Optimized extraction of phenolic antioxidants from red pitaya (Hylocereus polyrhizus) seeds by subcritical water extraction using response surface methodology

A growing trend now involves the comprehensive use of by-products from fruit processing, especially of pitaya, whose production has increased worldwide in recent years. The aim of this study was to evaluate the optimal process for applying subcritical water extraction (SWE) to obtaining polyphenols from red pitaya (Hylocereus polyrhizus) seeds using response surface methodology (RSM). The antioxidant activities of polyphenol extracts obtained by SWE and three other conventional solvent extraction methods (CSE: water, ethanol and acetone extraction) were compared. The study was divided into two steps. In the first, the Box-Behnken design was used to determine the effects of extraction time, temperature, and solid-solvent ratio on antioxidant activity (ABTS, DPPH and FRAP) and the yield of total phenolic compounds (TPC). Protein and total sugar content of the crude polyphenol extract was significantly reduced after purification with NKA-9 type macroporous resin column. Optimal extraction conditions for subcritical water extraction of polyphenols from red pitaya seeds were 15 min reaction time at 220 °C with 2% solid-solvent ratio (TPC: 63.14 mg GAE/g). In the second step, purified SWE polyphenol extract revealed excellent scavenging efficiency in four antioxidant experiments compared with CSE extracts. And phenolic composition of extracts obtained by SWE revealed high amounts of catechins and E-p-Coumaric acid. Characteristic reactive groups in SWE extract were mainly hydroxyl, benzene group, methoxy group, and oxygen-containing heterocyclic ring. Therefore, SWE proved to be a useful extraction technique for the recovery of polyphenols from red pitaya seeds.