# Analytical Methods for Exploring Nutraceuticalticals Based on Phenolic Acids and Polypphenols
## 2 Polyphenols and Related Compounds  
They are primarily synthesized as signaling and defensive molecules against external stressors, such as climatic and environmental factors (drought, high temperature, solar radiation, and pollution) and pathogens, thus allowing plants to respond promptly to multiple aggressions of different origins [11,12]. Strictly speaking, polyphenols consist of organic compounds containing several phenolic groups. Other related compounds are sometimes included under this generic designation so that, in a broader context, the family of polyphenols and related compounds is as follows: phenolic acids, flavonoids, stilbenes, lignans, as well as a more heterogeneous group of structurally related compounds such as chalcones, humulones, and alcohols [12,13]. The number of polyphenol molecules that have been reported, identified, or characterized is close to 9000. A brief explanation of the four main classes is given as follows (see Figure 1):  
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2. Phenolic acids comprise hydroxybenzoic and hydroxycinnamic acids. They account for 30% of the total dietary phenolics, and, in general, cinnamic derivatives, such as caffeic, ferulic, and coumaric acids, are more abundant than the benzoic ones. Tartaric esters of these acids (e.g., cafraric and coutaric acids) are especially abundant in grape, vine, and wines. In addition, quinic acid derivatives such as chlorogenic acids are present in a broad range of products (e.g., coffee, tea, and pear). Phenolic acids also include hydrolyzable tannins, which consist of sugar residues e  
Figure 1: Classification of polyphenols according to their structures with some representative molecules of each family. -ELiggic acids. This type of tannin can be found at 10 to 100 mg kg\({}^{-1}\) levels in coffee, fruits, nuts, tea, and wine. 2. Flavonoids are the largest family of polyphenols both qualitatively and quantitatively as more than 5000 different molecules have been described, accounting for 60% of the total dietary polyphenols in humans [14]. The flavonoid backbone consists of two aromatic rings connected through a linking heterocycle with an oxygen and three carbon atoms (C6-C3-C6 skeleton). Flavonoids can be divided into six subclasses, namely: flavonoids, flavones, isoflavones, flavanones, anthocyanin, and flavanols. Additional details are given below. Flavonols are based on the hydroxyflavone backbone in which the phenyl residues can be variedly hydroxylated (or metchoxylated), thus resulting in various remarkable aglycones such as quercetin, kaempfer, fisetin, and morin [15; 16; 17]. In nature, they are predominantly linked to sugars as a way to enhance their solubility in intra- and extracellular media. Hence, glucosides, galactosides, rhamnosides, and rutinosides are frequently found in plant matrices. Although flavonols are found at low concentrations, they play fundamental roles in growth regulation mechanisms. Flavones consist of the 2-phenylchromen-4-one backbone, structurally similar to flavonols but without the hydroxyl group [18; 19]. Some representative molecules are apigenin, luteolin, and tangeritin, generally occurring at levels below 1 mg kg\({}^{-1}\) in vegetables such a broccoli, celery, carrot, parsley, or spinach. C- and O-glycos have been reported in cereals such as sorghum and millet at levels from 0.05 to 10 mg kg\({}^{-1}\). Isoflavones are also based on the 2-phenylchromen-4-one backbone, differing from the flavones in the position of the phenyl group. Isoflavones are almost specific to the Fabaceae family, occurring at concentrations of 1 mg kg\({}^{-1}\) in soybean, chickpea, peanut, and sunflower seeds [20; 21]. Some typical aglycones are daidzin and genistin, often glycosylated to a high extent. Despite their low concentrations in natural sources, they have great interest due to their structural resemblance with steroid hormones. Hence, isoflaves act as estrogens to alleviate postmenopausal effects (e.g., hot flashes and bone loss) in women and to lower cholesterol levels in blood.