Advanced Insights into Walnut Protein: Structure, Physiochemical Properties and Applications

The effect of enzymatic deamidation on the solubility and emulsifying properties of walnut protein isolate

BACKGROUND

Alkaline-extracted walnut protein isolates (WPI) exhibit limited solubility, which poses challenges for their application in the food industry. The present study investigated the effects of protein-glutaminase (PG) deamidation on the physicochemical characteristics, solubility and emulsifying properties of WPI.

RESULTS

The deamidation process of WPI was monitored by assessing the release of free ammonia and the reduction in solution turbidity. PG deamidation significantly increased the surface charge of WPI and modified its surface hydrophobicity with increasing deamidation degree (DD), resulting in a gradual improvement in solubility by approximately 50-70%. Furthermore, the emulsifying capacity of deamidated WPI (DeWPI), specifically at 0.25 h (DeWPI0.25, DD 7%) and 9 h (DeWPI9, DD 23%), was evaluated for stabilizing low internal phase emulsions (LIPEs) and high internal phase emulsions (HIPEs). LIPEs stabilized by WPI and DeWPI0.25 exhibited significant flocculation of oil droplets, leading to decreased stability against heat, salt treatment and storage compared to those stabilized by DeWPI9. DeWPI-stabilized HIPEs showed a 2-2.5-fold higher storage modulus compared to those stabilized by WPI. However, HIPEs stabilized by DeWPI0.25 displayed higher flow stress and flow strain compared to DeWPI9-stabilized HIPEs. Overall, DeWPI-stabilized HIPEs demonstrated relatively high physical stability against storage, heat treatment and high ionic strength.

CONCLUSION

PG deamidation significantly enhanced the solubility and influenced the emulsifying properties of WPI in a manner dependent on the DD. © 2024 Society of Chemical Industry.

Improving physicochemical properties and gel formation mechanism of nutty plant-based yogurt with Tremella fuciformis polysaccharides

Developing nutty plant-based yogurt (NPBY) with desired texture and sensory properties has been challenging. This study sought to investigate the effects of Tremella fuciformis polysaccharides (TFPS) on the physicochemical, textural, rheological, and microstructural properties of NPBY. The introduction of TFPS enhanced the accumulation of organic acids, water holding capacity, and antioxidant activity. The firmness of NPBY with 0.85 % TFPS increased from 187.77 × 10-3 N to 259.90 × 10-3 N, with significant enhancements in elastic modulus (G', G'') and apparent viscosity. Furthermore, the introduction of 0.85 % TFPS significantly improved liking scores in sensory evaluations. Microstructural analysis revealed that TFPS promoted the formation of proteins and oil body clusters, resulting in a more compact gel network. The synergetic effects of electrostatic and hydrophobic interactions were identified as primary driving forces for NPBY gel formation. This study provides valuable insights into the role of natural polysaccharides in strengthening plant-based yogurt gel.

Quality characteristics and fibrous structure formation mechanism of walnut protein and wheat gluten meat analogues during high-moisture extrusion cooking process

Blending two or more materials to create better high-moisture meat analogues has been actively studied in the food science and technology field. Walnut protein is a high-quality plant-based protein resource, yet its full potential remains underexploited. Thus, this study focused on exploring the quality characteristics and fibrous structure formation mechanism of walnut protein (WP) and wheat gluten (WG) meat analogues during high-moisture extrusion cooking process. Results showed that the optimized WP and WG-blended high-moisture meat analogues exhibited a more pronounced anisotropic and oriented fibrous structure. The blending of WP and WG can protect the molecular chains from the thermal transition, and promote the aggregation of protein molecules mainly by enhancing the interaction between hydrophobic interactions and hydrogen bonds, increasing the apparent viscosity and forming protein subunits with larger molecular weights (>100 kDa) to stabilize the newly formed conformation. Additionally, the content of α-helix was the highest among the secondary structures. This study provides a theoretical basis for the application of WG and WP to produce HMMAs with rich fibrous structures.

Preparation and Properties of Walnut Protein Isolate-Whey Protein Isolate Nanoparticles Stabilizing High Internal Phase Pickering Emulsions

To enhance the functional properties of walnut protein isolate (WalPI), hydrophilic whey protein isolate (WPI) was selected to formulate WalPI-WPI nanoparticles (nano-WalPI-WPI) via a pH cycling technique. These nano-WalPI-WPI particles were subsequently employed to stabilize high internal phase Pickering emulsions (HIPEs). By adjusting the mass ratio of WalPI to WPI from 9:1 to 1:1, the resultant nano-WalPI-WPI exhibited sizes ranging from 70.98 to 124.57 nm, with a polydispersity index of less than 0.326. When the mass ratio of WalPI to WPI was 7:3, there were significant enhancements in various functional properties: the solubility, denaturation peak temperature, emulsifying activity index, and emulsifying stability index increased by 6.09 times, 0.54 °C, 318.94 m2/g, and 552.95 min, respectively, and the surface hydrophobicity decreased by 59.23%, compared with that of WalPI nanoparticles (nano-WalPI), with the best overall performance. The nano-WalPI-WPI were held together by hydrophobic interactions, hydrogen bonding, and electrostatic forces, which preserved the intact primary structure and improved resistance to structural changes during the neutralization process. The HIPEs stabilized by nano-WalPI-WPI exhibited an average droplet size of less than 30 μm, with droplets uniformly dispersed and maintaining an intact spherical structure, demonstrating superior storage stability. All HIPEs exhibited pseudoplastic behavior with good thixotropic properties. This study provides a theoretical foundation for enhancing the functional properties of hydrophobic proteins and introduces a novel approach for constructing emulsion systems stabilized by composite proteins as emulsifiers.

Modulation of cecal microbiota and fecal metabolism in mice by walnut protein

Walnut meal is a by-product of walnut oil pressing, in which the protein content is more than 40%, which is an excellent food raw material, but at present, it is basically used as animal feed or discarded, which results in a great waste of resources, and its modulating effect on the intestinal microbiota is not clear. In this study, we used supercritically extracted walnut meal as a raw material, prepared walnut meal isolate protein (WP) by alkaline extraction and acid precipitation, and systematically analyzed its structure by Fourier infrared spectroscopy (FTIR), Raman spectroscopy (Raman), and scanning electron microscopy (SEM); meanwhile, we explored the effects of WP on the cecal bacterial flora and fecal metabolites of mice by microbiological and metabolomic techniques. The results showed that the protein content of WP prepared using alkaline extraction and acid precipitation was as high as 83.7%, in which arginine and glutamic acid were abundant, and it has the potential to be used as a raw material for weight-loss meal replacement food; FTIR and Raman analyses showed that the absorption peaks of WP's characteristic functional groups were obvious, and that the content of the α-helix and β-fold in the secondary structure was greater than 30%, which indicated that it was structurally stable; differential scanning calorimetry (DSC) and SEM analyses showed that WP is a typical spherical particle, its denaturation temperature is 73.6 °C, and it has good thermal stability. Supplementation of WP significantly altered the composition of the intestinal flora in mice, with an increase in beneficial bacteria and a decrease in harmful bacteria; the strongest modulation of the intestinal flora was achieved by altering the composition of the intestinal flora and by increasing the number of Akkermansia (p < 0.01), which consequently affects the function of the microbiota. Based on LC-MS metabolomic results, we identified a total of 87 WP-regulated metabolites, mainly enriched in the bile secretion pathway, which had the highest relevance, followed by benzoxazine biosynthesis. In summary, walnut protein is an important plant protein and has a positive impact on intestinal health, which may provide new ideas for the development of functional foods.

Quality Evaluation of Walnuts from Different Regions in China

This study analyzed and evaluated the basic crude fat contents, crude protein contents, phenolic compounds, lipid compositions (fatty acids, phytosterols, and tocopherols), and amino acid compositions of 26 walnut samples from 11 walnut-growing provinces in China. The results indicate that the oil contents of the samples varied from 60.08% to 71.06%, and their protein contents ranged from 7.26 g/100 g to 19.50 g/100 g. The composition of fatty acids corresponded to palmitic acid at 4.61-8.27%, stearic acid at 1.90-3.55%, oleic acid at 15.50-32.28%, linoleic acid at 53.44-67.64%, and α-linolenic acid at 2.45-12.77%. The samples provided micronutrients in widely varying amounts, including tocopherol, phytosterol, and total phenolic content, which were found in the walnut oil samples in amounts ranging from 356.49 to 930.43 mg/kg, from 1248.61 to 2155.24 mg/kg, and from 15.85 to 68.51 mg/kg, respectively. A comprehensive evaluation of walnut oil quality in the samples from the 11 provinces using a principal component analysis was conducted. The findings revealed that the samples from Henan, Gansu, and Zhejiang had the highest composite scores among all provinces. Overall, Yunnan-produced walnuts had high levels of crude fat, polyunsaturated fatty acids, and total tocopherols, making them more suitable for producing high-quality oil, whereas Henan-produced walnuts, although lower in crude fat, had a higher crude protein content and composite score, thus showing the best walnut characteristics.