## 3 Results and Discussion
### DPPH Free Radical Scavenging Effect and the Reduction of ROS Production by the Samples  
First, in Figure 4, antioxidant effects of AF were observed as DPPH free radical scavenging activities and compared with AW and quercetin, a flavonoid with strong antioxidant effects on human skin cells [42; 43], to confirm the hypothesis that AF exerts positive effects on mitochondrial activities that are similar to quercetin. Treatment with 10 ug/mL quercetin, which corresponds to a concentration of 0.3% of the AF as described in Section 3.1, an ca. 72.56 \(\pm\) 3.03% increase in the free radical scavenging activity was measured compared with the addition of 50 ug/mL Trolox as a positive control, while 74.45 \(\pm\) 1.68% higher antioxidant activity was observed after the addition of 0.3% of the AW. Compared to antioxidant activities of the AW, a ca. 110.2 \(\pm\) 0.71% higher antioxidant effect was observed after treatment with 0.3% of AF as the same concentration of the AW that showed the highest efficacy. Interestingly, at doses greater than 0.3%, little increase in antioxidant activities was observed for both the AW and AF, and the antioxidant activity even decreased slightly, indicating that an optimum dose might be required for maintaining high antioxidant activities. Moreover, the AF showed higher antioxidant activities, although similar or even lower quercetin contents were present in the AF than in the AW. In the present study, AF, a crude extract, showed higher antioxidant activity than quercetin itself at the same dosage, which strongly implies a synergistic effect of quercetin and other bioactive substances  
Figure 4: DPPH free radical scavenging activities of various concentrations of two different extracts of _Aloe vera_ leaf skin compared with 10 ug/mL quercetin and 0.05% Trolox as positive controls. AW: hot water extraction of _Aloe vera_ leaf skin at 100 \({}^{\circ}\)C for 24 h. AF: _L. plantarum_ fermentation of _Aloe vera_ leaf skin at 37 \({}^{\circ}\)C for 3 days and extraction with an ultrasonicator at 40 kHz for 1 h at room temperature. The mean values \(\pm\) SD for separate triplicate experiments are shown, and error bars indicate the SD. The significance of the difference was set to \({}^{\ast}\)_p_\(<\) 0.05, \({}^{\ast\ast}\)_p_\(<\) 0.01 and \({}^{\ast\ast\ast}\)_p_\(<\) 0.001 compared with the Trolox group. in the extracts that were possibly derived from the fermentation of _Aloe vera_ leaf skin by lactic acid bacteria. However, only quercetin itself also exerted various positive effects on human skin cells [42; 44]. This result indicates the advantages of the fermentation process in employing by-products of aloe processing. In general, ROS generation is presumed to mediate cell death through oxidative stress, such as UVb, and subsequently causes mitochondrial dysfunction, which eventually results in the senescence of skin cells [45; 46; 47]. Therefore, Figure 5 confirms that the increased antioxidant activity of AF is attributed to the reduced intracellular ROS production from the mitochondria of UVb-irradiated human skin fibroblasts. More detailed efficacy could be explained by directly measuring mitochondrial ROS generation. The AF substantially inhibited ROS generation compared with the other two samples, the AW and quercetin, at the same dose. As expected, quercetin, a well-known antioxidant, decreased the intracellular ROS generation down to 82.33 \(\pm\) 5.04% compared to the control (not exposed to UVb light), while a 244 \(\pm\) 6.33% increase in ROS production was observed from UVb-irradiated human skin fibroblasts. After treatment with 0.3% of the AW, an approximately 80.25 \(\pm\) 6.88% decrease was observed, and an 81.50 \(\pm\) 2.12% decrease was observed after treatment with 0.5% of the AW, values that were similar to the 82% observed when adding the same concentration of quercetin itself to the extract.