Effect of exogenous ascorbic acid (ASA)on walnut browning

【Objective】Walnut (Juglans regia L.), belonging to the Juglandaceae family and Juglans genus, is a significant economic forest tree species in China, playing a crucial role in the economic forest industry. One of the main cultivars in Xinjiang, Wen 185, is widely cultivated for its high yield, superior quality, and strong stress resistance. However, as a thin-shelled variety, Wen 185 is susceptible to kernel pellicle browning during harvesting, as air penetrates into the fruit interior through the suture lines and micropores on the hard shell. The color of walnut kernels is an important appearance indicator for Wen 185, and pellicle browning negatively affects flavor, nutritional value, and marketability, thereby reducing economic value and causing direct economic losses for farmers. Therefore, this study aimed to pre- liminarily elucidate the browning occurrence and mechanism in Wen 185 kernels, establishing a theoretical foundation for addressing browning issues. The study also investigated the inhibitory effects of exogenous ascorbic acid (ASA) at different concentrations and treatment durations on walnut kernel browning, providing potential techiques for postharvest storage and quality preservation of walnuts. 【Methods】Wen 185 walnut kernels were used as the experimental material. Postharvest treatments involved immersing the kernels in ASA solutions at different concentrations and durations. A three-factor (concentration) and four-level (time) experimental design was adopted, with ASA concentrations of 50, 100, and 200 mg L-1 and immersion durations of 2, 6, 12, and 18 minutes, using water treatment as the control (CK). Measured parameters included browning index, color parameters (L, a, b, h), contents of total phenols, tannins, total flavonoids, and malondialdehyde (MDA), and activities of key enzymes such as polyphenol oxidase (PPO), peroxidase (POD), ascorbic acid oxidase (AAO), and catalase (CAT).【Results】ASA treatment had a significant positive effect on maintaining the color of Wen 185 walnut kernels after harvest. After treatment with ASA at different times and concentrations, the brightness values of walnut kernel color were consistently higher than those of the control. The brightness reached 1.36 times that of the control under 200 mg L-1 ASA treatment for 18 minutes, while the highest hue angle was observed under 100 mg L-1 treatment for 18 minutes. Overall, the browning indices under all ASA concentrations and treatment durations were lower than those of the control, showing a decreasing trend with prolonged treatment time. The maximum difference from the control (0.14) was observed under ASA treatment at 100 mg L-1 (A2) for 18 minutes, which yielded the optimal browning index. Total phenol content peaked at 22.93 mg g-1 at 18 minutes of the A2 treatment, representing an 18.44% increase over the control. Tannin content showed no significant difference from the control during the early stages of A2 treatment but began to decrease with prolonged treatment time. When the processing time was 6 minutes, the overall flavonoid content showed an increasing trend compared to CK. After 6 minutes of processing, at a concentration of 50 mg L- 1 , the total flavonoid content was 20.33 mg g- 1 , which is the lowest value, but still 2% higher than CK. At 18 minutes of treatment, total flavonoid content under 200 mg L-1 (A3) reached 23.99 mg g-1 , an 11% increase over the control. As important antioxidant components, the accumulation of flavonoids helps enhance the free radical scavenging capacity of the tissue. PPO activity decreased to its minimum value of 20.64 U g- 1 under 200 mg L- 1 treatment for 12 minutes, which was 20% lower than the control. AAO activity was lower than the control under all concentrations at 6 minutes of treatment and gradually decreased with increasing concentration. The maximum AAO activity (155.24 nmol min- 1 g- 1 ) was observed under 50 mg L- 1 treatment but was still 1% lower than the control. CAT activity showed an upward trend compared to CK at different concentrations when the treatment time was 12 minutes. After 12 minutes, the activity was lowest at 50 mg L-1 , with a CAT activity of（38.91 μmol g- 1 ）, but still 2% higher than CK. Afterwards, it gradually increased, reaching the highest value at 200 mg L-1 , with a CAT activity of（40.39 mol g-1 ）, which is 6% higher than CK. This indicates that ASA enhances cellular reactive oxygen species scavenging capacity, contributing to the maintenance of redox homeostasis. Correlation analysis further confirmed that the browning index was significantly positively correlated with tannin content and MDA accumulation, while negatively correlated with total phenol content, total flavonoid content, and CAT activity. This suggests that ASA comprehensively delays the browning process in walnut kernels by synergistically regulating phenolic metabolism, inhibiting oxidase activity, and enhancing the antioxidant defense system.【Conclusion】Exogenous ascorbic acid treatment can significantly inhibit enzymatic browning in Wen 185 walnut kernels, with more pronounced inhibitory effects observed at 12 minutes and concentra-tions of 100 mg L-1 . As a postharvest treatment method, ASA offers distinct advantages, including simplicity of operation, high efficiency, and low cost. Its non-toxic and biodegradable nature aligns with food safety regulations and sustainable agricultural practices, making it a promising green alternative to synthetic chemical treatments. Therefore, ASA treatment represents a scientifically grounded and economically viable strategy for extending the shelf life and preserving the sensory and bioactive qualities of thin-shelled walnut varieties such as Wen 185, with potential applicability to other browning-sensitive horticultural crops.