Optimization of Justicia gendarussa Burm.f. fermentation by Aspergillus oryzae based on total phenolic, total flavonoid, and antioxidant capacity responses

This study aimed to determine the optimal fermentation conditions for Justicia gendarussa leaves to produce compounds with the highest antioxidant activity. Solid-state fermentation was conducted using Aspergillus oryzae, with treatment variables including incubation time, temperature, moisture content, and inoculum concentration optimized using Design-Expert v.13.0 software. Methanol was used to extract the fermented products, which were analyzed for total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric antioxidant power (FRAP) methods. The results revealed that fermentation generally increased TPC by approximately threefold and antioxidant activity while decreasing TFC by half. Correlation analysis indicated a negative relationship between inoculum concentration and TPC. TFC increased with prolonged fermentation time but decreased at higher inoculum concentrations. Antioxidant activity, as determined by the DPPH method, increased at higher fermentation temperatures, whereas the FRAP method showed higher antioxidant capacity at lower inoculum concentrations. The optimal fermentation conditions for achieving the highest TPC, TFC, and antioxidant activity were an incubation period of 14 d at 45°C, 80% moisture content, and an inoculum concentration of 20%.