying ability and capacity of β-conglycinin were enhanced with involvement EGCG, however no effect was found for glycinin. Our findings deliver insights in understanding of the interaction mechanisms between β-conglycinin/glycinin and EGCG.

Comparative structural and emulsifying properties of ultrasound-treated pea (Pisum sativum L.) protein isolate and the legumin and vicilin fractions

The structural properties, interfacial behavior, and emulsifying ability of ultrasound-treated pea protein isolate (PPI) and the legumin (11S) and vicilin (7S) globulin fractions prepared with a salt-solubilization procedure were investigated. Of the three protein groups, PPI was strongly responsive to ultrasound perturbation (20 kHz, 57-60 W·cm-2) showing the greatest solubility increase, particle size reduction, structure destabilization, and conformational change. Similar but less remarkable effects were observed on 11S globulins; 7S proteins, already highly soluble (>99%), were generally less sensitive to ultrasound. The ultrasound treatment significantly improved emulsifying activity, which resulted in greater emulsifying capacity and stronger interfacial adsorption for all protein samples. PPI exhibited the higher activity increase (70.8%) compared to approximately 30% for 11S and 7S. For both control and ultrasound treated proteins, the emulsifying capacity was in the order of 7S > 11S > PPI, inversely related to the trend of protein loading at the interface, indicating efficiency differences. The latter was attributed to emulsion clusters formed through protein-protein interaction in PPI and 11S emulsions which were visibly absent in 7S emulsions.

Effects of Ultrasound Combined with Preheating Treatment to Improve the Thermal Stability of Coconut Milk by Modifying the Physicochemical Properties of Coconut Protein

In the food industry, coconut milk has a unique flavor and rich nutritional value. However, the poor emulsifying properties of coconut proteins restrict its development. In this study, the effect of ultrasound combined with preheating on coconut globulin and coconut milk was evaluated by physicochemical properties and structural characteristics. The results showed that ultrasound and 90 °C preheating gave coconut protein better emulsifying and thermal properties, demonstrated by higher solubility (45.2% to 53.5%), fewer free sulfhydryl groups (33.24 to 28.05 μmol/g) and higher surface hydrophobicity (7658.6 to 10,815.1). Additionally, Fourier transform infrared spectroscopy and scanning electron microscopy showed obvious changes in the secondary structure. Furthermore, the change in the physicochemical properties of the protein brought a higher zeta potential (−11 to −23 mV), decreased the thermal aggregation rate (148.5% to 13.4%) and increased the viscosity (126.9 to 1103.0 m·Pa·s) of the coconut milk, which indicates that ultrasound combined with preheating treatment provided coconut milk with better thermal stability. In conclusion, ultrasound combined with preheating will have a better influence on modifying coconut globulin and increasing the thermal stability of coconut milk. This study provides evidence that ultrasound and other modification technologies can be combined to solve the problems encountered in the processing of coconut protein products.

Modification of structure and functionalities of ginkgo seed proteins by pH-shifting treatment

Modification and improvement of protein functionalities are important for expanding the applications of proteins in food. The objective of this study was to investigate the effects of pH-shifting treatments on the structural and functional properties of ginkgo seed protein isolate (GSPI). GSPI was exposed to acidic (pH 2.0-4.0) and basic (pH 10.0-12.0) pHs for 0, 0.5, 1, 2, and 4 h and subsequently neutralized for refolding. The pH-shifting treatments significantly increased GSPI solubility by 43-141% except for the treatment at pH 2.0, which decreased protein solubility by 16-39%. All pH-shifting treatments more than doubled the surface hydrophobicity of GSPI and significantly improved the emulsifying activity. The highest emulsifying activity was observed in the pH 2.0-treated GSPI, which was 4.9-fold higher than the control. Acid-induced GSPI degradation likely promoted protein adsorption to the oil-water interface. In summary, the pH-shifting-modified GSPI may serve as a promising emulsifier in various food systems.

Modification of rapeseed protein by ultrasound-assisted pH shift treatment: Ultrasonic mode and frequency screening, changes in protein solubility and structural characteristics

We investigated the effect of ultrasound-assisted pH shift treatment on the micro-particle, molecular, and spatial structure of rapeseed protein isolates (RPI). Various ultrasonic frequency modes (fixed, and sweep) was used. Protein characterization by the indexes: particle size, zeta potential, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), scanning electron microscopy (SEM), free sulfhydryl (SH), surface hydrophobicity (Ho), Fourier transform infrared Spectrum (FTIR) and fluorescence intensity was studied to elucidate the changes in solubility and structural attributes of RPI. The results showed that ultrasonic frequency and working modes substantially altered the structure, and modified the solubility of RPI. Ultra + pH mode at fixed frequency of 20 kHz had the best effect on the solubility of RPI. Under the condition of ultra + pH mode, 20 kHz at pH 12.5, solubility, compared to control, increased from 8.90% to 66.84%; and the change in molecular structure of RPI was characterized by smaller particles (from 330.90 to 115.77 nm), high zeta potential (from -17.95 to -14.43 mV, p < 0.05), and increased free sulfhydryl (from 11.63 to 24.50 µmol/g) compared to control. Likewise, surface hydrophobicity increased (from 2053.9 to 2649.4, p < 0.05), whilst ɑ-helix and random coil decreased (p < 0.05), compared to control. The fluorescence spectroscopy and FTIR spectroscopy showed that the secondary and tertiary structure of the RPI were altered. These observations revealed that changes in RPI structure was the direct factor affecting solubility. In conclusion, ultrasound assisted pH shift treatment was proven to be an effective method for the modification of protein, with promising application in food industry.

Storage stability of soy protein isolate powders containing soluble protein aggregates formed at varying pH

Soy protein is wildly used in food industry due to its high nutritional value and good functionalities. However, the poor storage stability of commercial soy protein products has puzzled both the producers and the users for a long time. The current study assessed the changes in protein solubility, aggregation, oxidation, and conformation of soy protein isolate (SPI) with various soluble aggregates formed at different pH values (pH 5-8) during storage. During storage, SPI samples showed a reduced protein solubility (p < .05), an increased protein oxidation (p < .05), and an attenuated conformational enthalpy (∆H). SPI with a higher pH produced more disulfide-mediated aggregates at the expense of sulfhydryl groups and experienced greater losses of protein tertiary structure and a faster reduction in solubility. Yet, all samples nearly shared similar rising trend during 8-week storage, which indicated the production of protein carbonyls was insensitive to pH. Soluble aggregates present in fresh SPI samples appeared to induce instability of SPI during storage. These findings suggested SPI prepared at pH 6 was in favor of its storage stability, and soluble aggregates presented in fresh samples should be paid more attention for further study of storage stability kinetics.

Low pH-shifting treatment would improve functional properties of black turtle bean (Phaseolus vulgaris L.) protein isolate with immunoreactivity reduction

Low pH-shifting was firstly applied in the black turtle bean (Phaseolus vulgaris L.) protein isolate treatment by acidic (pH 1.0-3.0) buffer incubation for 8 h, then was adjusted to pH 7.2 and kept 3 h for protein stabilizing. Mild loss of secondary structure was confirmed in the protein isolate after low pH-shifting treatment by CD and FT-IR analyses. Intrinsic fluorescence, UV spectra, surface hydrophobicity, SH content and SDS-PAGE analyses indicated the protein conformation was unfolded with the exposure of much more buried hydrophobic residues, which would result in the enhancement of emulsifying properties, foaming properties and fat holding capacity, and lead to the reduction of solubility and water holding capacity. Furthermore, lower immunoreactivity was observed by the ELISA, and improved digestibility was found in in vitro digestion assay. Our results suggested the low pH-shifting treatments would broaden the application of bean protein isolate with better hydrophobic processing functions and safety.

Effect of pH-shifting treatment on structural and heat induced gel properties of peanut protein isolate

Modifying proteins with new methods to improve their functional properties is essential to extend their application in food related field. In this study, the effect of pH-shifting treatment on structural and heat induced gel properties of peanut protein isolate (PPI) were investigated. PPI was subject to different pH conditions (pH 2, pH 4, pH 10, and pH 12) for 1 h, then adjusted back to pH 7 and they were marked as PPI2, PPI4, PPI10, and PPI12, respectively. It was found that the breaking force (BF) and water holding capacity (WHC) of PPI10 gel were significantly improved due to the decreased particle size, increased solubility, free sulfhydryl group content and surface hydrophobicity of PPI10. While PPI2 or PPI12 lost their gel ability due to the formation of large protein aggregates. These results indicated that pH-shifting treatment might be a convenient and economical method to prepare PPI with different gel and structural properties.

Structural characterization and in-silico analysis of Momordica charantia 7S globulin for stability and ACE inhibition

Momordica charantia (Mc) seeds are widely used edible crop with high nutritional quality. The food and pharmaceutical industries use it as a natural anti-oxygenic agent. Herein, a ~52 kDa protein, which is a major part of seed proteome has been purified, biochemically characterized and structure has been determined. MALDI-ESI-MS identified peptide fragments and contig-deduced sequence suggested the protein to be homologous to 7S globulins. The crystal structure shows that protein has a bicupin fold similar to 7S globulins and the electron density for a copper and acetate ligand were observed in the C-terminal barrel domain. In silico study reveals that a tripeptide (VFK) from Mc7S possess a higher binding affinity for angiotensin converting enzyme (ACE) than already reported drug Lisinopril (LPR). The protein is a glycoprotein and highly stable under varying thermal and pH conditions due to its secondary structures. The DPPH (2,2-diphenyl-1-picryl-hydrazyl-hydrate) assay showed the protein to have an anti-oxygenic nature and can aid in scavenging free radical from sample. The protein can assist to enhance the nutritional and functional value of food by acting as a food antioxidant. Further, characterization of Mc7S required which might add in importance of Mc7S as antioxidant, anti-diabetic and anti-hypertensive.

Relationship between Molecular Flexibility and Emulsifying Properties of Soy Protein Isolate-Glucose Conjugates

At present, the structure-activity relationships of soy protein isolate are still not well understood. In this paper, the relationship between molecular flexibility and emulsifying properties of soy protein isolate and soy protein isolate-glucose conjugates were investigated. The Maillard reaction was carried out at different temperature conditions (50 °C, 60 °C, 70 °C, 80 °C, and 90 °C) under a specific wet condition. Meanwhile, structural properties including surface hydrophobicity ( H₀), molecular flexibility and secondary, tertiary, quaternary structures, and the free sulfhydryl group ( -SH) content were measured. The results showed that there was a good correlation between molecular flexibility and emulsifying properties, and the correlation coefficients was 0.920 ( P < 0.01) for emulsifying activity and 0.952 ( P < 0.01) for emulsion stability. Compared with soy protein isolate, the H₀ of samples at different temperatures first increased and then decreased reaching a maximum at 70 °C, a red shift occurred during the whole given reaction conditions shown by the intrinsic fluorescence spectrum, and the free sulfhydryl content also displayed a marked increase ( P < 0.05). At the same time, the particle size gradually became smaller as the degree of grafting increased. The contents of β-turn and random coil increased at the cost of α-helix and β-sheet contents, as evidenced by Fourier transform infrared results. The findings could provide a deep insight into the structure-function relationship of soy protein isolate-glucose conjugates, thus providing theoretical guidance for further research of soy proteins.

The Molecular Properties of Peanut Protein: Impact of Temperature, Relative Humidity and Vacuum Packaging during Storage

This study aimed to investigate the variation of molecular functional properties of peanut protein isolate (PPI) over the storage process and reveal the correlation between the PPI secondary structure and properties in the storage procedure. After storage, the molecular properties of PPI changed significantly (p < 0.05). Extending storage time resulted in a decrease in free sulfhydryl content, fluorescence intensity, surface hydrophobicity and emulsifying properties, which was accompanied by an increase in protein particle size. The results of infrared spectroscopy suggested the content decline of α-helix and β-sheet, and the content rise of β-turn and random coil. Based on bivariate correlation analysis, it was revealed that surface hydrophobicity and emulsifying activity of PPI was significantly affected by α-helix and by β-turn (p < 0.05), respectively. This research supplied more information for the relationship between the peanut protein’s secondary structure and functional properties over the stored process.

Ionic Strength and pH Responsive Permeability of Soy Glycinin Microcapsules

Recently, hollow protein microcapsules have been made simply by heating the microphase separated soy glycinin microdomains. However, the properties (e.g., mechanical properties and permeability) that relate to the application of these microcapsules are unknown. In this study, the permeability of the soy glycinin microcapsules was investigated by confocal laser scanning microscopy (CLSM), as influenced by ionic strength and pH using fluorescein isothiocyanate-dextran (FITC-dextran). The glycinin microcapsules kept the integrity between pH 1 and 11.5, swelled when pH was below 3 or above pH 11, dissociated at pH above 11.5 and deswelled slightly at pH 1. When the pH increased above 11, the permeability of the microcapsule significantly increased. Remarkably, when the pH was below the isoelectric point of glycinin (≈pH 5), FITC-dextran spontaneously accumulated inside the microcapsule with a significantly higher concentration than that in bulk solution, as evidenced by the strong intensity increase of fluorescence. This unique feature significantly increased the loading amount of FITC-dextran. The permeability of microcapsules was also increased by adding salt but less significant than by adjusting pH. The surface of the microcapsules became coarser when the permeability increased, which was revealed by scanning electron microscopy. These results show that soy glycinin has a great potential to be used as a wall material to fabricate hollow microcapsules that could find applications in biomedicine and food industry.