Optimization of Ultrasonic Enzyme-Assisted Extraction for the Recovery of Phenolic Compounds and Soluble Solids from Apple Pomace

Apple pomace is a significant by-product of the juice processing industry and a rich source of bioactive compounds; however, its potential as a valuable resource is currently largely untapped. In this work, the ultrasound-enzyme-assisted extraction (UEAE) was evaluated as an alternative method for the extraction of phenolic compounds and soluble solids from apple pomace. For this purpose, an optimization study was carried out using a Box-Behnken factorial design combined with the response surface methodology to assess the influence of enzyme/substrate ratio (0-10% <i>v</i>/<i>w</i>), extraction time (1-5 h) and temperature (25-55 °C) on three response variables: total phenolic content, DPPH radical scavenging activity and soluble solids content of the extracts. In addition, the phenolic profile of the extracts was also investigated. According to the model, DPPH radical scavenging activity will record the maximum value (0.69 mmol Trolox/L) for a 10% enzyme/substrate ratio, at 42 °C and 1 h extraction time. Extraction with an enzyme/substrate ratio of 8.5% at 41 °C for 1 h gave the highest retrieval of soluble solids content (4.1%) in the extracts. Based on HPLC results, chlorogenic acid, caffeic acid, rutin, and epicatechin were the predominant polyphenols in the extracts. The results confirmed the great potential of apple pomace as an economical source of bioactive compounds, and UEAE enhanced the recovery of phenolic compounds and soluble solids from this underutilized by-product.