Optimized Paocai Fermentation of Rehmannia Radix: Physicochemical Quality, Untargeted Metabolomics, and Antioxidant Activity.

Rehmannia Radix is an important medicinal and edible plant resource in China. However, the production of paocai by fermenting Rehmannia Radix has rarely been reported. This study employed response surface methodology to optimize fermentation conditions, and investigated the changes in physicochemical properties, bioactive components, and metabolic profiles of paocai before and after fermentation. The optimal fermentation conditions were 5% salt, 9% sugar, and fermentation at 20 °C for 5 days. During the fermentation process, total acidity increased, pH decreased, and crispness and chewiness declined significantly, indicating a softer texture. Electronic tongue analysis demonstrated that fermentation decreased bitterness and astringency, while enhancing sourness, sweetness, and umami. Fermentation significantly increased total phenolic content, total flavonoid content, and antioxidant activity, with phenolic content increasing from 0.80 to 1.63 mg/g and flavonoid content increasing from 2.67 to 3.67 mg/g, while the contents of iridoid glycosides and phenylethanoid glycosides decreased significantly after fermentation. Untargeted metabolomics analysis identified 719 metabolites, of which 417 were differentially abundant metabolites, indicating that fermentation altered the metabolic profile of paocai. In conclusion, fermentation exerted a bidirectional regulatory effect: mitigating the bitterness and astringency of Rehmannia Radix paocai while improving its texture and flavor. This study not only developed the optimal fermentation process for Rehmannia Radix-based medicinal and edible products, but also provided the scientific basis for the sustainable processing and high-value utilization of Rehmannia Radix as a functional food.