*2.1. Effects of FIR Treatment on TPC in Ginseng Leaves*

The effects of FIR treatment on TPC in ginseng leaves are shown in Figure 1. As the FIR temperature increased, the TPC gradually increased up to 180 ◦C and then decreased. The highest TPC in ginseng leaves was 25.27 mg/g d.w. at FIR-180, which is about 1.56 times that of the untreated control. The lowest TPC (15.16 mg/g d.w.) was observed at FIR-200, with no significant difference being observed between FIR-200 and the untreated control. It can be concluded that suitable FIR treatment conditions can increase the TPC in ginseng leaves. The effect of FIR treatment on TPC improvement has also been reported in *Angelica gigas* Nakai, *Arachis hypogaea* L., *Camellia sinensis* var. sinensis, and *Hibiscus cannabinus* L. [15,19–21]. The increase in the TPC can be explained by the release of small polyphenols due to the breaking of molecular bonds in large polyphenols at high FIR temperatures [12,22]. However, relatively higher temperatures (FIR-190 and FIR-200) caused a decrease in the TPC in ginseng leaves compared to low temperatures (FIR-160, FIR-170, and FIR-180), which might be explained by the destruction of polyphenol structures by high FIR energy [18]. Based on our results, it can be concluded that 180 ◦C is a suitable temperature for FIR treatment for the processing of ginseng leaves to obtain more polyphenols. Polyphenols are an important phytochemical group in edible plants due to their various biological activities and health benefits, gaining increasingly more attention of researchers to investigate their content variation in plant tissues during different processing methods [23–25]. For ginseng, previous studies have mainly focused on their root polyphenol content changes during processing [26,27]. To the best of our knowledge, this is the first study to show the excellent effect of suitable FIR treatment on improving the TPC in ginseng leaves.

**Figure 1.** Changes in the total polyphenol content of ginseng leaves treated by FIR at different temperatures. The values are expressed as the mean with standard error (*n* = 3). Different letters (a–e) above the bar graphs indicate significant differences at *p* < 0.05 on Tukey's HSD test.
