High-yield recovery of highly bioactive compounds from red ginseng marc using subcritical water extraction

Highlights

• Recovery of highly bioactive compounds form red ginseng marc using subH₂O extraction.

• 48 wt% extraction yield, which was 2–4 times higher than Soxhlet extraction.

• SubH₂O extract exhibited 3–79 times higher antioxidant activity than Soxhlet.

• Total phenolic contents were strongly correlated with the antioxidant activities.

• Reaction pathways for transformation of minor ginsenosides were proposed.

Abstract

Red ginseng marc (RGM), a byproduct obtained during manufacturing various ginseng products, which is typically discarded as waste, contains numerous residual bioactive compounds. However, the recovery of bioactive compounds, including transformed ginsenosides, from RGM using conventional extraction techniques is difficult. In this study, subcritical water was used for a quick and high-yield extraction of the bioactive compounds in RGM. Furthermore, the chemical species and antioxidant activities of the extracts were analyzed. Extraction was performed in the temperature range and duration of 140–200 °C and 15–90 min, respectively, to determine the optimal conditions for achieving the highest extraction yield and bioactivity. Under the optimized conditions (200 °C and 15 min), an extraction yield of 48 wt% was achieved, which was 1.8 and 4.1 times higher than those achieved via Soxhlet extraction with water and 80% ethanol, respectively (8 h). In addition, the antioxidant activity of the subcritical water extract was 2.7–78.7 times and 2.8–9.8 times higher than those of the extracts obtained using the Soxhlet method with water and 80% ethanol, respectively. The total ginsenoside content of the extract was 30 mg/g, and G-Rf, a transformed ginsenoside, was the primary component of the extract.

Introduction

Ginseng root has been widely used as a herbal tonic in Chinese traditional medicine for over 3000 years owing to its various pharmacological and therapeutic actions in the human body, such as its effects on the central nervous, cardiovascular, and immune systems and its high anti-stress and anti-aging properties [1]. The most common species of ginseng are Panax ginseng, Panax quinquefolius, and Panax japonicus, which are indigenous to East Asia (China and Korea), North America, and Japan, respectively [2]. In Korea, red ginseng is a product obtained from fresh ginseng using superheated steam, followed by drying, whereas white ginseng is produced via simple sun drying. The biological activity of red ginseng is higher than those of fresh and white ginseng. Moreover, the amount of transformed ginsenosides derived from the primary ginsenosides in red ginseng is more than that from fresh ginseng [3].

The residual byproduct generated after the extraction of ginsenosides and acidic polysaccharides from red ginseng using hot water or water–alcohol mixtures is referred to as red ginseng marc (RGM), and it represents approximately 65–70 wt% of unextracted solid residue [4], [5]. The increasing interest in red ginseng products has led to an increase in the amount of generated RGM, most of which is discarded as biomass waste. However, because RGM contains highly bioactive carbohydrates, proteins, and other valuable components (e.g., unextracted saponin, minerals, and vitamins) [6], researchers should develop efficient extraction methods for reusing RGM in various sectors, including the food, cosmetics, and functional materials industries and medicine [7].

To date, several studies have been conducted to recover various valuable components from RGM using traditional Soxhlet-based extraction methods with hot water and water–organic solvent mixtures [5], [8]; however, they have been unsatisfactory owing to high energy consumption and long extraction times. Moreover, the extraction yields of the bioactive compounds are low. To overcome these drawbacks, in this study, we used highly efficient and environment friendly subcritical water extraction (SWE). To optimize the extraction process, the physicochemical properties of subcritical water (subH₂O) can be modified by adjusting the temperature and pressure (Table S1). For example, as the water temperature increased from 140 to 200 °C at 10 MPa, the dielectric constant of water (${\epsilon }_{H_{2}O}$) decreased from 46.6 to 35.1; therefore, a wide range of polar to slightly polar compounds can be extracted in this temperature range. At 200 °C and 10 MPa, the ${\epsilon }_{H_{2}O}$ value was slightly higher than those of ethanol (${\epsilon }_{\mathit{EtOH}}$ = 25) and methanol (${\epsilon }_{\mathit{MeOH}}$ = 33) [9]. In addition, upon increasing the temperature from 140 to 200 °C at 10 MPa, the viscosity of subH₂O decreased from 0.20 to 0.14 mPa·s, and the self-diffusivity of subH₂O increased from 13.8 to 23.8 × 10⁻⁹ m²/s. The high diffusivity of subcritical water molecules allowed them to easily and quickly penetrate the substrate matrix and facilitated the extraction of target compounds into the fluid phase. The high concentration of H⁺ ions in subH₂O renders it a good medium for hydrolysis in the absence of external acidic catalysts [10]. The unique physicochemical properties of subH₂O make the extraction of various types of bioactive compounds possible [11], [12], [13], [14]. For example, biologically active compounds were extracted from herb and plant materials, such as Centella asiatica [15], Mahkota dewa [16], Glycyrrhiza uralensis Fish. [17], rosemary [18], and Allium hookeri root [19]. In addition, active ingredients were extracted from various types of plant waste, such as grape residue [20], onion skin [21], potato peel [22], and marigold flower residue [23], using subH₂O. The recovery yields of bioactive compounds can be increased by optimizing the SWE conditions. Moreover, the time and energy required for SWE are shorter and lower, respectively, than those required for conventional extraction methods [24], [25]. Recently, there have been a few studies reported on the SWE of bioactive components from ginseng leaves [26] and roots [27], [28], [29], [30]. However, based on our knowledge, there has been no study reported on the recovery of bioactive compounds from RGM in subH₂O. Therefore, this is the first study to obtain high-yield extracts with high antioxidant activity from RGM using SWE for a short time.

The primary goals of this study were to identify the efficiency of the SWE method for the extraction of biologically active compounds from RGM powder and to gain insight into the extraction mechanism. SWE was performed in the temperature range of 140–200 °C for an extraction time in the range of 15–90 min. The extraction yields, total carbohydrates contents (TCCs), total phenolic contents (TPCs), and browning intensities of the extracts and the types and contents of ginsenosides in the extracts obtained under various extraction conditions were analyzed. In addition, the chemical species in the extracts were analyzed in detail using high-performance liquid chromatography (HPLC) and gas chromatography–time-of-flight mass spectrometry (GC–TOF/MS). Lastly, the 2,2ʹ-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS), 2,2-diphenyl-1-picrylhydrazyl (DPPH), and ferric reducing antioxidant power (FRAP) assays were used to determine the antioxidant activities of the RGM extracts. Moreover, the extraction yields, chemical species, and antioxidant activities of the subH₂O extracts were compared with those of the extracts obtained using the conventional Soxhlet method with water and 80% ethanol.