Optimizing γ-aminobutyric acid content of fermented mulberry leaves by using Taguchi design and analyzing antioxidant capacity of whole extract

This study utilized the Taguchi method to optimize the conditions for the hydrothermal extraction of γ-aminobutyric acid (GABA) from fermented mulberry leaves (FMLs) and evaluated the antioxidant capacity of FML extract powder (EP). Four extraction factors-the leaves-to-water ratio, temperature, duration, and pH-were considered, and a series of experiments were conducted in accordance with a Taguchi design. The results indicated that the optimal hydrothermal extraction conditions were a leaves-to-water ratio of 1:10, temperature of 100 °C, duration of 15 min, and pH of 4. Under these conditions, the GABA content of an FML infusion was 75.04 mg GABA/g EP. Further analysis revealed that the pH had the strongest impact on GABA extraction, with the corresponding signal/noise ratio ranging from 27.36 to 31.60. Additionally, experimental results for antioxidant capacity showed that the total phenolic and flavonoid content of the EP were 12.94 mg gallic acid equivalent/g EP and 81.77 mg quercetin equivalent/g EP, respectively. The IC 50 values for ABTS and DPPH free-radical-scavenging ability were 0.58 mg/mL and 0.20 mg/mL, respectively. The reducing power of the EP was 81.69 mg vitamin C equivalent/g EP. These findings provide an important foundation for further applications of FMLs.