**1. Introduction**

Ginseng (*Panax ginseng* Meyer) roots and their processed products are widely consumed because of their excellent health benefits, which are attributed to their bioactive ginsenosides [1]. Qualified ginseng roots require a relatively long cultivation period and are typically harvested between 4 and 6 years. Consequently, ginseng leaves are produced in large quantities annually and disposed of as waste at the end of the growing season. However, ginseng leaves possess similar pharmacological activities to the roots [2]. The distribution of ginsenosides in ginseng leaves has been investigated, which suggests that the leaves have similar ginsenoside compositions to the roots [3–5]. In addition, Chung et al. [6] reported an extremely high total phenolic content of ginseng leaves compared to the roots. Yin et al. [7] suggested that ginseng leaves contain higher total flavonoids compared to the roots, especially flavonol glycosides of panasenoside and kaempferol-3-Oglucoside. However, although ginseng leaves are potentially good sources for producing

**Citation:** Duan, S.; Liu, J.R.; Wang, X.; Sun, X.M.; Gong, H.S.; Jin, C.W.; Eom, S.H. Thermal Control Using Far-Infrared Irradiation for Producing Deglycosylated Bioactive Compounds from Korean Ginseng Leaves. *Molecules* **2022**, *27*, 4782. https://doi.org/10.3390/ molecules27154782

Academic Editor: Nour Eddine Es-Safi

Received: 28 June 2022 Accepted: 23 July 2022 Published: 26 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

phytochemicals, especially polyphenols and ginsenosides, and have advantages in annual yield production, ginseng leaves have not been fully studied and used.

Thermal processing methods play a vital role in the global ginseng market. Numerous studies have shown significant processing results related to the deglycosylation pattern of ginsenosides and various health-beneficial biological activities, exhibiting bioconversion from major ginsenosides to minor ginsenosides, as well as antioxidant, antitumor, and anti-inflammatory effects [8–11]. Nevertheless, considerable efforts are required to reasonably use ginseng leaves, because few studies have focused on phytochemical variations, especially polyphenols, during dry-heat thermal processing.

Far-infrared irradiation (FIR) treatment is widely used in the food-processing industry because it is easily applicable in terms of fast and efficient heating of plant materials for inducing the decomposition of multi-chain molecular clusters [12]. Our previous studies have suggested that FIR is an efficient method for accelerating ginsenoside conversions in ginseng roots and leaves [10,13], which is significantly different from traditional steaming treatment at the same temperature [13]. An increase in the content of bioactive compounds, such as phenolic acids and flavonoids, by FIR has been reported in rice, angelica, gamguk flowers, grapes, and buckwheat sprouts [14–18]. However, there is still no detailed study related to defining an adequate FIR temperature for producing the maximum range of bioactive compounds from ginseng leaves.

When considering cost, source availability, and sustainability, ginseng leaves are a more appropriate choice for obtaining phytochemicals, especially polyphenols and ginsenosides, compared to their roots. The aim of this study was to investigate the effects of FIR treatment on the bioactive compounds (total phenolics, flavonoids, and ginsenosides) and human health benefits (antioxidant activities) of ginseng leaves. Our results will provide useful information for the further use and application of ginseng leaves in the health product industry and food science.

#### **2. Results and Discussion**
