Improvement of Functional Compounds and Antioxidant Activity in <i>Houttuynia cordata</i> Thunb. Leaf Extracts Through <i>Lactiplantibacillus plantarum</i> Fermentation

This study aimed to compare the functional compounds and antioxidant activity of <i>Houttuynia cordata</i> Thunb. (HC) leaves and stems harvested in summer and winter and to select the optimal sample to determine its growth-promoting effects on <i>Lactiplantibacillus plantarum</i>. Dried HC leaves and stems were extracted using boiling water, and colorimetric methods were used to measure their total polyphenol and flavonoid contents and antioxidant capacity. High-performance liquid chromatography was used to determine the content of active compounds, including quercitrin, hyperoside, and quercetin. The results showed that the raw materials of HC leaves harvested in summer had significantly higher quercitrin and hyperoside contents compared with those of HC stems harvested in winter. Boiling water extracts of HC (BWEHC) leaves harvested in summer also exhibited higher total polyphenol and flavonoid contents and antioxidant activity than stems harvested in winter. During fermentation, BWEHC leaves were fermented with <i>L. plantarum</i> at solid-solvent ratios of 0.3%－1.5% (w/v) for 24－72 h. The optimal fermentation condition was a solid-liquid ratio of 0.9% (w/v) with a fermentation time of 48 h as this condition resulted in the highest bacterial counts. After fermentation, the quercetin content, total polyphenol and flavonoid content, and antioxidant activity significantly improved, whereas the quercitrin and hyperoside contents decreased compared with those in nonfermented samples. The <i>L. plantarum</i> count was considerably higher in BWEHC leaves than in the control group. In conclusion, BWEHC leaves could serve as a potential prebiotic for <i>L. plantarum</i> and may offer benefits as a functional food.