Although ginseng leaf is a good source of health-beneficial phytochemicals, such as polyphenols and ginsenosides, few studies have focused on the variation in compounds and bioactivities during leaf thermal processing. The efficiency of far-infrared irradiation (FIR) between 160 °C and 200 °C on the deglycosylation of bioactive compounds in ginseng leaves was analyzed. FIR treatment significantly increased the total polyphenol content (TPC) and kaempferol production from panasenoside conversion. The highest content or conversion ratio was observed at 180 °C (FIR-180). Major ginsenoside contents gradually decreased as the FIR temperature increased, while minor ginsenoside contents significantly increased. FIR exhibited high efficiency to produce dehydrated minor ginsenosides, of which F4, Rg6, Rh4, Rk3, Rk1, and Rg5 increased to their highest levels at FIR-190, by 278-, 149-, 176-, 275-, 64-, and 81-fold, respectively. Moreover, significantly increased antioxidant activities were also observed in FIR-treated leaves, particularly FIR-180, mainly due to the breakage of phenolic polymers to release antioxidants. These results suggest that FIR treatment is a rapid and efficient processing method for producing various health-beneficial bioactive compounds from ginseng leaves. After 30 min of treatment without leaf burning, FIR-190 was the optimum temperature for producing minor ginsenosides, whereas FIR-180 was the optimum temperature for producing polyphenols and kaempferol. In addition, the results suggested that the antioxidant benefits of ginseng leaves are mainly due to polyphenols rather than ginsenosides.

The roots, leaves, and other parts of American ginseng contain ginsenosides and polyphenols, among other bioactive components. So far, reports on exploring the changes in bioactive components within its leaves after heat treatment have been limited. In the present study, we investigated the effects of far-infrared irradiation (FIR) heat treatment from 160 to 200 ℃ on the deglycosylation and dehydration rates of the bioactive components in American ginseng leaves. As the temperature was increased, the amounts of constant ginsenosides decreased while those of rare ginsenosides increased. After FIR heat treatment of the American ginseng leaves at an optimal 190 ℃, the highest total polyphenolic content and kaempferol content were detected, the antioxidant activity was significantly enhanced, and the amounts of rare ginsenosides F4, Rg6, Rh4, Rk3, Rk1, Rg3, and Rg5 were 41, 5, 37, 64, 222, 17, and 266 times higher than those in the untreated leaves, respectively. Moreover, the radical scavenging rates for 2,2-diphenyl-1-picrylhydrazyl and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), and the reducing power of the treated leaf extract, were 2.17, 1.86, and 1.77 times higher, respectively. Hence, optimal FIR heat treatment at 190 ℃ is an efficient method for producing beneficial bioactive components from American ginseng leaves.