Limited hydrolysis of dehulled walnut (Juglans regia L.) proteins using trypsin: Functional properties and structural characteristics

Tailoring soy protein/corn zein mixture by limited enzymatic hydrolysis to improve digestibility and functionality

This study aimed to modify plant protein mixture to improve their functionality and digestibility by limited hydrolysis. Soy protein isolate and corn zein were mixed at the ratio of 5:1 (w/w), followed by limited hydrolysis using papain from 15 to 30 min. The structural characteristics, in vitro digestibility, and functional properties were evaluated. Also, DPPH radical scavenging activity was determined. The results indicated that the molecular weight of different modified samples was largely reduced by limited hydrolysis, and the proportion of random coil was significantly increased. Furthermore, the solubility, foaming, emulsifying and water-holding capacity of hydrolyzed protein mixture were significantly improved, which were close to those of whey protein isolate. In vitro digestibility after 30-min limited hydrolysis was remarkably elevated. In addition, the hydrolyzed protein mixture exhibited a higher antioxidant activity than those of untreated proteins. Overall, limited hydrolysis of protein mixture led to improved digestibility, functionality and antioxidant activity.

Walnut protein isolate-epigallocatechin gallate nanoparticles: A functional carrier enhanced stability and antioxidant activity of lycopene

Walnut isolate protein (WPI)-epigallocatechin gallate (EGCG) conjugates can be employed to creat food-grade delivery systems for preserving bioactive compounds. In this study, WPI-EGCG nanoparticles (WENPs) were developed for encapsulating lycopene (LYC) using the ultrasound-assisted method. The results indicated successful loading of LYC into these WENPs, forming the WENPs/LYC (cylinder with 200-300 nm in length and 14.81-30.05 nm in diameter). Encapsulating LYC in WENPs led to a notable decrease in release rate and improved stability in terms of thermal, ultraviolet (UV), and storage conditions compared to free LYC. Simultaneously, WENPs/LYC exhibited a synergistic and significantly higher antioxidant activity with an EC50 value of 23.98 μg/mL in HepG2 cells compared to free LYC's 31.54 μg/mL. Treatment with WENPs/LYC led to a dose-dependent restoration of intracellular antioxidant enzyme activities (SOD, CAT, and GSH-Px) and inhibition of intracellular malondialdehyde (MDA) formation. Furthermore, transcriptome analysis indicated that enrichment in glutathione metabolism and peroxisome processes following WENPs/LYC addition. Quantitative real-time reverse transcription PCR (qRT-PCR) verified the expression levels of related genes involved in the antioxidant resistance pathway of WENPs/LYC on AAPH-induced oxidative stress. This study offers novel perspectives into the antioxidant resistance pathway of WENPs/LYC, holding significant potential in food industry.

Enhancing functionality and bioactivity of walnut protein through limited enzyme digestion

BACKGROUND

Walnut protein (WP) is recognized as a valuable plant protein. However, the poor solubility and functional properties limit its application in the food industry. It is a great requirement to improve the physicochemical properties of WP.

RESULTS

Following a 90 min restricted enzymatic hydrolysis period, the solubility of WP significantly increased from 3.24% to 54.54%, with the majority of WP hydrolysates (WPHs) possessing a molecular weight exceeding 50 kDa. Circular dichroism spectra showed that post-hydrolysis, the structure of the protein became more flexible, while the hydrolysis time did not significantly alter the protein's secondary structure. After hydrolysis, WP's surface hydrophobicity significantly increased from 2279 to 6100. Furthermore, WPHs exhibited a strong capacity for icariin loading and micelle formation with critical micelle concentration values of 0.71, 0.99 and 1.09 mg mL-1, respectively. Moreover, similar immuno-enhancement activities were observed in WPHs. After exposure to WPHs, the pinocytosis of RAW264.7 macrophages was significantly improved. WPH treatment also increased the production of nitric oxide, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) in macrophages. Up-regulation of mRNA expressions of IL-6, inducible nitric oxide synthase (iNOS) and TNF-α was observed in a dose-dependent manner.

CONCLUSION

The enhancement of functionality and bioactivity in WP can be achieved through the application of limited enzyme digestion with trypsin. This process effectively augments the nutritional value and utility of the protein, making it a valuable component in various dietary applications. © 2024 Society of Chemical Industry.

Performance of Azolla pinnata fern protein hydrolysates as an emulsifier and nutraceutical ingredient in an O/W emulsion system and their effect on human gut microbiota and mammalian cells

This study investigated the dual potential of Azolla pinnata fern protein hydrolysates (AFPHs) as functional and nutraceutical ingredients in an oil/water emulsion system. The AFPH-stabilised emulsion (AFPH-E) displayed a small and uniform droplet distribution and was stable to aggregation and creaming over a wide range of pH (5-8), salt concentrations ≤ 100 mM, and heat treatment ≤ 70 °C. Besides, the AFPH-E possessed and maintained strong biological activities, including antihypertensive, antidiabetic, and antioxidant, under different food processing conditions (pH 5-8; NaCl: 50-150 mM, and heat treatment: 30-100 °C). Following in vitro gastrointestinal digestion, the antihypertensive and antioxidant activities were unchanged, while a notable increase of 8% was observed for DPPH. However, the antidiabetic activities were partially reduced in the range of 5-11%. Notably, AFPH-E modulated the gut microbiota and short-chain fatty acids (SCFAs), promoting the growth of beneficial bacteria, particularly Bifidobacteria and Lactobacilli, along with increased SCFA acetate, propionate, and butyrate. Also, AFPH-E up to 10 mg mL-1 did not affect the proliferation of the normal colon cells. In the current work, AFPH demonstrated dual functionality as a plant-based emulsifier with strong biological activities in an oil/water emulsion system and promoted healthy changes in the human gut microbiota.

Screening and Constructing of Novel Angiotensin I-Converting Enzyme Inhibiting Peptides from Walnut Protein Isolate and Their Mechanisms of Action: A Merged In Silico and In Vitro Study

Angiotensin I-converting enzyme (ACE)-inhibiting peptides were isolated from walnut protein isolate (WPI) using ultrasound-assisted extraction. This study aimed to assess the impact of ultrasonic pretreatment on the physicochemical properties of WPI. The optimal extraction conditions for WPI were determined as a 15-min ultrasonic treatment at 400 W. Subsequently, the hydrolysate exhibiting the highest in vitro ACE-inhibiting activity underwent further processing and separation steps, including ultrafiltration, ion exchange chromatography, liquid chromatography-tandem mass spectrometry, ADMET screening, and molecular docking. As a result of this comprehensive process, two previously unidentified ACE-inhibiting peptides, namely Tyr-Ile-Gln (YIQ) and Ile-Tyr-Gln (IYQ), were identified. In addition, a novel peptide, Ile-Lys-Gln (IKQ), was synthesized, demonstrating superior ACE-inhibiting activity and temperature stability. In silico analysis estimated an in vivo utilization rate of 21.7% for IKQ. These peptides were observed to inhibit ACE through an anti-competitive mechanism, with molecular docking simulations suggesting an interaction mechanism involving hydrogen bonding. Notably, both IYQ and IKQ peptides exhibited no discernible toxicity to HUVECs cells and promoted nitric oxide (NO) generation. These findings underscore the potential of ultrasonicated WPI in the separation of ACE-inhibiting peptides and their utility in the development of novel ACE inhibitors for functional food applications.

Characterization of walnut protein isolate-polyphenol nanoconjugates for the developing a food-grade delivery system

This study investigated the effects of the interaction of walnut protein isolate (WPI) with epigallocatechin gallate (EGCG), chlorogenic acid (CLA), (+)-catechin (CA), and ellagic acid (EA) on the structural and functional properties of proteins. The results for polyphenol binding equivalents and content of free amino and sulfhydryl groups as well as those from sodium dodecyl sulfate‒polyacrylamide gel electrophoresis confirmed the covalent interaction between WPI and the polyphenols. The binding capacities of the WPI-polyphenol mixtures and conjugates were as follows: WPI-EGCG > WPI-CLA > WPI-CA > WPI-EA. Fourier transform infrared spectroscopy (FTIR) and fluorescence spectrum analysis identified changes in the protein structure. The conjugation process obviously increased the polyphenols' antioxidant properties and the surface hydrophobicity was substantially reduced. WPI-EGCG conjugates had the best functional properties, followed by WPI-CLA, WPI-CA, and WPI-EA. Lycopene (LYC) was loaded into nanocarriers by WPI-EGCG self-assembly. These results indicated that WPI-polyphenol conjugates can be utilized to develop food-grade delivery systems to protect chemically lipophilic bioactive compounds.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05768-2.

Structural and textural properties of walnut protein gels induced by ultrasound and transglutaminase: encapsulation and release of tea polyphenols

This study investigated the synergy of ultrasonic and transglutaminase (TGase) treatment on the structural, physicochemical, rheological, gelation properties and controlled release properties of dehulled walnut proteins (DWP). The results showed that after ultrasonic-TGase treatment, the surface hydrophobicity was decreased, indicating the involvement of disulfide bonds in gel formation. Scanning electron microscopy (SEM) showed that ultrasonic-TGase treatment resulted in a more uniform and denser microstructure of DWP gels. Ultrasonic-TGase treatment changed the secondary structure of the DWP gels as determined by Fourier transform infrared spectroscopy, with an increase in α-helix, β-turn and random coils and a decrease in β-sheets. In addition, in vitro drug release profiles showed that ultrasonic-TGase treatment promoted the cross-linking of protein molecules and formed a dense network to embed tea polyphenols (TP), thereby slowing down the digestion of TP in simulated gastric fluid and achieving the purpose of slow-release in simulated intestinal fluid. Thus, the synergy of ultrasonic and TGase treatment might be an effective method to improve gel properties and expand the application of protein gels in the food industries.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05756-6.

The interaction of an effector protein Hap secreted by Aeromonas salmonicida and myofibrillar protein of meat: Possible mechanisms from structural changes to sites of molecular docking

Microbial spoilage of meat products is a significant problem in the food industry. Aeromonas salmonicida is a significant microorganism responsible for spoilage in chilled meat. Its effector protein, hemagglutinin protease (Hap), has been identified as an effective substance for degrading meat proteins. The ability of Hap to hydrolyze myofibrillar proteins (MPs) in vitro demonstrated that Hap has obvious proteolytic activity, which could alter MPs' tertiary structure, secondary structure, and sulfhydryl groups. Moreover, Hap could significantly degrade MPs, focusing primarily on myosin heavy chain (MHC) and actin. Active site analysis and molecular docking revealed that the active center of Hap was bound to MPs via hydrophobic interaction and hydrogen bonding. It may preferentially cleave peptide bonds between Gly44-Val45 in actin, and Ala825-Phe826 in MHC. These findings suggest that Hap may be involved in the spoilage mechanism of microorganisms and provide crucial insights into the mechanisms of meat spoilage induced by bacteria.

Identification of Key Lipogenesis Stages and Proteins Involved in Walnut Kernel Development

Walnuts are abundant in oil content, especially for polyunsaturated fatty acids, but the understanding of their formation is limited. We collected walnut (Juglans regia L.) kernels at 60, 74, 88, 102, 116, 130, and 144 days after pollination (designated S1-S7). The ultrastructure and accumulation of oil bodies (OBs) were observed using transmission electron microscopy (TEM), and the oil content, fatty acid composition, and proteomic changes in walnut kernels were determined. The oil content and OB accumulation increased during the development and rose sharply from S1 to S3 stages, which are considered the key lipogenesis stage. A total of 5442 proteins were identified and determined as differentially expressed proteins (DEPs) using label-free proteomic analysis. Fatty acid desaturases (FAD) 2, FAD3, oleosin, and caleosin were essential and upregulated from the S1 to S3 stages. Furthermore, the highly expressed oleosin gene JrOLE14.7 from walnuts was cloned and overexpressed in transgenic Brassica napus. The overexpression of JrOLE14.7 increased the oil content, diameter, hundred weight of seeds and changed the fatty acid composition and OB size of Brassica napus seeds. These findings provide insights into the molecular mechanism of oil biosynthesis and the basis for the genetic improvement of walnuts.

Formation mechanism and functional properties of walnut protein isolate and soy protein isolate nanoparticles using the pH-cycle technology

Walnut protein isolate (WPI) is a nutritious protein with poor solubility, which severely limits its application. In this study, composite nanoparticles were prepared from WPI and soy protein isolate (SPI) using the pH-cycle technology. The WPI solubility increased from 12.64 to 88.53% with a WPI: SPI ratio increased from 1: 0.01 to 1: 1. Morphological and structural analyses illustrated that interaction forces with hydrogen bonding as the main effect jointly drive the binding of WPI to SPI and that protein co-folding occurs during the neutralization process, resulting in a hydrophilic rigid structure. In addition, the interfacial characterization showed that the composite nanoparticle with a large surface charge enhanced the affinity with water molecules, prevented protein aggregation, and protected the new hydrophilic structure from damage. All these parameters helped to maintain the stability of the composite nanoparticles in a neutral environment. Amino acid analysis, emulsification capacity, foaming, and stability analysis showed that the prepared WPI-based nanoparticles exhibited good nutritional and functional properties. Overall, this study could provide a technical reference for the value-added use of WPI and an alternative strategy for delivering natural food ingredients.

Novel angiotensin I-converting enzyme (ACE) inhibitory peptides from walnut protein isolate: Separation, identification and molecular docking study

Walnut protein isolate was hydrolyzed using alcalase® to obtain angiotensin-I-converting enzyme (ACE) inhibitory (ACEI) peptides. The components with high ACEI activity were successfully purified from walnut protein isolate hydrolysates (WPIH) by ultrafiltration and G-25 gel chromatography. The 1520 peptides were identified by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Then the screening model of ACEI peptides was established by in silico approach. It was found that four ACEI active peptides (PPKP, YPQY, YLPP, and PKPP) were obtained with IC₅₀ values ranging from 506 to 89 μmol/L, among which PPKP had the highest ACEI activity (IC₅₀  = 89 ± 1 μmol/L). The four peptides mentioned above were novel, non-toxic, and resistant to gastrointestinal digestion. The molecular docking studies showed that the ACEI effect of ACEI peptide was mainly due to the interaction with residues of Gln281 and His353 in the ACE active pockets. In vivo availability of ACEI peptides showed that the probability of PPKP binding to ACE was 37.9% in the human body. Our studies suggest that the ACEI peptides derived from the WPIH can be considered functional foods that can prevent hypertension. PRACTICAL APPLICATIONS: Hypertension is a significant risk factor for cardiovascular and cerebrovascular disease, the leading cause of death worldwide. This study used a cost-effective method to isolate and identify potential ACEI peptides from the walnut meal. Since the walnut meal is often discarded in the processing of walnut products and thus pollutes the environment, the preparation of walnut meal into ACEI peptides can reduce the impact of hypertension on people and reduce environmental pollution. The experimental results show that walnut ACEI peptides are a safe and healthy nutritional product.

Recent application of protein hydrolysates in food texture modification

The demand for clean labels has increased the importance of natural texture modifying ingredients. Proteins are unique compounds that can impart unique textural and structural changes in food. However, lack of solubility and extensive aggregability of proteins have increased the demand for enzymatically hydrolyzed proteins, to impart functional and structural modifications to food products. The review elaborates the recent application of various proteins, protein hydrolysates, and their role in texture modification. The impact of protein hydrolysates interaction with other food macromolecules, the effect of pretreatments, and dependence of various protein functionalities on textural and structural modification of food products with controlled enzymatic hydrolysis are explained in detail. Many researchers have acknowledged the positive effect of enzymatically hydrolyzed proteins on texture modification over natural protein. With enzymatic hydrolysis, various textural properties including foaming, gelling, emulsifying, water holding capacity have been effectively improved. It is evident that each protein is unique and imparts exceptional structural changes to different food products. Thus, selection of protein requires a fundamental understanding of its structure-substrate property relation. For wider applicability in the industrial sector, more studies on interactions at the molecular level, dosage, functionality changes, and sensorial attributes of protein hydrolysates in food systems are required.

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.

Impact of Hydro-Alcoholic Solvents on the Oil and Phenolics Extraction from Walnut (Juglans regia L.) Press-Cake and the Self-Emulsification of Extracts

The objective was to evaluate the performance of four hydro-alcoholic solvents to simultaneously extract oil and more polar molecules as phenolics, among others, to produce complex extracts that eventually could self-emulsify after solvent removal. Walnut press-cake was selected as the sourcing material. Extractions were performed as a semi-continuous operation up to a solvent-to-solid ratio of 28, with a fractional collection of the effluent. Among the solvents, labelled by their alcohol content EtOH 58, EtOH 86, iPro 60 and iPro 90 for ethanol (EtOH) and isopropanol (iPro), iPro 90 allowed to reach an oil extraction efficiency of 97% while the recovery for the other solvents was in the range of 30–40%. For both alcohols, the increase of the solvent hydration negatively influenced the oil extraction but positively increased the recovery of phenolics that reached 17.6 mg GAE/g_cake when EtOH 58 was used. Several fractions contained enough surface-active material and oil to self-assemble as emulsions. IPro 90 and EtOH 86 showed better performances in the sense that most extracts were able to emulsify, though extraction kinetics pointed out differences. The most hydrated solvents behaved equally, with extraction yields in the same range and a similar but limited emulsifying capacity of only few fractions.

Modulation of the structural and functional properties of perilla protein isolate from oilseed residues by dynamic high-pressure microfluidization

Dynamic high-pressure microfluidization (DHPM) is an alternative method to physically modify proteins to improve their functional properties. In this study, perilla protein isolate (PPI) was treated by DHPM at different pressures. Results showed that DHPM treatment reduced the particle size and absolute potential of PPI by 75.90% and 22.28%. The increased surface hydrophobicity and free sulfhydryl content were observed in DHPM-treated PPI, which may be caused by the comformation changes of PPI. Furthermore, DHPM treatment would not cause the degradation of the main subunits and the variation of crystalline regions in PPI, but enhancing the thermal stability of PPI at 90 MPa and 120 MPa. Functional properties analysis indicated that DHPM treatment at 120 MPa was more effective in improving the solubility, foaming and emulsifying capacities of PPI. The results suggested that DHPM can be used to enhance the functional properties of PPI and expand its application in food systems.