Ultrasound-Assisted Fermentation by Lactiplantibacillus plantarum Promotes Phytochemical Transformation and Antioxidant Activity of Citrus aurantium L. Through Metabolic Alterations

Citrus aurantium L. is a citrus-derived functional food rich in various phenolic compounds, including flavonoids. However, the bioavailability of its phytochemicals and sensory quality remain limited. This study investigated the effects of ultrasound-assisted fermentation by Lactiplantibacillus plantarum on microbial growth, phytochemical transformation, antioxidant activity, and metabolic profiles of C. aurantium. Ultrasound treatments were applied at different fermentation stages and power levels, among which treatment at 100 W during the exponential growth phase (ULP4) exhibited the best overall performance. Compared with conventional fermentation by L. plantarum (LP), ULP4 significantly promoted microbial growth, accelerated acidification, enhanced carbohydrate utilization, and stimulated β-glucosidase activity compared with conventional fermentation. Consequently, total phenolic content, total flavonoid content, and antioxidant capacities were markedly improved. Untargeted full-MS/MS metabolomic analysis revealed extensive metabolic alterations following ultrasound treatment, with 335 metabolites significantly altered between LP and ULP4. Differential metabolites were mainly associated with flavone and flavonol biosynthesis, secondary metabolite biosynthesis, amino acid metabolism, and lipid metabolism. These metabolic changes were closely associated with improved antioxidant properties and functional quality. Overall, ultrasound treatment during the exponential growth phase effectively enhanced microbial metabolism and phytochemical transformation, offering a promising strategy to improve the functional value of fermented C. aurantium products.