Walnut Quality Research Articles

Glycerol ginger extract addition to edible coating formulation for preventing oxidation and fungal spoilage of stored walnuts

During storage, walnuts can undergo oxidation processes that lead to a decay of product quality and the development of rancidity and off-flavor. The present study investigated how the application of a chitosan coating, containing a glycerol ginger extract (GGE) rich in bioactive compounds, can preserve walnut quality during storage. Glycerol can be considered a green extraction solvent, which allows to obtain extract rich in polyphenols and with high radical scavenging activity. GGE employed in the formulation of a chitosan-based coating determined significantly lower (p < 0.05) lipid oxidation and peroxides values in walnuts stored under stressing temperature conditions (45 °C), in comparison to Control (uncoated sample) and even in comparison to the walnuts coated with sole chitosan. Moreover, when artificially inoculated with Aspergillus flavus spore suspension, walnuts treated with GGE-enriched coating evidenced the lowest disease incidence and the lowest spore concentration. Results obtained encourage the application of coating containing GGE even in other rich lipid matrices that can easily undergo oxidation process or spoilage caused by phytopathogenic fungi.

Impact of phosphine and of sulfuryl fluoride fumigation on walnut quality

Food products are often fumigated to protect them from pest damage and to meet phytosanitary requirements. While fumigants undergo a risk assessment prior to approval for use, additional studies are necessary to evaluate their effects on food quality. We investigated the effect of the fumigants phosphine (PH3) and sulfuryl fluoride (SO2F2) on three markers of walnut quality: peroxide value (PV), p-anisidine value (pAV) and fatty acid profile. Walnuts were treated with PH3, SO2F2, or left untreated. They were allowed to desorb any excess gas at 20°C for 2 weeks and then stored at 60°C for 4 weeks to simulate the walnuts' shelf life in an accelerated way. Samples were tested at four time points for PV, pAV, and fatty acid profile. Storage at 60°C caused increases in PV (2-way ANOVA, F3 = 700, P = 3E-57) and in pAV (2-way ANOVA, F3 = 54, P = 4E-18), with walnuts reaching PVs exceeding the industry standards for sale. No significant difference in PV was observed between PH3-treated and untreated walnuts or between SO2F2-treated and untreated walnuts at any time point (Tukey's HSD test for multiple comparisons, α = 0.05). pAV was not affected by fumigation at all (2-way ANOVA, F3 = 0.02, P = 1.0). Storage did not significantly change the levels of any of the five main fatty acids in the walnuts (2-way ANOVA, α = 0.05). Phosphine fumigation resulted in a 2% increase in linolenic acid content during the 2 weeks at 20°C, but this small effect disappeared afterwards (Tukey's HSD test, α = 0.05). In summary, no major effects of PH3 or SO2F2 on walnuts' commercial value or nutritional profile were found as assessed with PV, pAV and fatty acid profile.

Isolation, Identification, and Characterization of Endophytic Bacillus from Walnut (Juglans sigillata) Root and Its Biocontrol Effects on Walnut Anthracnose

Anthracnose is a major disease of walnut, which seriously reduces the yield and quality of walnut in Yunnan province. Therefore, it is necessary to explore and find a biological control agent for the prevention and control of anthracnose disease. In this study, an endophytic Bacillus WB1, with broad-spectrum antibacterial activity was isolated and screened from healthy walnut roots. The strain WB1 was identified as Bacillus siamensis WB1 based on morphological characteristics, physiological and biochemical tests, and 16S rRNA gene sequence analysis. Bacillus siamensis WB1 produces siderophores and indole-3-acetic acid and solubilizes inorganic phosphate. The strain WB1 not only showed a significant inhibition effect on fourteen phytopathogens, but also showed obvious inhibition on the spore germination of Colletotrichum acutatum. Meanwhile, strain WB1 can code genes for the production of antifungal lipopeptides and generate extracellular hydrolytic enzymes (protease, β-1, 3-glucanase, cellulase, and amylase). In addition, WB1 activated the systemic resistance of the host plant by enhancing the activity of defense enzymes, including phenylalanine ammonia lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO). The results of greenhouse assays also revealed that B. siamensis WB1 can effectively reduce the occurrence and severity of walnut anthracnose disease. These results also indicated that B. siamensis WB1 is a potential biocontrol agent for walnut anthracnose.

Genome-Wide Identification and Expression Analysis of the CAD Gene Family in Walnut (Juglans regia L.).

Lignin deficiency in the endocarp of walnuts causes kernel bare, leads to inconvenient processing and transportation of walnuts, and easily produces insect damage and mildew, thereby affecting the quality of walnuts. Cinnamyl alcohol dehydrogenase (CAD) is one of the key rate-limiting enzymes in lignin synthesis and plays an important role in the synthesis of lignin in the endocarp of walnut. However, knowledge about CAD gene family members and their evolutionary and functional characteristics in walnuts is limited. In this study, all 18 JrCADs were identified, and phylogenetic relationships, gene structure, protein motifs, collinearity analysis, and expression patterns of the JrCADs were also analyzed. All JrCADs could be divided into three groups based on the phylogenetic tree, gene structure, and motif analysis also support this grouping. Transcriptome data demonstrated that JrCADs have different expression patterns in walnut endocarps at different developmental stages. Combined with qRT-PCR data, we finally identified several candidate JrCADs involved in the process of endocarp sclerosis. This study showed that the JrCAD family members are highly conservative in evolutionary characteristics and they might participate in a variety of hormone responses. JrCAD17 and JrCAD18 are highly expressed in all periods of walnut endocarp harding, they are closely related to lignin accumulation.

Influence of the Extraction Method on the Quality and Chemical Composition of Walnut (Juglans regia L.) Oil.

The present study investigated and compared the quality and chemical composition of Moroccan walnut (Juglans regia L.) oil. This study used three extraction techniques: cold pressing (CP), soxhlet extraction (SE), and ultrasonic extraction (UE). The findings showed that soxhlet extraction gave a significantly higher oil yield compared to the other techniques used in this work (65.10% with p < 0.05), while cold pressing and ultrasonic extraction gave similar yields: 54.51% and 56.66%, respectively (p > 0.05). Chemical composition analysis was carried out by GC−MS and allowed 11 compounds to be identified, of which the major compound was linoleic acid (C18:2), with a similar percentage (between 57.08% and 57.84%) for the three extractions (p > 0.05). Regarding the carotenoid pigment, the extraction technique significantly affected its content (p < 0.05) with values between 10.11 mg/kg and 14.83 mg/kg. The chlorophyll pigment presented a similar content in both oils extracted by SE and UE (p > 0.05), 0.20 mg/kg and 0.16 mg/kg, respectively, while the lowest content was recorded in the cold-pressed oil with 0.13 mg/kg. Moreover, the analysis of phytosterols in walnut oil revealed significantly different contents (p < 0.05) for the three extraction techniques (between 1168.55 mg/kg and 1306.03 mg/kg). In addition, the analyses of tocopherol composition revealed that γ-tocopherol represented the main tocopherol isomer in all studied oils and the CP technique provided the highest content of total tocopherol with 857.65 mg/kg, followed by SE and UE with contents of 454.97 mg/kg and 146.31 mg/kg, respectively, which were significantly different (p < 0.05). This study presents essential information for producers of nutritional oils and, in particular, walnut oil; this information helps to select the appropriate method to produce walnut oil with the targeted quality properties and chemical compositions for the desired purpose. It also helps to form a scientific basis for further research on this plant in order to provide a vision for the possibility of exploiting these oils in the pharmaceutical, cosmetic, and food fields.

Study on the Identification and Detection of Walnut Quality Based on Terahertz Imaging.

Walnuts have rich nutritional value and are favored by the majority of consumers. As walnuts are shelled nuts, they are prone to suffer from defects such as mildew during storage. The fullness and mildew of the fruit impose effects on the quality of the walnuts. Therefore, it is of great economic significance to carry out a study on the rapid, non-destructive detection of walnut quality. Terahertz spectroscopy, with wavelengths between infrared and electromagnetic waves, has unique detection advantages. In this paper, the rapid and nondestructive detection of walnut mildew and fullness based on terahertz spectroscopy is carried out using the emerging terahertz transmission spectroscopy imaging technology. First, the normal walnuts and mildewed walnuts are identified and analyzed. At the same time, the image processing is carried out on the physical samples with different kernel sizes to calculate the fullness of the walnut kernels. The THz image of the walnuts is collected to extract the spectral information in different regions of interest. Four kinds of time domain signals in different regions of interest are extracted, and three qualitative discrimination models are established, including the support vector machine (SVM), random forest (RF), and k-nearest neighbor (KNN) algorithms. In addition, in order to realize the visual expression of walnut fullness, the terahertz images of the walnut are segmented with a binarization threshold, and the walnut fullness is calculated by the proportion of the shell and kernel pixels. In the frequency domain signal, the amplitude intensity from high to low is the mildew sample, walnut kernel, and walnut shell, and the distinction between walnut kernel, shell samples, and mildew samples is high. The overall identification accuracy of the aforementioned three models is 90.83%, 97.38%, and 97.87%, respectively. Among them, KNN has the best qualitative discrimination effect. In a single category, the recognition accuracy of the model for the walnut kernel, walnut shell, mildew sample, and reference group (background) reaches 94%, 100%, 97.43%, and 100%, respectively. The terahertz transmission images of the five categories of walnut samples with different kernel sizes are processed to visualize the detection of kernel fullness inside walnuts, and the errors are less than 5% compared to the actual fullness of walnuts. This study illustrates that terahertz spectroscopy detection can achieve the detection of walnut mildew, and terahertz imaging technology can realize the visual expression and fullness calculation of walnut kernels. Terahertz spectroscopy and imaging provides a non-destructive detection method for walnut quality, which can provide a reference for the quality detection of other dried nuts with shells, thus having significant practical value.

Storage Temperature, Relative Humidity, and Time Effects on the Organoleptic Profile of Walnut Kernels

Four cultivars of English walnut (Juglans regia) were evaluated by a trained taste panel after 6 and 12 months of storage. English walnuts were stored at 5, 15, or 25 °C, and at 40%, 60%, or 80% relative humidity within each temperature. Principal component analysis was used to compare taste, texture, and aroma attributes evaluated by the taste panel to objective indicators of English walnut quality including water activity, moisture content, free fatty acids, peroxide value, hexanal content, and kernel color. Temperature was found to significantly impact English walnut oxidation and perceived rancidity, whereas storage at high relative humidity affected English walnut texture and accelerated quality loss. Water activity was more strongly correlated to textural changes than moisture content. The effect of relative humidity was more pronounced at lower temperatures, leading to increased hydrolytic rancidity and free fatty acids. Peroxide value had higher and more significant correlation to sensory attributes related to rancidity than hexanal. Free fatty acids were not correlated to the rancid sensory attribute, but were significantly correlated to bitter. English walnuts stored at 5 °C with 40% or 60% relative humidity were associated with the sweet sensory attribute and L* value (light color). Kernel darkening was associated with bitter and rancid, but a causal relationship is unknown. Sensory quality of English walnuts is complex and requires further study to establish thresholds for chemical indices of English walnut quality loss based on organoleptic perception.

Energy and exergy analysis of the temperature and relative humidity controlled infrared convective walnut drying

In this study, energy and exergy analyses of the closed-cycle infrared (IR) convective walnut drying were performed. The effects of the combined heat transfer, product–source distance, and temperature–relative humidity (RH) control on the food quality of walnuts and performance were investigated. Experiments were conducted with and without an IR lamp. The temperature and RH of the drying air were set at 40°C and 20%, respectively. The distance between the product and the IR source was changed to 20 cm and 30 cm. The coefficient of performance values of the whole system changed between 1.56 and 2.34. Exergy efficiencies varied between 60.81% and 88.19% without IR and 72.73%–78.54% with IR. Energy and time savings increased by 14.7% and 26.1%, respectively, with using IR at a 20 cm distance compared with the convective system. The fatty acid composition of walnut samples and chemical properties of walnut samples such as total phenolic content, oil, ash, color, antioxidant activity, peroxide values, free fatty acid, conjugated trienes, conjugated dienes, saponification value, and iodine number parameters were analyzed. Food analysis results of all dried walnuts were in the acceptable range. As a result, energy and time savings were achieved with the desired food quality. Novelty impact statement This application makes it possible to achieve the desired conditions with less energy. In this study, the effects of heat transfer mechanisms, the effect of using two mechanisms together, and the effects of the variables within each mechanism on the drying of walnuts were investigated. It has been observed that the addition of an infrared heat source with the right and suitable application shortens the drying time as well as saves energy. As a result of the food quality analysis, it was determined that all products dried without deteriorating the food quality. The closed-cycle intermittent infrared convective drying system is recommended for industrial use.

Inactivation of Foodborne Pathogens on Inshell Walnuts by UV-C Radiation

Inshell walnuts can be contaminated with pathogens through direct contact or cross-contamination during harvesting and postharvest hulling, drying, or storage. This study aimed to assess the efficacy of UV-C radiation in inactivating foodborne pathogens on inshell walnut surfaces. Intact inshell walnut surfaces were inoculated separately with Salmonella,Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus and then were subjected to UV-C radiation at doses of 29.4, 147.0, 294.0, 588.0, and 882.0 mJ/cm2. UV-C radiation inactivated the inoculated pathogens in a dose-dependent manner, and a tailing effect was observed for the inactivation of pathogens. UV-C radiation at 29.4 and 882.0 mJ/cm2 reduced the populations of Salmonella Enteritidis PT 30, Salmonella Typhimurium, E. coli O157:H7, L. monocytogenes, and S. aureus on inshell walnut surfaces by 0.82 to 1.25 and 1.76 to 2.41 log CFU per walnut, respectively. Scanning electron photomicrographs showed pathogenic bacterial cells in the cracks and crevices of the inshell walnut surface, and the shielding of microorganisms by the cracks and crevices may have contributed to the tailing effect observed during UV-C inactivation. No significant changes (P > 0.05) were found in walnut lipid oxidation following UV-C radiation at doses up to 882.0 mJ/cm2. Together, the results indicate that UV-C radiation could be a potential technology for reducing the populations of various foodborne pathogens on inshell walnut surfaces while maintaining the quality of walnuts.

Analysis of walnut fruit quality based on source-sink relationships

In the paper, we investigate the relationship between the walnut fruit quality and the leaf-to-fruit ratio (LFR). The LFR had a significant effect on the physical quality of walnuts (P &amp;lt; 0.05), and the fruit volume, fruit fresh weight, fruit dry weight, nut dry weight and kernel dry weight increased with an increasing LFR. However, the LFR had no significant effect on the crude protein and crude fat kernel contents (P &amp;gt; 0.05). The number of cells per unit area and the cross-sectional area of the green husk cells with 5L : 1F were significantly higher than those with 1L : 3F and 2L : 3F (P &amp;lt; 0.05). The number of cells per unit area of the kernel with 1L : 3F and 2L : 3F was significantly higher than that with the other LFRs (P &amp;lt; 0.05). There was no significant difference in the NDW between the natural and girdled fruit-bearing shoots with 2L : 1F (P &amp;gt; 0.05). We concluded that the high carbohydrate availability with the high LFR augmented the fruit size by increasing the number of cells in the green husk and kernels. There was no change in the crude protein and crude fat contents in the kernels, possibly due to the proportion of the sugar to the fat distribution not affected by the LFR. Two leaves with good light were necessary to ensure the normal growth and development of one walnut fruit on girdled fruit-bearing shoots.

Transcriptome analysis of walnut quality formation and color change mechanism of pellicle during walnut development

Walnuts (including those covered with a pellicle) are loved for their rich nutritional value. And the popular varieties of walnut cultivation are Juglans sigillata L. The pellicle (seed coat) of these walnut cultivars has different colors and has an indispensable influence on the walnut quality formation. However, there are few reports on the pellicle color and quality formation in different developmental stages of walnut (Juglans sigillata L.). Therefore, in this study, three walnut cultivars (F, Q, and T) with different pellicle colors were selected for transcriptome sequencing and physiological index analysis of the color and quality formation mechanisms at different development stages. The results showed that with the development of walnut fruit, the starch sucrose metabolism pathway in the pellicle was activated and promoted starch hydrolysis. Meanwhile, the expression levels of genes related to the alpha-linolenic acid metabolism pathway were significantly increased during walnut maturation, especially in F2. Some physiological indicators related to lipid oxidation were also detected and analyzed in this study, such as MDA, CAT, POD and DPPH. These results were similar to the expression patterns of corresponding regulatory genes in the RNA-Seq profile. In addition, lignin synthesis genes were up-regulated in the phenylpropanoid metabolic pathway, while key genes enriched in the flavonoid and anthocyanin synthesis pathways were down-regulated. The results were consistent with the results of total anthocyanins and flavonoid content detection during walnut development. Therefore, this experiment suggested that with the maturation of walnut pellicle, the gene expression in the phenylpropanoid metabolic pathway flowed to the branch of lignin synthesis, especially in the Q variety, resulting in lower flavonoid and anthocyanin content at the maturity stage than immature. This is also the main reason for the pale pellicle of the three walnut varieties after mature. The findings of this study showed that changes in the expression levels of regulating genes for lipid, starch, sugar, and flavonoid synthesis during walnut development influenced the accumulation of the related metabolite for walnut quality formation and pellicle color. The results of this experiment provided the molecular basis and reference for the breeding of high nutritional quality walnut varieties.

Design and Key Parameter Optimization of Conic Roller Shelling Device Based on Walnut Moisture-Regulating Treatments

The quality of walnuts deteriorates owing to the poor quality of the shelling equipment. The improvement of shelling quality is urgently required for walnut processing. In this study, systematic research was carried out on the changes in walnut mechanical properties, mechanical model of walnut shelling, and the key parameters of the equipment. The key parameters were determined as the angles of the shelling conic roller (X1), speeds of the shelling conic roller (X2), clearance between the shelling conic roller and the static roller (X3), and the moisture content of walnuts (X4). The Box–Behnken design method was used for the experimental design, an analysis of variance was applied to determine the graded significance of each variable on the rate of high-quality kernel (RHQK) and rate of shell rushing (RSC), and the multi-objective optimization method was used to obtain the optimal parameters for RHQK and RSC. The ranking of factors affecting RHQK and RSC were: (X3) &gt; (X4) &gt; (X2) &gt; (X1) for RHQK, and (X2) &gt; (X3) &gt; (X4) &gt; (X1) for RSC. The ranks of significant interactive effects among the factors were as follows: (X1 X2) &gt; (X2 X3) for RHQK and (X2 X3) &gt; (X3 X4) &gt; (X2 X4) for RSC. The multi-objective optimization results showed that the optimal combination was X1 = 15.83°, X2 = 17.93 rpm, X3 = 45 mm, and X4 = 9.5%, yielding RHQK = 84.54%, and RSC = 99.15%. The verification test of the optimal results further illustrates the accuracy of the optimization. The obtained results showed that the quality of walnut shelling can be improved by adjusting the moisture content of walnuts and optimizing key parameters of the equipment. This method also represents a potential solution for improving the shelling quality of other nuts.

The phytochemical components of walnuts and their application for geographical origin based on chemical markers

Abstract Place of origin has an important influence on walnut quality and commercial value, which results in the requirement of rapid geographical traceability method. Thus, a method for geographical origin identification of walnuts on the basis of nutritional quality of walnuts from China was conducted. The concentrations of 43 phytochemical components were analyzed in walnut samples from five different walnut-producing regions of China. Based on 14 chemical markers selected by the Random Forest method from these phytochemical components, a new discriminant model for geographical origin was built, with the corresponding correct classification rate of 99.3%. In addition, the quantitative quality differences of walnuts from five regions were analyzed, with values of 0.17–1.43. Moreover, the top three chemical markers for the geographical origin discriminant analysis were Mo, V, and stearic acid, with contribution rates of 26.8%, 18.9%, and 10.9%, respectively. This study provides a potentially viable method for application in food authentication.

MYCOFLORA AND AFLATOXIN LEVELS IN WALNUT SAMPLES STORED IN DIFFERENT PACKAGING MATERIALS IN OGUN STATE

Aflatoxin levels in nuts and fruits pose a great threat to food storage and availability. This study aimed at determining the mycoflora and aflatoxin level in stored walnut under various conditions. Freshly harvested walnuts were stored for 90 days in three different media: jute bags, Plastic containers and sterile polythene bags under room (37ᵒC) and refrigeration (4ᵒC) temperature. After 90 days of storage, the stored walnuts were examined for fungal growth at one month interval after which colonial and morphological characterization were carried out to identify the fungi present. Proximate analysis and aflatoxin content of the walnut samples were determined by standard methods. Fungi isolated from the walnuts include; Aspergillus niger, Saccharomyces cerevisae, Penicillum notatum, Aspergillus sydowi, Fusarium oxysporium, and Rhizopus stolonifer. Walnuts stored in plastic container at room temperature had the lowest aflatoxin level of 0.002 ng/kg while that stored in polythene bags had the highest (0.054 ng/kg). Proximate analysis also revealed that walnuts stored in polythene bag reduced significantly (p&lt;0.05) in protein, ash content, fat, fibre, moisture and carbohydrates while samples stored in jute bag and plastic container remained unchanged. This study shows that storage methods contributed to the overall quality, shelf life and aflatoxin content of walnut and hence care should be taken during post harvesting process.&#x0D; &#x0D;

Effect of Special Compound Microbial Fertilizer on Growth and Quality of Walnut

On the basis of the research on the site condition of mineral yellow soil in the early stage, the purple soil walnut planting area in Xide County of Liangshan Prefecture was selected to carry out the fertilizer contrast experimen, in order to put forward reasonable fertilization suggestions for walnut planting in the purple soil widely distributed area. The 4-year-old shuoxing saplings and 10-year-old hanging fruit trees were selected as the experimental materials, and four treatments (T1: no fertilization, T2: application of common compound fertilizer, T3: application of special inorganic fertilizer, T4: application of compound microbial fertilizer) were set up to explore the effects of special compound microbial fertilizer on the plant height, fruit yield, quality and economic benefits of young trees. The results showed that the application of special compound microbial fertilizer could increase the height and fruit yield of young walnut trees. The growth of young walnut tree height and fruit yield in purple soil was higher than that in mineral yellow soil. The application of compound fertilizer was 38.03% higher than that of no fertilizer, 27.27% higher than that of common compound fertilizer and 10.11% higher than that of inorganic special fertilizer. The yield of walnut fruit increased by 44.56% and 28.04% compared with no fertilizer and common compound fertilizer, and the transverse diameter, weight, kernel yield, fat content and protein content of walnut fruit increased by 10.99%, 14.89%, 10.31%, 8.09% and 7.27%, respectively.

Brief Research Report: Estimation of the Protein Digestibility-Corrected Amino Acid Score of Defatted Walnuts.

Introduction: Walnuts are considered a good source of essential fatty acids, which is unique among tree nuts. Walnuts are also composed of about 10–15% protein, but the quality of this protein has not been evaluated. Pistachios and almonds have been evaluated for their protein content using a protein digestibility-corrected amino acid score (PDCAAS), but it is unclear how the quality of protein in walnuts relates to that in other commonly consumed tree nuts. The objective of this study was to substantiate the protein quality of walnuts by determining their PDCAAS.Methods: A small, 10-day dietary intervention trial was conducted using male Sprague-Dawley rats (n = 8, 4 per group) with two diets: a nitrogen-free diet and a diet containing protein exclusively from defatted walnuts. Feed intake and fecal output of nitrogen were measured to estimate the true protein digestibility, and the amino acid compositions of walnuts compared to child and adult populations were used to calculate amino acid scores (AAS) and PDCAAS.Results: The true protein digestibility score of raw walnuts was calculated to be 86.22%. Raw walnuts contained 15.6 g protein/g walnut with AAS of 0.45 and 0.63 for children aged 6 months to 3 years and 3–10 years, respectively. For each population, a PDCAAS of 39 and 46% was calculated, respectively, using a protein conversion constant of 5.30. Using a protein constant of 6.25, a PDCAAS of 39% (6 months - 3 years) or 46% (3-10 years) was calculated.Conclusions: This is the first known assessment of the PDCAAS of walnuts. Like almonds, they appear to have a low-to-moderate score, indicating they are not a quality source of protein.

Physiochemical Responses of the Kernel Quality, Total Phenols and Antioxidant Enzymes of Walnut in Different Forms to the Low-Temperature Storage.

Fresh walnut is obtaining high attention due to its pleasant taste and health benefits. This study aimed to evaluate the influence of storage temperatures (0 °C and −20 °C) on the kernel quality, total phenols, and antioxidant enzyme activities of walnuts in three forms (fresh kernels, walnuts with green husk, and walnuts with shell). For a short storage within 3 months at 0 °C, the results revealed that walnuts with green husk provided a better walnut kernel quality resulting from its lower acid value and peroxide value, together with a higher total phenol content and total antioxidant activity, compared with other forms of walnuts. In comparison, frozen storage at −20 °C for a long duration (up to 10 months), found that walnuts with shell showed advantages in improving the kernel quality (fatty acid content, total phenols, and total antioxidant activity) and antioxidant enzyme (peroxidase, catalase, and superoxide dismutase) activities in the kernels, leading to an acceptable range of acid value and peroxide value, compared with other forms of walnuts. Thus, frozen storage at −20 °C showed a potential application in maintaining the walnut kernel quality, especially the walnuts with shell.

Effects of different baking techniques on the quality of walnut and its oil

The baking conditions of walnut kernels were optimized based on different cultivars and baking methods. The influence of the different baking techniques on the chemical properties of walnut oils was determined. The results showed that acid value, peroxide value and induction period (IP) all significantly increased in the baked samples compared to the unbaked ones. The highest increase in IP was from 6 to 17 h indicating that baking can improve the oxidative stability of walnut oils and prolong their shelf-life. Several aroma components increased after baking. However, among the different baking conditions, the strongest aroma in walnut oil was observed after baking was done for 20 min with sucrose (107%) at 153 ℃. Nevertheless, baking had little effect on the fatty acid composition of walnuts.

Role of kaolin on drought tolerance and nut quality of Persian walnut

Global warming has endangered crop production by reducing water availability and increasing temperature. Kaolin may protect plants against these conditions by reflecting the light and decreasing plant temperature. This study was conducted to evaluate the protective effects of kaolin spraying on kernel yield and quality of four walnut cultivars and genotype. The experiment was conducted as a split-plot factorial design in time factor with four factors including irrigation levels (100% and 50% of regular irrigation), kaolin solution concentrations (0, 2.5, 5 and 7.5%), walnut varieties (‘Chandler’, ‘Franquette’, ‘K72’ and ‘SG’) in two consecutive years (2012–2013). By increasing the kaolin concentration under well-watered irrigation, gas exchange, chlorophyll concentration, leaf potassium content, and kernel quality were improved. Moreover, increasing the kaolin concentration reduced sunburn and leaf temperature under different irrigation regimes. Based on results, genotypes ‘K72’ and ‘SG’ showed more severe sunburn symptoms. Water stress and no kaolin application had negative effect on the kernel color. Leaf temperature increased with reducing the irrigation and concentration of kaolin. Under water stress, ‘Chandler’ had the highest relative water content (58.25%) and ‘Franquette’ had the lowest RWC and therefore considered as the most drought sensitive cultivar. The lowest fruits sunburn in ‘Franquette’, was related to the thickness of its fruits’ husk. In general, the foliar application of kaolin 5% and 7.5% alleviated the negative effect of drought stress and improved the walnut kernels quality under water deficit condition by improving chlorophyll content, RWC, and gas exchange and reducing leaf temperature.

Walnut (Juglans regia L.) Volatile Compounds Indicate Kernel and Oil Oxidation.

Kernel oxidation susceptibility and pellicle darkening are among the biggest concerns regarding walnut quality. Monitoring oxidation is crucial to preserve quality from production to consumption. Chemical oxidation parameters (peroxide value and UV absorbances), fatty acid profile, tocopherols, phenols, and volatiles in ‘Chandler’ and ‘Howard’ kernels were studied at different time points during 28 weeks of storage to evaluate potential oxidation markers. During storage, peroxide value, UV absorbances, and volatiles concentration increased; oxidative stability, phenols, and tocopherols decreased, while fatty acid profile was unaffected. ‘Chandler’ had a lower peroxide value, K232, and K268; and higher kernel and oil oxidative stability compared to ‘Howard’. Phenols and tocopherols decreased 1.2-fold in ‘Chandler’ and 1.3-fold in ‘Howard’. Using multivariate analysis, samples were discriminated in three groups according with their oxidative levels. Increases of volatiles in oil and kernel were associated with higher oxidative levels. Pentanal, 2-methylpropanal, hexanal, (E)-2-pentenal, 3-octanone, octanal, (Z)-2-penten-1-ol, hexanol, (E)-2-octenal, 1-octen-3-ol, benzaldehyde, (E,E)-2,4-nonadienal, and hexanoic acid in kernels were adequate at distinguishing oxidation levels and as oxidative markers in walnuts. Kernel volatiles is a useful measurement for walnut oxidation during storage without any prior fat extraction.