**2. Polyphenols**

Plant phenols are among the most abundant and widely represented class of existing plant natural products [40] thanks to the continuous evolution of new genes brought about by gene duplication and mutation and subsequent recruitment and adaptation to specific functions.

The amino acids phenylalanine and tyrosine (derived from the shikimic acid pathway) are the most common origin of polyphenols [41, 42]. Chemically, polyphenols belong to four main classes (**Figure 1**): flavonoids, phenolic acids (hydroxy derivatives of benzoic acid and cinnamic acid, i.e., p-hydroxybenzoic, protocatechuic, vanillic and syringic acids) and their esters (chlorogenic, caftaric, coutaric and fertaric acids), stilbenes (resveratrol, pterostilbene, piceatannol), characterized by a double bond (1,2-diarylethene) connecting the phenolic rings and lignans (pinoresinol, podophyllotoxin, steganacin), characterized by a 1,4-diarylbutane structure, i.e., having 2-phenylpropane units. Flavonoids and phenolic acids account for 60 and 30%, respectively, of the total dietary polyphenols [43].

When the phenolic molecules are not attached to sugar moieties are known as the aglycone form; while those molecules conjugated with one or more sugar residues are called glycosides. Most phenolic compounds are found in nature associated with mono- or polysaccharides or functional derivatives [44] such as esters or methyl esters, varying widely in their hydroxylation pattern and can be glycosylated or acylated.

Despite the fact that most of the literature on phenolic compounds focuses mainly on those found in fruits, vegetables, wines and teas. However, many phenolic compounds present in fruits and vegetables (phenolic acids and flavonoids) are also found in cereals [45, 46].

For decades, this family of compounds has attracted the attention [47] since three British scientists open the door to understand separation, structural elucidation and taxonomical distribution of phenolic compounds. As mentioned above, traditionally, the interest was focused on organoleptical properties of polyphenol such as color, astringency, bitterness, astringency and a range of other tactile or "mouth feel" characteristics [48, 49], as well as their physiological role in plants in the reproduction, pathogenesis and symbiosis. In last decades, polyphenols

**Figure 1.** Types of phytochemicals [38].

 metabolites of a phenolic nature [5–12]. The importance of secondary metabolites and their crucial role in many important functional aspects of plant life was recognized for the first time in the second-half of the ninetieth century by Julius Sachs (1873) [13, 14]. Polyphenols are natural compounds occurring in plants [15–18], including foods such as fruits, vegetables,

The study mainly focuses on organoleptical properties of polyphenols [19] and their physiological importance to plants [20]. Later, polyphenols are found to be recognized by their nutritional value, since they may help reduce the risk of chronic diseases [1, 21–24] and, in general, have a positive effect on health, because of their free radical scavenging capacity [25–27], which, among other biological effects, increases antioxidant activity and prevents cellular oxidation. The research on phenolic compounds is mainly focused on anthocyanins [28–29], natural pigments and common components of the human diet (foods, fruits and vegetables, especially in berries and in red wine), As they provide for much of the red to blue pigmentation of flowers and fruits and have physiological functions in vegetative tissues. Their biosynthetic pathway has been the subject of much research and the associated biosynthetic and regulatory genes are well defined. Besides considerable interest in coloring properties of anthocyanins, they have also attracted attention due to their antioxidant activity [30–34] and their property is closely related, to a large extent, with their chemical structure. The pH-dependent groundstate chemistry of anthocyanins is extremely rich. In the past 20 years, the health benefits of

Analytical chemistry plays an importance role in this context [35–37] which determines the identity and quantities of anthocyanins in natural products, as well as their effects *in vivo* and *in vitro*. This chapter intends to reflect the interdisciplinary nature of the research that is currently carried out in anthocyanin pigments through an update of the state-of-art of a series of previously published reviews on this field in the year 2012 [28, 29, 38, 39]. First, general considerations concerning polyphenols with emphasis on their role as secondary metabolites are made. Flavonoid classification, structure, biological activities, databases, intake and dietary sources are also contemplated. Second, aspects of anthocyanin concerning its early history and chemical structure, color and intake are dealt. It should be noted that anthocyanins are readily distinguished from other flavonoids as they undergo rearrangements in response to pH. The antioxidant activity of anthocyanins is depending on their chemical structure. Finally, special attention is paid to analytical methodologies involved in the isolation, determination and characterization of bioactive polyphenols in plants, fruits and vegetables, herbal drugs, medicinal plants and wines, including sample-handling strategies, a feature of analysis often ignored. The use of nonthermal technologies in the assisted extraction of anthocyanins will be covered in future reports.

Plant phenols are among the most abundant and widely represented class of existing plant natural products [40] thanks to the continuous evolution of new genes brought about by gene duplication and mutation and subsequent recruitment and adaptation to specific functions.

anthocyanins have become the subject of intensive research.

cereals, tea, coffee and wine.

118 Phenolic Compounds - Natural Sources, Importance and Applications

**2. Polyphenols**

are increasingly recognized due their nutritional value, since they may help reduce the risk of chronic diseases [50–53]. The capacity of phenolic compounds to trap free radicals depends upon their structure, in particular of the hydrogen atoms of the aromatic group that can be transferred to the free radicals [54, 55] and of the capacity of the aromatic compound to cope with the uncoupling of electrons as a result of the surrounding displacement of the electron-π system [27]. The polyphenols are still gaining attention.

Although the percentage of absorbed natural polyphenols is usually quite low [56], researchers have seen a large quantity of metabolites of polyphenols in the form of simple phenolic acids in the blood. The amount and form in which plant phenolic substances are administered influence greatly the physiological effects connected with their consumption [57]. About 1 g of polyphenols per day is commonly ingested with foods, being the most abundant antioxidant in the diet (about 10 times higher than the intake of vitamin C and 100 times that of vitamin E) [28, 29, 58]. The daily intake of polyphenols is difficult to estimate and depend on several factors. In the literature there are about 1000 peer-reviewed publications [28] concerning the polyphenol content in food. Recently, the construction and application of a database with polyphenols content in foods has facilitated this task [59]. The Institute Nationale de la Recherche Agronomique [60] has developed a new Phenol-Explorer database [59] covering over 60,000 foods useful to epidemiologists, food scientists and food manufacturers. The content of polyphenols of the 100 richest dietary sources can range from 15,000 mg per 100 g in cloves to 10 mg per 100 mL in rose wine [61]. EuroFir [62] is another database to build national food composition in different countries within the framework of the European Food Information Resources Network (i.e., Spanish [63] and Irish [64] databases).
