Objective To address the problems of enzymatic browning prone to occur in longan pulp during drying processing, which causes deteriorated color and reduced nutritional quality of finished products, this study aims to develop an efficient and green color-protection technology for longan pulp and provide technical support for industrial drying of longan.

Method The inhibitory effects of various physical treatments and common color protectants on the browning of longan pulp were compared to screen appropriate physical treatment modes and color protectant combinations. On this basis, taking the relative residual activities of polyphenol oxidase (PPO) and peroxidase (POD) as well as the browning degree of products as primary evaluation indices, single-factor experiments and an orthogonal array design L₉(3⁴) were adopted to optimize the process parameters of ultra-high pressure combined with color protection treatment. Samples treated with the optimized process were further subjected to hot-air drying, followed by determinations of color parameters, PPO and POD relative residual activities, browning degree, free and bound phenolic contents, and antioxidant capacities.

Result Screening results demonstrated that ultra-high pressure treatment exhibited superior color-preserving performance among all tested physical methods, while D-isoascorbic acid and glutathione constituted the optimal color protectant combination. The optimized integrated process was determined as follows: longan pulp was soaked in a mixed solution containing 0.5% D-isoascorbic acid and 0.4% glutathione, vacuum-packed, and then treated under 500 MPa for 25 min. Under these conditions, the relative residual activities of PPO and POD were significantly reduced to 28.72% and 9.34%, respectively (P < 0.05), and the browning degree decreased by 45.71% compared with the untreated control group. Further analyses revealed that the combined color protection treatment significantly elevated the contents of free phenols, bound phenols and total phenols in dried longan pulp. Specifically, free and bound phenols contents increased by 18.28% and 137.82% relative to the untreated group, respectively. The ferric reducing antioxidant power (FRAP) and 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity of treated samples were increased 5.14 times and 1.29 times of the untreated group.

Conclusion The integrated process of ultra-high pressure combined with color protectants effectively alleviates browning of longan and enhances its FRAP and DPPH radical scavenging capacity, which provides a technical reference for color protection during the drying processing of longan.