## 1 Introduction  
Skin aging has been explained as intrinsic or extrinsic aging that is often induced by photo-aging. Compared to intrinsic aging due to the senescence of the human body, photo-aging is caused by chronic exposure to environmental factors, mainly ultraviolet (UV) radiation. It eventually results in skin fragility, wrinkling, roughness, dryness, and hyperpigmentation [1; 2; 3]. In general, skin aging is considered most closely related to photo-aging [2]. Among the three types of solar UV rays, UVa, UVb, and UVc, UVb radiation in the wavelength range of 280 to 315 nm is considered most damaging to the skin because it results in sunburns, erythema, acute inflammation, and skin cancers, which are often caused by photo-aging of the skin [4; 5]. Exposure of human skin to UV radiation increases the synthesis of various matrix metalloproteinases (MMPs), which are implicated in photo-aging and the destruction of the extracellular matrix (ECM) due to the loss of collagen [6]. Oxidative stress due to exposure to UVb radiation substantially increases the levels of reactive oxygen species (ROS), which generate oxidative photo-lesions in DNA, proteins, and lipids in the skin [4; 7]. UV-induced oxidative damage also directly induces mitochondrial ROS production, since mitochondria are the primary organelle affected by UV-induced skin aging. Several results support the hypothesis of the close association between mitochondria and skin health either via an increase in ATP production or free radical scavenging activity [8; 9]. Therefore, an increase in mitochondrial activities and a decrease in ROS levels would be a worthwhile approach that has never been used to prevent skin aging as an extensive review on the relationship between flavonoids and mitochondrial activation for cytoprotection [10]. This goal could be achieved by administering aloe, a medicinal plant that has mainly been used to treat skin disorders for thousands of years. Among the many aloe species, the gel of _Aloe barbadensis_ Miller, commonly called _Aloe vera_, obtained by peeling the skin off the leaves and has been most widely used as a food supplement and cosmetic ingredient due to its various biological effects, such as the treatment of burns and anti-inflammatory and antibacterial properties [11; 12; 13]. However, the remaining parts of _Aloe vera_ have not been utilized much due to their very bitter tastes and hardness, although they contain more biologically active substances, such as aloins, aloe-amodin, and polyphenols [12; 13]. Moreover, more than 97% of the gel is composed of water [14], and the remaining constituents mainly include polysaccharides such as acetaminophen and mannose [15; 16]. In addition, anthraquinones, which are considered to possess strong anti-inflammatory, antiprotozoan, and antioxidant activities, only exist on the leaf [17]. Therefore, it is necessary to develop a technique for processing the relatively hard parts of _Aloe vera_ while maintaining its good efficacies to utilize this leaf skin efficiently. Lactic acid fermentation could primarily be considered since this process is operated under low temperature and mild extraction conditions, which would not degrade the bioactive substances in leaf skin [18; 19]. The fermentation process has also been shown to improve the elution yields of active components from natural resources, especially the hard structures of the plants, along with the bioconversion of its metabolites, which increase its efficacies by promoting synergistic effects with various components in the extracts [19; 20]. Lactobacillus family members have generally been employed for this purpose because the extracts obtained from the fermentation of plant resources with Lactobacillus species exert many useful effects on treating skin disorders through metabolites obtained during fermentation processes [21; 22; 23]. Therefore, in this study, the fermentation of the skin of _Aloe vera_ leaves by _L.