**2.1 Concentrations of polyphenols, flavonoids, and anthocyanins and antioxidant activity in common foods**

Polyphenols are among the most numerous products of the secondary metabolism of plants and are an integral part of the human diet [12]. They constitute more than 8000 phenolic structures and can be divided into at least ten different classes, depending on their basic chemical structure. The recent interest in food phenolics has increased greatly owing to their possible benefits to human health, such as prevention of cancer, cardiovascular disease, and other pathologies. The polyphenolic concentration of plant foods can vary by several orders of magnitude [12], with the cereals barley and sorghum having considerably higher levels than those found in some more commonly consumed foods (e.g., fruits, nuts, and vegetables). In the fruits mentioned in the review [12], blackcurrant has the highest polyphenolic concentration, followed by grape, raspberry, and apple. Therefore, apples can serve as an important source of polyphenols as dietary foods.

Plant flavonoids constitute one of the largest groups of naturally occurring phenolics, and they possess an ideal chemical structure for antioxidant activity. They commonly contain diphenylpropanes with a 15-carbon skeleton (C6▬C3▬C6), which comprises two aromatic rings linked through three carbons that usually form an oxygenated heterocycle. The flavonoids in foods comprise 13 types of basic structure, one of which is anthocyanidin [12]. The glycosides of anthocyanidin are referred to as anthocyanins.

Anthocyanins vary in the number and position of hydroxyl and methoxyl groups on the basic anthocyanidin skeleton. There are over 600 naturally occurring anthocyanins. Approximately 17 anthocyanidins are found in nature, and 6 of them—cyanidin, delphinidin, petunidin, peonidin, pelargonidin, and malvidin are ubiquitously distributed in nature.

Antioxidants are present at high levels in apples [13], and the antioxidant activity of apples has significant positive relationships with total phenolic and flavonoid concentrations [10]. When compared with many other commonly consumed fruits, apples have the second-highest level of antioxidant activity [13]. Antioxidant activity provides a measure of protection that slows the process of oxidative damage, which is an important cause of disease initiation and progression in the human body [14].
