High internal phase emulsions stabilized by alkaline-extracted walnut protein isolates and their application in food 3D printing.

Alkaline-extracted walnut protein isolates showed relatively poor solubility and emulsifying properties in many previous studies. However, whether they can be used as potential emulsifiers to stabilize high internal phase emulsions (HIPEs) remains unknown. Herein, walnut protein isolates were prepared by alkaline extraction from walnut kernels with or without pellicles (named PAWPI and AWPI, respectively). PAWPI conjugated with pellicle polyphenols showed improved solubility and higher antioxidant capacity than AWPI. HIPEs were fabricated via a one-step method using AWPI or PAWPI as the sole protein emulsifier. HIPEs (oil fraction of 0.8, with 0.1% ß-carotene) could be stabilized by PAWPI at a relatively low concentration of 0.2% (w/v), while at least 1% (w/v) AWPI was required to effectively stabilize HIPEs. HIPEs stabilized by PAWPI had smaller oil droplet sizes than those stabilized by AWPI. Rheological analysis indicated that PAWPI-stabilized HIPEs showed higher viscosity and better viscoelasticity than AWPI-stabilized HIPEs. Large-amplitude oscillation shearing analysis suggested that PAWPI-stabilized HIPEs were stiffer but more brittle than AWPI-stabilized HIPEs. Moreover, both PAWPI- and AWPI-stabilized HIPEs exhibited good storage stability and were relatively stable against heat treatment and ionic strength. PAWPI-stabilized HIPEs showed a higher protective capacity for encapsulated ß-carotene than AWPI-stabilized HIPEs. In addition, PAWPI-stabilized HIPEs showed good 3D printability and could be used as a promising edible ink.

The effects of conjugation of walnut protein isolate with polyphenols on protein solubility, antioxidant activity, and emulsifying properties.

Alkaline-extracted walnut protein isolate (AWPI) shows poor solubility in aqueous solutions, resulting in relatively low emulsion capacity. This work investigated the influence of ellagic acid (EA) or epigallocatechin-3-gallate (EGCG) conjugation on the solubility and emulsifying properties of AWPI. The increase in polyphenol content and decrease in free amino and thiol groups of walnut proteins confirmed successful conjugation. AWPI polyphenol conjugates showed significantly reduced surface hydrophobicity, greatly enhanced surface charge, and consequently improved solubility. Circular dichroism (CD) results indicated that the polyphenol-conjugated AWPI contained relatively higher α-helical and lower ß-sheet contents than AWPI. The antioxidant capacity of the polyphenol-conjugated AWPI was significantly enhanced. Additionally, polyphenol conjugation resulted in decreased mean particle sizes and increased surface charges of the walnut protein-covered oil droplets. The emulsions stabilized by AWPI-polyphenol conjugates were relatively more stable over a range of pH values (7.0-11.0) and thermal treatments (25 °C-90 °C). Moreover, they exhibited greater storage stability than those stabilized by unmodified AWPI.

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 ß-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.

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 ß-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.

Ellagic acid and pentagalloylglucose are potential inhibitors of prion protein fibrillization.

Prion diseases are fatal neurodegenerative diseases caused by the conformational transition of the cellular prion protein (PrPC) to the abnormal pathological prion protein (PrPSc). In this work, the effects of ellagic acid (EA) and pentagalloylglucose (PGG) on prion protein (PrP) fibrillization were investigated. Fluorescence quenching experiments indicated that both EA and PGG could specifically interact with native human PrP with binding affinities of 1.92 × 105 and 2.36 × 105 L·mol-1, respectively. Thioflavin-T (ThT) fluorescence assays showed that the binding of EA or PPG could effectively inhibit the nucleation and elongation of PrP fibrilization and reduce the amount of PrP fibrils generated. EA and PGG could also lead to a significant disaggregation of PrP fibrils. Circular dichroism (CD) measurements suggested that EA- or PPG-bound PrP could preserve a higher content of α-helical structures than ß-sheet-rich PrP fibrils. The PrP aggregates formed in the presence of EA or PGG showed lower resistance to proteinase K (PK) digestion. Overall, the present work reported the inhibitory effect of EA and PGG on PrP fibrillization. These two natural polyphenols could be potential prodrug molecules for the prevention and treatment of prion diseases.