## 3 Results and Discussion
### Single-Factor Investigation of Extraction Conditions  
#### 3.2.1 Effects of Extraction Time on Extraction Yield  
The efficiency of TPC recovery from different COG rhizome extracts varied with the different extraction durations (Figure 2A). The highest polyphenol recovery efficiency was achieved after 120 min of extraction and tended to decrease when the process was extended. A similar trend was also observed (Figure 2B) for the extraction yield of TSC, but the peak  
Figure 1: Extraction yields of TPC (**A**), TSC (**B**) and DPPH antioxidant activity (**C**) obtained by different solvents (ethyl acetate, ethanol 96%, ethanol 80%, acetone and water). Different characters indicate statistically significant differences among the values (\(p<0.05\)).  
of TSC recovery was reached earlier (60 min of extraction). This result indicates that the saponin compounds in COG rhizomes can diffuse into the solvent faster than the phenolic compounds. The extension of the extraction time to 120 min did not significantly increase the amount of extracted TSC and a longer time even led to decreases in the TSC extraction yield instead. Therefore, it is recommended that the process should not be extended beyond 180 min for the extraction of TSC from COG rhizomes.  
The literature on the extraction of bioactive compounds has claimed that the appropriate extraction time to achieve the efficient recovery of phenolic compounds depends on the nature of the raw material and the solvent used [14,21]. For example, [PERSON] et al. (2007) [22] conducted a study on the extraction of _Cladina stellaris_ and found that shorter extraction times resulted in the better recovery of phenolic compounds. In addition, other studies have also reported that prolonging the extraction time may increase the exposure of the sample to environmental factors such as oxygen and light, which leads to lower extraction efficiency [23,24].  
Figure 2C shows that the antioxidant activity of the COG rhizome extract tended to increase gradually from 30 to 120 min of extraction and decreased thereafter. Previous studies on the extraction of antioxidants from plants have also concluded that soaking the samples for 90 to 180 min typically yields the highest antioxidant activity in the extracts [17,25]. During extraction, the diffusion time of solutes from the solid phase (material) into the liquid phase (solvent) is one of the crucial factors, so a very short extraction time is often insufficient for dissolution. According to [PERSON]'s second law of diffusion, the diffusion of substances into a solvent gradually reaches equilibrium after a certain period [26,27], so extraction beyond the optimal time can lead to the degradation of bioactive compounds due to the effects of oxygen, light and the temperature [27]. For example, a study investigating the stability of polyphenols in plant extracts under thermal and photo-oxidation treatments found that polyphenols were relatively thermal stable (degradation of 15% to 30% after 240 min of exposure to a temperature from 60 to 100 \({}^{\circ}\)C) [28]. However, these compounds were very sensitive to UV-C exposure, with 50% degradation of gallic acid and 83% loss of catechin after 180 min of treatment, which resulted in a significant reduction in the DPPH radical scavenging activity correspondingly. The extraction of _Polyscias fruticosa_ roots also revealed that although higher total soluble solid content could be achieved until 120 min of extraction, the TSS recovery and DPPH antioxidant activity of the extract reached the highest values at 90 min and remained unchanged during extended extraction periods [29].  
Thus, an extraction time ranging from 60 to 120 min is recommended for the recovery of all three desired outputs, namely the TPC, TSC and DPPH antioxidant activity, from COG rhizomes.