*3.5. Optimization of the Extraction Condition using the Box–Behnken Design*

Based on the results of the screening test, the BBD experimental design of the RSM was performed using the Design-Expert 12.0 software to optimize the four selected factors, extraction temperature (◦C), extraction time (h), DES content (%), and stirring speed (rpm), that influenced the extraction of PUL and could enhance the extraction efficiency of the flavonoids. The four independent factors were investigated at three different levels (−1, 0, and +1). The complete experimental design consisted of 29 runs. Each run was repeated three times.
