Chemical composition, molecular weight distribution, secondary structure and effect of NaCl on functional properties of walnut (Juglans regia L) protein isolates and concentrates

Walnut Protein: A Rising Source of High-Quality Protein and Its Updated Comprehensive Review

Recently, plant protein as a necessary nutrient source for human beings, a common ingredient of traditional processed food, and an important element of new functional food has gained prominence due to the increasing demand for healthy food. Walnut protein (WP) is obtained from walnut kernels and walnut oil-pressing waste and has better nutritional, functional, and essential amino acids in comparison with other vegetable and grain proteins. WP can be conveniently obtained by various extraction techniques, including alkali-soluble acid precipitation, salting-out, and ultrasonic-assisted extraction, among others. The functional properties of WP can be modified for desired purposes by using some novel methods, including free radical oxidation, enzymatic modification, high hydrostatic pressure, etc. Moreover, walnut peptides play an important biological role both in vitro and in vivo. The main activities of the walnut peptides are antihypertensive, antioxidant, learning improvement, and anticancer, among others. Furthermore, WP could be applied in the development of functional foods or dietary supplements, such as delivery systems and food additives, among others. This review summarizes recent knowledge on the nutritional, functional, and bioactive peptide aspects of WP and possible future products, providing a theoretical reference for the utilization and development of oil crop waste.

Isolation and Characterization of Protein Fractions for Valorization of Sacha Inchi Oil Press-Cake

The growing interest in plant-based food protein sources has provided opportunities for the valorization of agri-food by-products, driving the food industry towards more sustainable development. In this study, three extraction procedures (varying the pH value (7.0 and 11.0) and the addition of salt (0 and 5%)) were investigated to obtain seven different protein fractions (SIPF) from Sacha Inchi oil press-cake (SIPC), which were characterized in terms of their protein content, electrophoretic profile, secondary structure, and techno-functional properties. Extractions at pH 11.0 without salt addition produced the highest values of protein content, extraction yield, protein recovery, and protein concentration increase (84.0%, 24.7%, 36.5%, and 1.5-fold, respectively). Under these extraction conditions, the electrophoretic analysis indicated that most of the SIPC proteins were extracted. SIPF displayed an excellent oil absorption capacity (4.3-9.0 w/w), and interesting foam activity (36.4-133.3%). The solubility and emulsifying activity of the albumin fractions were significantly higher than those of the other fractions (~87 vs. <15.8%, and 280-370 vs. <140 m²/g, respectively). Correlation analysis showed that the secondary structure of the SIPF significantly influences their techno-functional properties. These results indicate that SIPC is a by-product of great potential for protein extraction processes, and as a valorization strategy for technical cycle solutions for the Sacha Inchi productive chain in the circular economy context.

Effect of Oxidative Modification by Peroxyl Radical on the Characterization and Identification of Oxidative Aggregates and In Vitro Digestion Products of Walnut (Juglans regia L.) Protein Isolates

Walnut protein is a key plant protein resource due to its high nutritional value, but walnuts are prone to oxidation during storage and processing. This article explored the oxidative modification and digestion mechanism of walnut protein isolates by peroxyl radical and obtained new findings. SDS-PAGE and spectral analysis were used to identify structural changes in the protein after oxidative modification, and LC-MS/MS was used to identify the digestion products. The findings demonstrated that as the AAPH concentration increased, protein carbonyl content increased from 2.36 to 5.12 nmol/mg, while free sulfhydryl content, free amino content, and surface hydrophobicity decreased from 4.30 nmol/mg, 1.47 μmol/mg, and 167.92 to 1.72 nmol/mg, 1.13 μmol/mg, and 40.93 nmol/mg, respectively. Furthermore, the result of Tricine-SDS-PAGE in vitro digestion revealed that protein oxidation could cause gastric digestion resistance and a tendency for intestinal digestion promotion. Carbonyl content increased dramatically during the early stages of gastric digestion and again after 90 min of intestine digestion, and LC-MS/MS identified the last digestive products of the stomach and intestine as essential seed storage proteins. Oxidation causes walnut proteins to form aggregates, which are then re-oxidized during digestion, and proper oxidative modification may benefit intestinal digestion.

Phosphorylated walnut protein isolate as a nanocarrier for enhanced water solubility and stability of curcumin

BACKGROUND

The low solubility and poor dispersion of alkaline-extracted walnut protein isolate (AWPI) limit its application as a protein-based carrier for the delivery of poorly soluble nutraceuticals, including curcumin. This work investigated the physicochemical characteristics of phosphorylated walnut protein isolate (PWPI) extracted using sodium tripolyphosphate (STP) and evaluated its encapsulation ability.

RESULTS

The results of phosphorus determination, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy confirmed the phosphorylation of the extracted PWPI. Circular dichroism (CD) analysis indicated that PWPI contained higher α-helix and lower β-sheet contents than AWPI. The PWPI prepared at pH 9.0 and 11.0 showed significantly improved solubility, similar surface hydrophobicity, and increased surface charges compared to the AWPI. Fluorescence quenching experiments indicated that the binding affinity of curcumin to PWPI was significantly higher than that of AWPI. When bound to PWPI, the solubility of curcumin in aqueous solution was greatly enhanced, with an 8700-fold increase at a nanocomplex concentration of 10 mg mL^-1 . The complexation of curcumin with PWPI significantly improved the storage stability of curcumin. Additionally, the PWPI-curcumin nanocomplexes showed significantly increased antioxidant capacity.

CONCLUSION

Phosphorylated walnut protein isolate showed greatly improved solubility and strong encapsulation ability, making it a promising nanocarrier for curcumin. © 2022 Society of Chemical Industry.

Multi-mode S-type ultrasound-assisted protein extraction from walnut dregs and in situ real-time process monitoring

This study aimed to investigate the impact of multi-mode S-type ultrasound treatment on the protein extraction level of walnut dregs. The structural properties of the walnut protein (WP) were characterized, and the correlation between protein structure and extraction level was analyzed. The in situ real-time monitoring model for the ultrasound-assisted WP extraction process was established by a miniature fiber near-infrared (NIR) spectrometer. Results showed that the protein yield, purity, and comprehensive extraction index (CEI) of extracted WP were 71.07 %, 72.69 %, and 71.72, respectively, under optimal conditions (dual-frequency 20/28 kHz, ultrasonic treatment duration 30 min, and ultrasound power density 120 W/L). The secondary structure of extracted WP displayed that the proportion of α-helix and β-sheet reduced, while the contents of β-turn and random coil increased after ultrasonic treatment. Besides, sonication decreased the disulfide bond content and increased free sulfhydryl (-SH) and surface hydrophobicity compared to the control. The microstructures of WP confirmed that appropriate sonication could unfold the protein aggregates and reduce the particle size. The extraction level of WP is positively correlated with the -SH content (p < 0.01). The quantitative prediction model of Si-PLS for -SH content in the ultrasound-assisted WP extraction process was established and performed a good correction and prediction performance (Rc = 0.9736; RMSECV = 0.446 μmol/L; Rp = 0.9342; RMSEP = 0.807 μmol/L). This study exploited a high-efficiency way for the WP extraction industry, and provided theoretical support for the development of the intelligent system in industrial protein extraction process.

Walnut (Juglans regia L.) Oligopeptide Effects on Enhancing Memory, Cognition and Improving Sleep Quality in Teenagers and Elderly People in a Randomized Double-Blind Controlled Trial

Walnut has been reported to have beneficial effects on improving cognitive performance. This randomized double-blind placebo-controlled clinical trial evaluates the clinical effectiveness and safety of walnut oligopeptide (WO) on memory enhancement, cognition, and sleep quality in teenagers and elderly people. Eighteen teenagers and 18 elderly people were, respectively, randomly allocated to placebo, low dosage (170 mg), and high dosage (340 mg) WO administration groups (n = 6 per group in each population). After 90 days of administration, the Wechsler Adult Intelligence Scale (WAIS) score was significantly increased and the global Pittsburgh Sleep Quality Index (PSQI) score was significantly decreased in the WO administration group. In addition, the average scores for test subjects of Chinese, Mathematics, and English examinations were significantly increased from the baseline for the teenagers in the WO administration group. Our results support the claim that WO has the potential to become a new option for nutritional intervention, to enhance the memory, cognitive ability, and sleep quality of teenagers and elderly people. This study was approved by the Institutional Review Board of the Shanghai Nutrition Society and registered at the Chinese Clinical Trial Registry ( http://www.chictr.org.cn ) with an ID number of ChiCTR1900028160.

Preparation, characterization and stability of nanoliposomes loaded with peptides from defatted walnut (Juglans regia L.) meal

This study aimed to encapsulate walnut peptides with different molecular weights (crude peptides, 5-10 kDa and < 5 kDa) within nanoliposomes. The peptides with molecular weight (MW) of 5-10 kDa (F2) was chosen as a representative sample to indicate the formation mechanism of nanoliposomes using scanning electron microscopy (SEM) and transmission electron microscope (TEM). The storage and simulated digestion experiment were carried out to evaluate the protective effect of nanoliposomes loading walnut peptides. Our results indicated that the amino acid composition was affected by peptide MW, and F2 exhibited the highest content of hydrophobic amino acids content. The MW of peptides also affected the distribution of the peptide of nanoliposomes, resulting in changes in particle size, ζ-potential, and encapsulation efficiency. The SEM exhibited that a high concentration of nanoliposomes might result in phospholipid fusion and larger particle diameters. The TEM showed individual nanoliposomes had spherical, smooth and full vesicle structures. The nanoliposomes could improve the stability of walnut peptides during storage. The maximum peptides retention after in vitro digestion was 61.6%, indicating a better sustained release in gastric digestion. The present study suggested that nanoliposomes can offer adequate protection to the walnut peptides during storage and digestion.

Preparation of Heat-Sensitivity Proteins from Walnut Meal by Sweep Frequency Ultrasound-Assisted Alkali Extraction

Sweep frequency ultrasound- (SFU-) assisted alkali extraction was conducted to increase the yield and content of heat-sensitive protein of walnut meal under a relatively mild condition. The physicochemical and structural characteristics of the proteins obtained by SFU-assisted alkali extraction and the conventional alkali extraction were compared. It was found that the optimal parameters for the SFU-assisted extraction were the solid-liquid ratio of 1 : 12, pH value of 9, initial temperature of 25°C, ultrasonic frequency of 28 kHz, sweep frequency amplitude of 1.5 kHz, sweep frequency cycle of 100 ms, duty ratio of 77%, and ultrasonic time of 90 min. Under this condition, a vast improvement in the walnut protein yield (34.9%) and the walnut protein content (9.8%) was observed. Such improvement was due to the structural changes of the sonicated protein; e.g., SFU decreased the intermolecular/intramolecular hydrogen bond force of proteins and, therefore, caused more order secondary structures and more loosen microstructures. This helped to improve the thermoplastic and solubility of the heat-sensitivity protein. Thus, SFU treatment could be an effective auxiliary technology in the alkali extraction of heat-sensitivity walnut protein. It might also be a promising technology for the extraction of heat-sensitivity protein from other agricultural by-products.

Walnut pellicle phenolics greatly influence the extraction and structural properties of walnut protein isolates

This study investigated the effects of walnut phenolics and extraction methods on the composition and structural properties of walnut protein isolates (WPIs). Fluorescence quenching experiments showed that walnut phenolics could bind to walnut globulins, albumins, and glutelins with apparent affinity constants of 5.49 × 10⁴ M^-1, 1.71 × 10⁴ M^-1, and 3.10 × 10⁴ M^-1, respectively. However, the UV turbidity and dynamic light scattering (DLS) measurements indicated that phenolics could lead to the severe precipitation of globulins and albumins but not glutelins. The removal of pellicles could significantly increase the yield rate of salt-soluble globulins to approximately 72.8%. Furthermore, salt- and alkaline-extraction methods could produce comparable WPIs yields when using pellicle-free walnut kernels. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and size-exclusive chromatography indicated that the major protein compositions of the salt- and alkaline-extracted WPIs from pellicle-free walnut kernels were similar, while alkaline-extracted WPIs from kernels with pellicles exhibited phenolic-induced protein aggregation. Fourier transform infrared (FTIR) spectroscopy indicated that WPIs produced from kernels with pellicles contained more α-helix and less β-sheet structures than WPIs produced from pellicle-free kernels. These results confirm that walnut pellicle phenolics and the extraction methods could greatly influence the composition and structural properties of WPIs.

Effect of dynamic ultra-high pressure homogenization on the structure and functional properties of whey protein

The effects of dynamic ultra-high pressure homogenization (UHPH) on the structure and functional properties of whey protein were investigated in this study. Whey protein solution of 10 mg/mL (1% w/w) was prepared and processed by a laboratory scale high pressure homogenizer with different pressures (25, 50, 100, 150, 200, and 250 MPa) at an initial temperature of 25 °C. Then, the solution samples were evaluated in terms of secondary structure, sulfhydryl and disulfide bond contents, surface hydrophobicity, average particle size, solubility, foaming capacity, emulsifying activity, and thermal properties. It was found that the secondary structure of whey protein changed with the dynamic UHPH treatment. The interchange reaction between the disulfide bond and the sulfhydryl group was promoted and the surface hydrophobicity significantly increased. The functional properties of the whey protein accordingly changed. Specifically, after dynamic UHPH treatment, the average particle size of the whey protein and emulsion decreased while the solubility, the foaming capability and the emulsification stability increased significantly. The results also revealed that with the dynamic UHPH at 150 MPa, the best improvement was observed in the whey protein functional properties. The whey protein solubility increased from 63.15 to 71.61% and the emulsification stability improved from 195 to 467 min.

Preparation, characterization and physicochemical properties of novel low-phosphorus egg yolk protein

BACKGROUND

In order to supply adequate dietary protein for chronic kidney disease (CKD) patients while simultaneously controlling phosphorus intake, a novel method was developed for the preparation of low-phosphorus egg yolk protein (LPYP) using alkaline protease auxiliary dephosphorization. In addition, the physicochemical properties of LPYP were studied.

RESULTS

In comparison with raw egg yolk protein (RYP) and defatted egg yolk protein (DFYP), LPYP was found to exhibit differences in amino acid (AA) composition, protein secondary structure, surface hydrophobicity, solubility and emulsion stability. It was observed that dephosphorization improved the AA composition, soluble protein content and dissolution stability of egg yolk protein. In addition, phosphate groups were found to impose a critical influence on the emulsion stability and particle size distribution. The final phosphorus to protein mass ratio (P/Pro) of LPYP was 5.64, which met the requirements of a protein diet for CKD patients. The FAO/WHO mode closeness and stability coefficient were 0.958 and 98.62% respectively.

CONCLUSION

LPYP can be effectively obtained by alkaline protease hydrolysis and subsequent alkali dephosphorization. The prepared LPYP can be considered to be a type of safe and suitable protein resource for CKD patients. © 2018 Society of Chemical Industry.

Comparative study of the quality characteristics of defatted soy flour treated by supercritical carbon dioxide and organic solvent

Defatted soy flour is a potential source of food protein, amino acids, ash and isoflavones. The supercritical carbon dioxide (SC-CO₂) and a traditional organic solvent extraction methods were used to remove fat from soy flour, and the quality characteristics of a control soy flour (CSF), defatted soy flour by SC-CO₂ (DSFSC-CO₂) and defatted soy flour by an organic solvent (DSF-OS) were compared. The SC-CO₂ process was carried out at a constant temperature of 45 °C, and a pressure of 40 MPa for 3 h with a CO₂ flow rate of 30 g/min. The DSFSC-CO₂ had significantly higher protein, ash, and amino acids content than CSF and DSF-OS. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis demonstrated that CSF and DSFSC-CO₂ had protein bands of similar intensity and area that indicated no denaturation of protein, whereas DSF-OS showed diffuse bands or no bands due to protein denaturation. In addition to higher nutritional value and protein contents, DSFSC-CO₂ showed superior functional properties in terms of total soluble solids content, water and oil absorption, emulsifying and foaming capacity. The SC-CO₂ method offers a nutritionally and environmentally friendly alternative extraction processing approach for the removal of oil from high-protein food sources. It has a great potential for producing high-protein fat-free, and low-calorie content diet than the traditional organic solvent extraction method.

Metabolomics reveals significant variations in metabolites and correlations regarding the maturation of walnuts (Juglans regia L.)

The content of walnut metabolites is related to its nutritive value and physiological characteristics, however, comprehensive information concerning the metabolome of walnut kernels is limited. In this study we analyzed the metabolites of walnut kernels at five developmental stages from filling to ripening using GC-MS-based untargeted metabolomics; of a total 252 peaks identified, 85 metabolites were positively identified. Further statistical analysis revealed that these 85 metabolites covered different types of metabolism pathways. PCA scores revealed that the metabolic compositions of the embryo are different at each stage, while the metabolic composition of the endotesta could not be significantly separated into distinct groups. Additionally, 7225 metabolite-metabolite correlations were detected in walnut kernel by a Pearson correlation coefficient approach; during screening of the calculated correlations, 463 and 1047 were determined to be significant with r²≥0.49 and had a false discovery rate (FDR) ≤0.05 in endotesta and embryo, respectively. This work provides the first comprehensive metabolomic study of walnut kernels and reveals that most of the carbohydrate and protein-derived carbon was transferred into other compounds, such as fatty acids, during the maturation of walnuts, which may potentially provide the basis for further studies on walnut kernel metabolism.

Summary: This work provides a comprehensive metabolomic study of walnut kernels and reveals most of the carbohydrate and protein-derived carbons were transferred into other compounds during the maturation of walnuts.

Physico-chemical properties, antioxidant activities and antihypertensive effects of walnut protein and its hydrolysate

BACKGROUND

Some food proteins hydrolysates are found to possess multiple health effects. In this study, walnut protein (WP) was enzymatically hydrolysed by alcalase and trypsin under optimal conditions. The physico-chemical properties, antioxidant and angiotensin-converting enzyme (ACE) inhibitory activities of WP, alcalase-generated walnut protein hydrolysate (AWPH) and trypsin-generated walnut protein hydrolysate (TWPH) were comparatively studied. Stability properties of the walnut protein hydrolysate (WPH) and the antihypertensive activity in spontaneously hypertensive rats (SHRs) were also investigated.

RESULTS

The WPH showed higher physico-chemical properties, antioxidant activities, ACE inhibitory activity and stability against thermal treatment and gastrointestinal digestion than WP. The results of antihypertensive effects in SHRs showed that the most potent decrease of AWPH and TWPH in the systolic blood pressure occurred at 4 h (-26 mmHg) and 6 h (-30 mmHg) after administration. The study indicated that the WPH could significantly decrease the systolic blood pressure (P < 0.05).

CONCLUSION

The WPH exhibited high physico-chemical properties, potent inhibitory activities and high stability. TWPH was more effective than AWPH in the detected properties. The results would be helpful for the comprehensive utilisation of the walnut resources. © 2015 Society of Chemical Industry.

Effect of maleylation on physicochemical and functional properties of rapeseed protein isolate

Influence of maleylation on the physicochemical and functional properties of rapeseed protein isolate was studied. Acylation increased whiteness value and dissociation of proteins, but reduced free sulfhydryl and disulfide content (p < 0.05). Intrinsic fluorescence emission and FTIR spectra revealed distinct perturbations in maleylated proteins' tertiary and secondary conformations. Increase in surface hydrophobicity, foaming capacity, emulsion stability, protein surface load at oil-water interface and decrease in surface tension at air-water interface, occurred till moderate level of modification. While maleylation impaired foam stability, protein solubility and emulsion capacity were markedly ameliorated (p < 0.05), which are concomitant with decreased droplet size distribution (d 32). In-vitro digestibility and cytotoxicity tests suggested no severe ill-effects of modified proteins, especially up to low degrees of maleylation. The study shows good potential for maleylated rapeseed proteins as functional food ingredient.