Mechanisms and Critical Thresholds of Cold Storage Duration-Modulated Postharvest Quality Deterioration in Litchi Fruit During Ambient Shelf Life

While cold storage is essential to extend the postharvest preservation of litchi fruit, the abrupt transfer to ambient temperature during supply chain transitions may trigger rapid quality degradation. However, the comprehensive mechanisms and critical threshold of post-transfer quality deterioration remain insufficiently characterized. In this study, litchi fruits were stored at 4 °C for 10, 20, and 30 days, followed by simulated shelf life at 25 °C. Key indicators, including appearance quality, antioxidant capacity, lipid peroxidation, and enzymatic oxidation, were monitored, and principal component analysis (PCA) was used to determine quality deterioration thresholds. Litchi subjected to 30 d of cold storage exhibited significantly accelerated pericarp browning compared to those stored for 20 d and 10 d, with the browning index increasing by 25.7% (vs. 20 d) and 41.9% (vs. 10 d), respectively, after 24 h of ambient exposure. This was accompanied by a significant impairment of the antioxidant system. Compared to the fruits stored for 10 d and 20 d, the activities of key antioxidant enzymes (SOD, CAT, and APX) were substantially decreased in the 30 d group, with reductions ranging from approximately 9% to 28%. Concurrently, the non-enzymatic antioxidant capacity also declined. Meanwhile, 30 d of storage activated the browning-related enzymes: anthocyanase and peroxidase (POD) activities increased by 1.2- to 3.6-fold, and poly-phenol oxidase (PPO) activity increased by 11% to 37%, compared to the 10 d and 20 d groups, respectively. In contrast, phenylalanine ammonia lyase (PAL) activity was inhibited by 56.9%. It also enhanced membrane lipid metabolism disorders, which aggravated cell structure damage and oxidative stress. For practical application, PCA identified 10 d (4 °C) + 6 h (25 °C), and 20 d (4 °C) + 12 h (25 °C) as the optimal and critical quality thresholds, respectively. This study reveals the interactive regulatory relationship between cold storage duration and ambient exposure time mediated by oxidative stress, enzymatic browning, and membrane lipid metabolism, providing a theoretical basis for developing time-temperature-quality models to reduce postharvest losses in litchi.