Introduction

Walnut (Juglans regia L.), a member of the Juglandaceae family, is one of the most important temperate nut crops widely cultivated worldwide, particularly in Iran, which is considered one of its primary centers of origin due to high genetic diversity (Aslamarz et al., 2009). Walnut kernels are rich in bioactive compounds, including unsaturated fatty acids (oleic, linoleic, and α-linolenic acids), phenols, tocopherols, phytosterols, and vitamins A, E, and C, contributing significantly to their antioxidant and nutritional value (Venkatachalam & Sathe, 2006; Hamidi et al., 2015). However, the presence of heavy metals such as iron and copper can accelerate oxidation, reducing shelf life (Belitz & Grosch, 1987). Consequently, the drying method plays a critical role in preserving chemical composition and kernel quality (Amini Rastabi & Mirzaey, 2018).

In Iran, walnuts are traditionally sun-dried either with the hard shell intact or as dehulled kernels. Direct exposure to sunlight, varying temperature, and oxygen contact can alter physical and chemical properties, including mineral content (Ozkan & Koyuncu, 2005; Martinez & Maestri, 2008). Previous studies indicate that drying method and kernel color influence mineral elements such as phosphorus, potassium, sodium, iron, zinc, copper, and manganese (Hamidi et al., 2015; Huang et al., 2014). Despite scattered research, comprehensive data on the direct impact of traditional drying methods on walnut mineral changes under Iranian climatic conditions remain limited.

The present study aimed to evaluate the effects of two traditional sun-drying methods (with hard shell vs. dehulled kernels) on mineral element concentrations in walnut kernels and identify the optimal traditional approach for maintaining nutritional and chemical quality.



Material and Methods

Walnuts of a local cultivar from Tuyserkan, Hamadan Province, Iran, were harvested from a single orchard at uniform maturity. Samples included 3 kg of in-shell walnuts and 1 kg of kernels. Drying was performed naturally in open air under direct sunlight for two consecutive days (average daily temperature 27–31°C, relative humidity 25–30%) in September, achieving final kernel moisture of 6–8% (Ozkan & Koyuncu, 2005). Post-drying, kernels were categorized by color into white (grade 1), amber (grade 2), and brown (grade 3).

Residual moisture was removed by oven-drying at 70°C for 24 h (Memmert UN55). Kernels were ground, and 1 g samples were ashed at 500°C for 4 h. Ash was digested in HCl:HNO₃ (1:3), filtered, and diluted to 50 mL. Potassium and sodium were measured by flame photometry (Jenway PFP7); phosphorus, iron, manganese, copper, and zinc by atomic absorption spectrophotometry (Shimadzu AA-7000). All analyses were conducted in triplicate. Data were analyzed using one-way ANOVA and Duncan's test in SPSS version 26 (p < 0.05).

Results and Discussion

Statistical analysis revealed significant effects of drying method on several mineral elements. Drying with the hard shell generally preserved higher mineral concentrations compared to drying dehulled kernels.

Phosphorus content was significantly higher (p = 0.037) in shell-dried walnuts (mean 2023 ± 27 mg/kg) than in kernel-dried samples (1883 ± 26 mg/kg). Similar trends were observed across color grades, with the highest values in amber kernels dried with shell.

Potassium showed the most pronounced difference (p = 0.033), with shell-dried samples averaging 3884 ± 45 mg/kg versus 3435 ± 42 mg/kg in kernel-dried walnuts. This aligns with Ozkan & Koyuncu (2005), who reported that direct exposure to light and air reduces alkali elements like potassium through leaching or oxidation.

Sodium levels exhibited no significant difference overall (p = 0.081), averaging 31.9 ± 0.09 mg/kg (shell-dried) and 24.7 ± 0.07 mg/kg (kernel-dried), indicating relative stability during sun-drying.

Iron concentration was significantly higher (p = 0.044) in shell-dried walnuts (2.83 ± 0.08 mg/kg) than kernel-dried (1.86 ± 0.07 mg/kg), likely due to the protective role of the shell against oxidation (Guinda, 2003).

Zinc showed no significant difference (p = 0.61), with means of 0.41 ± 0.01 mg/kg (shell-dried) and 0.43 ± 0.01 mg/kg (kernel-dried), suggesting greater thermal and light stability (Venkatachalam & Sathe, 2006).

Copper was significantly preserved in shell-dried samples (0.35 ± 0.01 mg/kg vs. 0.26 ± 0.01 mg/kg; p = 0.031), with losses in kernel-dried walnuts attributed to accelerated oxidation of copper-containing compounds (Martinez & Maestri, 2008).

Manganese displayed no significant difference (p = 0.29), though darker kernels tended to accumulate higher levels, consistent with Huang et al. (2014). Coefficient of variation analysis indicated greater variability in kernel-dried samples for most elements, reflecting less uniform mineral retention.

Overall, the hard shell acted as a barrier against direct sunlight, oxygen, and temperature fluctuations, reducing mineral degradation or leaching during traditional sun-drying.

Conclusions

The study demonstrated that potassium was the most abundant mineral in Tuyserkan walnuts, followed by phosphorus, while copper was present in the lowest concentration. Drying walnuts with the hard shell intact under sunlight significantly preserved higher levels of potassium, phosphorus, iron, and copper compared to drying dehulled kernels (p < 0.05), owing to the shell's protective effect against oxidation and direct environmental exposure. Zinc, sodium, and manganese remained relatively stable across methods.

Darker kernels generally contained more manganese and potassium, while lighter kernels were richer in iron. Given its simplicity, accessibility, and superior retention of nutritional quality, sun-drying with the hard shell represents the most effective traditional method under natural conditions. This approach can enhance shelf life, commercial value, and export potential of Iranian walnuts by maintaining superior chemical and nutritional properties.



Acknowledgements

Author Contributions

Conceptualization, methodology, investigation, and writing: research team (as per original Persian study).

Data Availability Statement

The data supporting the findings of this study are available within the article (tables and figures).

Ethical Considerations

This study involved no human or animal subjects; ethical approval was not required.

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Funding Statement

The author(s) received no specific funding for this research.