**1. Introduction**

The apple (*Malus domestica* Borkh.) is produced globally; about 89 million tonnes was produced in 2016, ranking it third in worldwide fruit production [1]. In Poland, most people across all age groups eat from one to six apples a week. Most Polish customers consider flavor, taste, and firmness at the top of the list and give less consideration to nutritional value, peel color, and size [2]. Nevertheless, Polish customers of all age groups prefer more redness over greenness [2]. In a study of Estonian customers from 2007 to 2012, the most important consumer preferences were apple taste, followed by appearance (e.g., color) and health benefits, which were rated of equal importance, and finally price; consumer preferences in taste and color remained unchanged, whereas preference for domestic and organic apples decreased, over the 5-year period survey [3]. In a UK study, consumers associated red apples with sweet sensory descriptors and green apples with grassy, astringent and drying, acidic, sour sensory, or unripe descriptors [4]. Therefore, the color of apple peel is important for market value, and well-reddened fruit has an increased price rating in Japan [5].

The redness of apple peel is due to the accumulation of anthocyanins, which are water-soluble plant pigments responsible for the blue, purple, and red in many plant tissues of fruits, flowers, and vegetables. Willett [6] showed that approximately 32% of cancers could be avoidable by changes in diet. People with low fruit and vegetable intake have about twice the risk of most cancers as those with high intake [7]. In addition, a case-control study of a group of 33 women recently diagnosed with breast cancer and a control group of 33 healthy women demonstrated that regular ingestion of apples, watermelons, and tomatoes was associated with protection against breast cancer [8]. Increased intake of apple fruits has been associated with reduced risk of disease, because apple fruits—especially their peel have strong antioxidant activity [9], which benefits human nutritional health.

This review provides a general overview of the nutritional impacts of polyphenols, flavonoids, and anthocyanins, which serve as antioxidants and counteract the prooxidant load of the human body [10]. The review focuses on anthocyanin in apple fruit peel and its health benefits and the means by which this anthocyanin concentration could be improved by regulating cultivation management and breeding cultivars with high anthocyanin levels. The literature on anthocyanin synthesis in red-fleshed apple fruits is not reviewed here; the details of genetic, environmental, and plant hormonal factors involved in anthocyanin synthesis have been reported by Honda and Moriya [11].
