## 1 Introduction  
Skin aging is typically distinguished by dryness, wrinkle formation, atrophy, sagging, rough texture, loss of elasticity, hyperpigmentation, and a thickened epidermis [1]. These changes, mainly observed in the dermal layer, are natural aging processes caused by sustained and irreversible degeneration of skin tissue, yet are also consequences of repeated exposure to sunlight, ultraviolet rays (UVR), drinking, smoking, and chemical toxins. Especially in skin areas exposed to sunlight, such as the face, neck, and the back of our hands, significant alterations are induced, such as wrinkle formation, spider veins, epidermal atrophy, melanogenesis, irregular pigmentation, and sunburn [2, 3]. Among the spectrum of sunlight, UVA (320-400 nm) and UVB (280-320 nm) are the main causes of photoaging [3]. Depending on its wavelength, UVA penetrates through the dermis, and causes deterioration and sagging, whereas UVB is absorbed in the epidermis and is the main cause of wrinkle formation [3; 4]. At the cellular level, UVB induces DNA and RNA damage, protein mutation, inflammation, and an imbalance in the extracellular matrix (ECM) remodeling through intracellular reactive oxygen species (ROS) production [3]. Over the previous decades, as the public interest in skin beauty and health has increased and the related industries have grown exponentially, functional products have been developed to prevent skin aging and maintain healthy skin conditions, including antioxidants, whiteners, moisturizers, wrinkle improvers, and UV protectors [5]. However, safety issues are continuously raised regarding harmful chemicals and toxins, which are potential risk factors for chemical syntheses or metal-based skin products. Consequently, there is an extensive demand to develop functional materials for the skin using natural resources, including effective and safe medicinal plants and herbs. Vitamin C (L-ascorbic acid, L-AA), an abundant compound in fruits and vegetables, is well-known for its beneficial effects on skin health and diseases. L-AA has been reported to exert skin wrinkle improvement and UV protection through skin epidermal barrier formation by collagen, elastin, and ceramide syntheses, skin-moisturizing effects, antioxidant effects, and skin-whitening effects through the inhibition of melanogenesis [6]. An apple, a mature fruit of the deciduous tree of the Rosaceae family (_Malus pumila_ Mill. ), contains various beneficial ingredients for health, such as dietary fibers, vitamins, and polyphenols. Accumulating evidence shows that these abundant bioactive components in apples exert beneficial effects on lipid metabolism, cardiovascular diseases, and allergic diseases, as well as skin health [7; 8; 9; 10]. Polyphenols are known to exert a potent ROS scavenging effect and anti-photoaging effect through inhibition of MMPs, elastase, and hyaluronidase [11]. Moreover, a recent study investigated the polyphenol applied with nanotechnology to exert antioxidant and UV protection effects that were superior than those of its parental polyphenol without cytotoxicity and genotoxicity [12]. A recent study revealed that both unripe and ripe apples contain various bioactive phenolic compounds such as chlorogenic acid, chlorogenic acid methyl ester, isoquercitrin, phloridzin, phlorotin, quercitrin, and reynoutrin [13]. The content of total phenolic compounds in unripe apples is four times higher than that in the ripe apples [13]. Unripe apples also contain higher amounts of chlorogenic acid, 5-O-\(p\)-cournaroylquinic acid, and phloridzin compared to ripe apples [13]. Oral intake or topical application of polyphenol-rich plant extracts can contribute to delaying the aging process of the skin [11]. Therefore, an unripe apple is expected to be a promising candidate as a functional food ingredient for the skin.