Green recovery of polyphenolic compounds from food industry apple waste using subcritical water

Subcritical water extraction (SWE) technology, as an environmentally sustainable technique, was applied to optimize the recovery of polyphenolic compounds from apple pomace (AP). Box–Behnken experimental design approach was used to investigate the effects of four process variables; extraction temperature (100-220 ºC), extraction time (30-120 min), mean sample particle size (500-1000 µm) and solid to solvent ratio (1-10 g/100 mL) on total polyphenolic compounds (TPC) yield. The SWE conditions for the optimal TPC of 39.08 ± 1.10 mg GAE per g of AP on dry basis (db) were determined to be temperature of 203.71 ºC, time of 75 min, solid to solvent ratio of 1 g/100 mL and a mean sample particle size of 500 µm. The temperature and solid to solvent ratio were determined as the most statistically significant process variables that influenced TPC yield. At the optimum conditions, antioxidant activity (% DPPH inhibition activity) of the polyphenolic extract was found in the range of 15.38–92.90% as compared to ascorbic acid (8.64–95.90%). The findings of this study proposed that SWE as a green and efficient technology provided significant advantage in term of polyphenolic compounds yield, and AP could be utilized as potential raw material for SWE of valuable antioxidant polyphenolic compounds.