Biotechnological upcycling of Laminaria japonica processing waste via Lactiplantibacillus plantarum fermentation: release and formation of antioxidant and tyrosinase-inhibitory compounds.

Laminaria japonica (L. japonica) is a valuable marine resource, and high-value utilization of its processing waste is of great significance. In this study, the waste was fermented by Lactiplantibacillus plantarum to investigate its bioactivities and metabolic changes, aiming to provide a theoretical foundation and application potential for the development of natural antioxidants or functional food ingredients. The fermented extract (LJF) displayed strong antioxidant and tyrosinase inhibitory activities. Specifically, LJF achieved an IC50 of 8.84 ± 0.04 mg/mL against tyrosinase under reversible mixed-type kinetics, utilizing copper ion chelation and fluorescence quenching as primary inhibitory mechanisms. Meanwhile, non-targeted metabolomics revealed elevated amino acids, organic acids, and benzene derivatives after fermentation. Further screening identified 3-HPA, BIC, and PTSA as potential inhibitors. Moreover, LJF effectively delayed shrimp melanosis during refrigerated storage. These findings provide a theoretical basis for utilizing L. japonica waste as a source of natural antioxidants and functional food ingredients.