Walnut (Juglans regia L.) oil chemical composition depending on variety, locality, extraction process and storage conditions: A comprehensive review

Exploring the lipid oxidation mechanisms during pumpkin seed kernels storage based on lipidomics: From phenomena, substances, and metabolic mechanisms

The study investigated the lipid oxidation of pumpkin seed kernels (PSK) under different storage conditions (room temperature, vacuum-room temperature, refrigeration, and vacuum-refrigeration) using HPLC-MS and GC-MS. Experimental results found the vacuum-refrigeration group showed the lowest PV (0.24 g/100 g), diene (8.68), hexanal (356.64 ± 16.06 ng/g), and nonanal (132.05 ± 8.38 ng/g) after a 9-month storage. A total of 586 lipids, including 6 classes and 27 subclasses, were detected, 46 of which showed significant differences. Refrigeration samples had the highest diacylglycerol content, while room temperature samples demonstrated the highest triacylglycerol and phosphatidylcholine content. Differential lipid metabolite analyses indicated that storage conditions mainly affected glycerolipid metabolism, glycerophospholipid metabolism, and sphingolipid metabolism pathways in PSK, while glycerolipid and glycerophospholipid metabolism were still dominant. It revealed that refrigeration was more effective than vacuum in inhibiting the oxidation of PSK. These findings could offer valuable references for the storage, transportation, preservation, and the development and utilization of PSK.

Chemical compositions and oxidative stabilities of cold-pressed walnut oils (Juglans regia L.): Effects of chemical refining, water degumming, and molecular distillation

Walnut oils are of important academic and economic value, and are becoming one of the most important woody oils. Accurate and moderate refining techniques are required to produce high-quality walnut oils. In this work, walnut oils obtained from cold processing were refined in three typical techniques, mainly chemical refining, water degumming, and molecular distillation. Physicochemical properties (acid value and peroxide value [POV]), minor components (tocopherol, polyphenols, and phytosterol), oxidative stability indices, and volatile compounds were analyzed to find out the appropriate refining method for the cold-pressed walnut oils. Quality indices of all the refined oils from the three different refining methods met the requirements of the national standard, of which the POV of chemically refined oil (0.241 g/100 g) was higher than crude oil (0.058 g/100 g). Water degumming was most suitable for retaining of bioactive compounds, for example, the tocopherol was 259.40 mg/kg, the polyphenols was 44.54 mg GAE/kg, and the phytosterol was 987.32 mg/kg, but oxidation stability of the obtained oil (3.09 h) was lower than that of molecular distilled oil (4.18 h). Initial physicochemical properties especially the POV had a significant impact on oxidation stability. There is a trade-off between the retention of nutrients and extending shelf life, indicating appropriate refining techniques should be developed; that is, water degumming is suggested to be involved in producing high-quality cold-pressed walnut oils.

Foliar application of biostimulants improves nutritional and bioactive quality of walnuts

BACKGROUND

Owing to their health benefits, walnuts are attracting interest as a good option for nutritious meals, thereby promoting their production. Furthermore, the adoption of ecologically and environmentally friendly agriculture strengthens biostimulant use as a sustainable complement to traditional fertilizers. This study evaluated the effects of different foliar-applied biostimulants in walnut tree orchards, in northeastern Portugal, on walnuts' chemical composition and bioactivity.

RESULTS

Walnut samples were rich in fat (particularly the polyunsaturated linoleic acid), dietary fiber and protein. Sucrose was the most prevalent soluble sugar, followed by glucose and fructose. Studied samples also showed an antioxidant activity comparable (or superior) to that of Trolox. Some plant biostimulants (e.g. Sprint Plus®) had a positive impact on the nutritional composition of walnuts, more specifically by boosting tocopherol levels, besides improving the bioactivity of walnut extracts against specific bacteria.

CONCLUSION

Overall, this research demonstrated that important quality traits of walnuts can be improved using sustainable agricultural bioproducts and practices. © 2024 Society of Chemical Industry.

Characterization of volatile organic compounds in walnut oil with various oxidation levels using olfactory analysis and HS-SPME-GC/MS

Walnut oil oxidizes and becomes rancid during storage, that could be significantly affecting flavor and quality. This study aimed to monitor the volatile compounds present in walnut oil during storage, identify the characteristic markers of walnut oil at different oxidation levels, and establish a correlation network analysis based on the relationship between the olfactory analyzer and the characteristic markers to understand their correlation. The results indicated that the oxidation level of walnut oil had a positive correlation with the response of the olfactory analyzer. 219 volatile compounds were identified in walnut oil, with 89 identified as key volatile compounds (VIP >1). Among these, compounds such as (E, E)-2,4-decadienal (6.10%-23.04%)，(E, E)-2,4-heptadienal (2.23%-13.61%)，(E)-2-octenal (0.95%-11.71%)， hexanoic acid (1.63%-4.30%)，1-octen-3-ol (2.53%-19.01%)，(Z)-2-heptenal (5.95%-25.01%)，2,3-dihydro-furan (1.08%-3.20%)，2-pentyl-furan (0.13%-0.54%), pyrazine (0.33%-1.32%), hexanal (24.52%-1.33%)，3-hethylbutylacetate (12.44%-1.29%)， 2-methyl butyl acetate (7.74%-1.56%) and ethenyl hexanoate (4.39%-0.41%) were found to be characteristic volatile compounds in the oxidation process of walnut oil. Furthermore, the correlation network analysis revealed a strong correlation between the olfactory analyzer sensors and the characteristic volatile compounds. The findings of this study can provide valuable data for the development of rapid determination of the oxidation level of walnut oil.

Unveiling the "hidden quality" of the walnut pellicle: a precious source of bioactive lipids

Tree nut consumption has been widely associated with various health benefits, with walnuts, in particular, being linked with improved cardiovascular and neurological health. These benefits have been attributed to walnuts' vast array of phenolic antioxidants and abundant polyunsaturated fatty acids. However, recent studies have revealed unexpected clinical outcomes related to walnut consumption, which cannot be explained simply with the aforementioned molecular hallmarks. With the goal of discovering potential molecular sources of these unexplained clinical outcomes, an exploratory untargeted metabolomics analysis of the isolated walnut pellicle was conducted. This analysis revealed a myriad of unusual lipids, including oxylipins and endocannabinoids. These lipid classes, which are likely present in the pellicle to enhance the seeds' defenses due to their antimicrobial properties, also have known potent bioactivities as mammalian signaling molecules and homeostatic regulators. Given the potential value of this tissue for human health, with respect to its "bioactive" lipid fraction, we sought to quantify the amounts of these compounds in pellicle-enriched waste by-products of mechanized walnut processing in California. An impressive repertoire of these compounds was revealed in these matrices, and in notably significant concentrations. This discovery establishes these low-value agriculture wastes promising candidates for valorization and translation into high-value, health-promoting products; as these molecules represent a potential explanation for the unexpected clinical outcomes of walnut consumption. This "hidden quality" of the walnut pellicle may encourage further consumption of walnuts, and walnut industries may benefit from a revaluation of abundant pellicle-enriched waste streams, leading to increased sustainability and profitability through waste upcycling.

Vegetable oils: Classification, quality analysis, nutritional value and lipidomics applications

Lipids are widespread in nature and play a pivotal role as a source of energy and nutrition for the human body. Vegetable oils (VOs) constitute a significant category in the food industry, containing various lipid components that have garnered attention for being natural, environmentally friendly and health-promoting. The review presented the classification of raw materials (RMs) from oil crops and quality analysis techniques of VOs, with the aim of improving comprehension and facilitating in-depth research of VOs. Brief descriptions were provided for four categories of VOs, and quality analysis techniques for both RMs and VOs were generalized. Furthermore, this study discussed the applications of lipidomics technology in component analysis, processing and utilization, quality determination, as well as nutritional function assessment of VOs. Through reviewing RMs and quality analysis techniques of VOs, this study aims to encourage further refinement and development in the processing and utilization of VOs, offering valuable references for theoretical and applied research in food chemistry and food science.

Characterization and metabolism pathway of volatile compounds in walnut oil obtained from various ripening stages via HS-GC-IMS and HS-SPME-GC-MS

Volatile organic compounds (VOCs) of walnut oil (WO) samples obtained from 5 ripening stages were analyzed by headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) and HS-solid phase microextraction-GC-mass spectrometry (HS-SPME-GC-MS). A total of 75 VOCs were identified in WO, of which 24 VOCs were found to be the key aroma-active compounds for WO by using odor activity values (OAVs) analysis. Based on chemometrics methods, flavor of WO samples can be characterized into three categories, i.e., early, mid-, and late stages. WO from early ripening stage had stronger green and sweet odor due to 1,8-cineole (OAV 280) and ethanol (OAV 134.5). While nonanal (OAV 181.82), (E)-2-octenol (OAV 160), and hexanal (OAV 103.78) were sources of intense fatty and oily odor in mid-ripening stage. For WO of later ripening stage, the flavor was affected by nonanal (OAV 192.28), 1-heptanol (OAV 150), heptanal (OAV 71.11) and some organic acids.

Effects of drying method and oil type on edible polyunsaturated oleogels co-structured by hydroxylpropyl methyl cellulose and xanthan gum

The subtle balance between the interactions of polysaccharide molecules and the interactions of polysaccharide molecules with oil molecules is significantly important for developing polysaccharide-based polyunsaturated oleogels. Here, hydroxylpropyl methyl cellulose and xanthan gum were used to structure edible oleogels via emulsion-template methodology, while the effects of drying methods (hot-air drying (AD) and vacuum-freeze drying (FD)) and oil types (walnut, flaxseed and Moringa seed oil) on the structure, oil binding capacity (OBC), rheological properties, thermal behaviors and stability of oleogels were specially investigated. Compared with AD oleogels, FD oleogels exhibited significantly better OBC, enhanced gelation strength (G' value) and better capacity to holding oil after high temperature processing, which was attributed to the possibly increased oil-polysaccharide interactions. However, the weakened polysaccharide-polysaccharide interactions in FD oleogels failed in providing stronger physical interface or enough rigidity to restrict the migration of oil molecules. Polyunsaturated triacylglycerols in vegetable oils deeply participated in the construction of the network of AD oleogels through weak intermolecular non-covalent interactions, which in turn greatly changed the crystallization and melting behaviors of vegetables oils. In brief, this research may provide useful information for the development of polysaccharide-based polyunsaturated oil oleogels.

In Situ Rapid Analysis of Squalene, Tocopherols, and Sterols in Walnut Oils Based on Supercritical Fluid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry

Quantification of liposoluble micronutrients in large-scale vegetable oil samples is urgently needed, because their health benefits are increasingly emphasized. However, current analytical methods are limited to either labor-intensive preparation processes or time-consuming chromatography separation. In this work, an online oil matrix separation strategy for direct, rapid, and simultaneous determination of squalene, tocopherols, and phytosterols in walnut oil (WO) was developed on the basis of the lipid class separation mode of supercritical fluid chromatography. A single run was completed in 13 min containing 6 min of column cleaning and balancing. Satisfactory limit of detections (0.05-0.20 ng/mL), limit of quantifications (0.15-0.45 ng/mL), recoveries (70.61-101.44%), and matrix effects (78.43-91.62%) were achieved, indicating the reliability of this method. In addition, eight sterol esters were identified in WO, which have not previously been reported. The proposed method was applied to characterize the liposoluble micronutrient profile of WO samples obtained from different walnut cultivars, geographical origins, and processes.

Optimized Single-Step Recovery of Lipophilic and Hydrophilic Compounds from Raspberry, Strawberry and Blackberry Pomaces Using a Simultaneous Ultrasound-Enzyme-Assisted Extraction (UEAE)

An ultrasound-enzyme-assisted extraction (UEAE) was optimized to extract, simultaneously, the hydrophilic and lipophilic compounds from three berry pomaces (raspberry, strawberry and blackberry). First, an enzyme screening designated a thermostable alkaline protease as the most suitable enzyme to recover, in an aqueous medium, the highest yields of polyphenols and oil in the most efficient way. Secondly, the selected enzyme was coupled to ultrasounds (US) in sequential and simultaneous combinations. The simultaneous US-alkaline enzyme combination was selected as a one-single-step process and was then optimized by definitive screening design (DSD). The optimized parameters were: US amplitude, 20% (raspberry pomace) or 70% (strawberry and blackberry pomaces); pH, 8; E/S ratio, 1% (w/w); S/L ratio, 6% (w/v); extraction time, 30 min; temperature, 60 °C. Compared to conventional extractions using organic solvents, the UEAE extracted all the polyphenols, with around 75% of the active polyphenols (measured by the DPPH● method) and up to 75% of the initial oil from the berry pomaces. Characterized lipophilic compounds were rich in polyunsaturated fatty acids (PUFAs), tocols and phytosterols. The polyphenolics were analyzed by UPLC-MS/MS; characteristic ellagitannins of the Rosaceae family (sanguiin H-6 or agrimoniin, sanguiin H-10, …) and ellagic acid conjugates were found as the major components.

Detection of walnut oil adulterated with high-linoleic acid vegetable oils using triacylglycerol pseudotargeted method based on SFC-QTOF-MS

Authentication of walnut oil (WO) is challenging due to the adulteration of high-linoleic acid vegetable oils (HLOs) with similar fatty acid composition. To allow the discrimination of WO adulteration, a rapid, sensitive and stable scanning method based on supercritical fluid chromatography quadrupole time-of-flight mass spectrometry (SFC-QTOF-MS) was established to profile 59 potential triacylglycerol (TAGs) in HLOs samples within 10 min. Limit of quantitation of the proposed method is 0.002 µg mL^-1 and the relative standard deviations range from 0.7% to 12.0%. TAGs profiles of WO samples from various varieties, geography origins, ripeness, and processing methods were used to construct orthogonal partial least squares-discriminant analysis (OPLS-DA) and OPLS models that were highly accurate in both qualitative and quantitative prediction at adulteration levels as low as 5% (w/w). This study advances the TAGs analysis to characterize vegetable oils and holds promise as an efficient method for oil authentication.

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% β-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 β-carotene than AWPI-stabilized HIPEs. In addition, PAWPI-stabilized HIPEs showed good 3D printability and could be used as a promising edible ink.

Effects of Packaging Materials on Structural and Simulated Digestive Characteristics of Walnut Protein during Accelerated Storage

Walnuts are rich in fat and proteins that become oxidized during the processing and storage conditions of their kernels. In this study, the effect of three packaging materials (e.g., polyethylene sealed packaging, polyamide/polyethylene vacuum packaging, and polyethylene terephthalate/aluminum foil/polyethylene vacuum packaging) were investigated on the oxidation, structural and digestive properties of walnut kernel proteins. Results showed that the amino acid content gradually decreased and carbonyl derivatives and dityrosine were formed during storage. The protein molecule structure became disordered as the α-helix decreased and the random coil increased. The endogenous fluorescence intensity decreased and the maximum fluorescence value was blue-shifted. After 15 days of storage, surface hydrophobicity decreased, while SDS-PAGE and HPLC indicated the formation of large protein aggregates, leading to a reduction in solubility. By simulating gastrointestinal digestion, we found that oxidation adversely affected the digestive properties of walnut protein isolate and protein digestibility was best for polyethylene terephthalate/aluminum foil/polyethylene vacuum packaging. The degree of protein oxidation in walnuts increased during storage, which showed that except for fat oxidation, the effect of protein oxidation on quality should be considered. The results of the study provided new ideas and methods for walnut quality control.

Roasting treatments affect oil extraction rate, fatty acids, oxidative stability, antioxidant activity, and flavor of walnut oil

Introduction

The quality of pressed walnut oil can be improved by moderate roasting treatment.

Methods

This study compared physicochemical characteristics and antioxidant ability of walnut oils pressed from differently roasted pretreated walnuts, analyzed the correlation among these indicators by using Pearson correlation coefficient and correlation coefficient heatmap, and evaluated the volatile organic compounds (VOCs) of walnut oil under optimal pretreatment roasting conditions using headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS).

Results

Hierarchical cluster analysis (HCA) and principal component analysis (PCA) were able to remarkably distinguish walnut oil produced by different roasting processes. In addition, correlation analysis showed that there was a significant impact among indicators. There were 73 VOCs were identified in the optimum roasted treated walnut oil, consisting of 30 aldehydes, 13 alcohols, 11 ketones, 10 esters, 5 acids, 2 oxygen-containing heterocycles, 1 nitrogen-containing heterocycle and 1 other compound. GC-IMS results showed that aldehydes contributed significantly to the volatile flavor profile of walnut oil, especially (E)-2-heptenal, (E)-2-pentenal and hexenal.

Discussion

The properties of walnut oil based on varying roasting pretreatment of walnut kernels were significantly differentiated. Roasting at 120°C for 20 min is a suitable pretreatment roasting condition for pressing walnut oil. Roasting at 120°C for 20 min is a suitable pretreatment roasting condition for pressing walnut oil.

Investigating the Tocopherol Contents of Walnut Seed Oils Produced in Different European Countries Analyzed by HPLC-UV: A Comparative Study on the Basis of Geographical Origin

A rapid HPLC-UV method was developed for the determination of tocopherols in walnut seed oils. The method was validated and the LODs ranged between 0.15 and 0.30 mg/kg, while the LOQs were calculated over the range of 0.50 to 1.00 mg/kg. The accuracy values ranged between 90.8 and 97.1% for the within-day assay (n = 6) and between 90.4 and 95.8% for the between-day assay (n = 3 × 3), respectively. The precision of the method was evaluated and the RSD% values were lower than 6.1 and 8.2, respectively. Overall, 40 samples of walnuts available on the Greek market, originating from four different European countries (Greece, Ukraine, France, and Bulgaria), were processed into oils and analyzed. One-way ANOVA was implemented in order to investigate potential statistically significant disparities between the concentrations of tocopherols in the walnut oils on the basis of the geographical origin, and Tukey's post hoc test was also performed to examine exactly which varieties differed. The statistical analysis of the results demonstrated that the Ukrainian walnut seed oils exhibited significantly higher total concentrations compared to the rest of the samples.

Variations in Antioxidant Capacity, Oxidative Stability, and Physicochemical Quality Parameters of Walnut (Juglans regia) Oil with Roasting and Accelerated Storage Conditions

Walnut oil, like all vegetable oils, is chemically unstable because of the sensitivity of its unsaturated fatty acids to the oxidation phenomenon. This phenomenon is based on a succession of chemical reactions, under the influence of temperature or storage conditions, that always lead to a considerable change in the quality of the oil by promoting the oxidation of unsaturated fatty acids through the degradation of their C-C double bonds, leading to the formation of secondary oxidation products that reduce the nutritional values of the oil. This research examines the oxidative stability of roasted and unroasted cold-pressed walnut oils under accelerated storage conditions. The oxidative stability of both oils was evaluated using physicochemical parameters: chemical composition (fatty acids, phytosterols, and tocopherols), pigment content (chlorophyll and carotenoids), specific extinction coefficients (K₂₃₂ and K₂₇₀), and quality indicators (acid and peroxide value) as well as the evaluation of radical scavenging activity by the DPPH method. The changes in these parameters were evaluated within 60 days at 60 ± 2 °C. The results showed that the levels of total phytosterols, the parameters of the acid and peroxide value, K₂₃₂ and K₂₇₀, increased slightly for both oils as well as the total tocopherol content and the antioxidant activity affected by the roasting process. In contrast, the fatty acid profiles did not change considerably during the 60 days of our study. After two months of oil treatment at 60 °C, the studied oils still showed an excellent physicochemical profile, which allows us to conclude that these oils are stable and can withstand such conditions. This may be due to the considerable content of tocopherols (vitamin E), which acts as an antioxidant.