Thermal Processing Effects on the Chemical Constituent and Antioxidant Activity of Okara Extracts Using Subcritical Water Extraction

Hongyi Sun,Xin Su,Xi Yuan,Min Shi,Zhenya Zhang

doi:10.1155/2018/6823789

Abstract

Subcritical water extraction (SWE) has been employed for the extraction of bioactive compounds from plant materials with cost-effectiveness, less consuming time, and environmental sustainability. To explore the effects of thermal processing during SWE, total organic content (TOC), total sugar, polysaccharides, total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity (ABTS and DPPH assays) of eight aqueous extracts have been quantitatively investigated. The results indicated that elevated temperatures indeed resulted in significant changes in the constituents and antioxidant activities of okara extracts. Among them, the extract obtained at 220°C exhibited the highest total phenolic, flavonoid content, DPPH (2,2-diphenyl-1-picrylhydrazyl) radical-scavenging activity, and ABTS [2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate)] radical-scavenging activity. However, phenolic compounds were destroyed after the treatment above 230°C, suggesting that any polymer processing is improper to undertake at higher than this value to achieve the high antioxidant activity. Moreover, a significant positive correlation between TPC or TFC and antioxidant capacity (DPPH and ABTS) values was detected.

Highlights

Okara is rich in bioactive compounds including dietary fiber, protein, polysaccharides, and phenolic compounds, many of which have been claimed to contribute to the antioxidant activity due to their redox properties derived from various possible mechanisms: free radical-scavenging activity, transition-metal-chelating activity, and/or singlet-oxygenquenching capacity [1]
Subcritical water extraction (SWE) is defined as hot water at temperatures ranging between 100°C and 374°C under high pressure to maintain water in the liquid state [3]
Erefore, the objective of this study was to investigate the effects of different temperatures under subcritical water extraction conditions on a range of potentially health-related chemical constituents and antioxidant capacities, in order to evaluate the potential of SWE for the production of high bioactive extracts from okara

Summary

Introduction

Okara is rich in bioactive compounds including dietary fiber, protein, polysaccharides, and phenolic compounds, many of which have been claimed to contribute to the antioxidant activity due to their redox properties derived from various possible mechanisms: free radical-scavenging activity, transition-metal-chelating activity, and/or singlet-oxygenquenching capacity [1]. The differences in the structures and substitutions may influence the phenoxyl radical stability and impair the antioxidant properties. Subcritical water extraction (SWE) is defined as hot water at temperatures ranging between 100°C and 374°C under high pressure to maintain water in the liquid state [3]. As one of an environmentally friendly and efficient technique, it has been a wide range of applications, such as extraction, hydrolysis, and wet oxidation of organic compounds [4, 5]. Elevated temperatures can modify the dielectric constant and ionization constant of water, resulting in the possibility of tuning its polarity and accelerating hydrolysis, thereby obtaining organic compounds instead of using organic solvents during food industry processing [6]

Objectives

Methods

Results