Abstract:

In this experiment, we optimized the conditions for extracting total flavonoids from stevia (Stevia rebaudiana)
residue left after stevioside extraction and determined the antioxidant activity of the total flavonoids. Firstly, we selected
ethanol concentration, ultrasound power, and ultrasound treatment time as factors to conduct single factor investigations for
the purpose of establishing the appropriate levels for each of these factors. Then, we developed a predictive mathematical
model of the yield of total flavonoids using Box-Behnken experimental design for the optimization of extraction conditions
by response surface analysis. The optimal extraction conditions were determined as follows: ethanol concentration, 70%;
ultrasound power, 420 W; and ultrasound treatment time, 42 min. Experiments conducted under these conditions gave the
maximum yield of total flavonoids of 5.928 mg/g, which was close to the predicted value of 5.949 mg/g. Moreover, antioxidant
experiments showed that total flavonoids from stevia residue left after stevioside had very strong scavenging capabilities for DPPH
and hydroxyl radicals with EC50 values of 7.096 and 22.065 μg/mL, respectively, which were approximately 1.37 and 1.61 times
that of vitamin C, respectively. Therefore, the total flavonoids had strong antioxidant capacity.

Key words: stevia residue left after stevioside extraction, ultrasonic extraction, total flavonoids, response surface methodology, antioxidant activity