**2. The phenolic compounds**

The biosynthesis of these compounds occurs via the shikimate pathway, from the amino acids phenylalanine and tyrosine and the participation of the enzyme phenylalanine ammonia lyase (PAL) that catalyzes the removal of the ammonia residue of the amino acids phenylalanine and tyrosine to produce cinnamic acid and 4‐coumaric acid, respectively [5]. Both compounds further enter the phenylpropanoid pathway and it is within the various branches of this path‐ way where the great diversity of phenolic compounds so far identified is synthesized.

Regarding their chemical structure, phenolic compounds possess at least one aromatic ring with one or more hydroxyl groups, including their functional derivatives [6]. The polyphe‐ nols are within the group of phenolic compounds, which according to Quideau et al. [7], the term "phenolics" should be used to define compounds derived exclusively from the shiki‐ mate/phenylpropanoid pathway and/or the route of polyketides, which include more than one phenolic unit (phenol). This restriction is necessary because substances from alternative metabolic pathways may also present more than one phenolic unit. In literature, the term polyphenols and phenolic compounds are often encountered, however, if the former term is used it would not include the phenolic acids, as their structure contains only one phenol. Therefore, throughout this work we will use the term "phenolic compounds" as we will be commenting on some flavonoids and phenolic acids.

The complexity of the phenolic compounds ranges from simple molecules as phenolic acids to highly polymerized compounds as tannins. Phenolic compounds are present in plants in conjugated form with one or more sugar residues bound to the hydroxyl groups, although in some cases direct connections between a sugar molecule and an aromatic carbon may occur. The most common way to find them in nature is as glycosides, conferring them solubility in water and in organic solvents. All phenolic compounds exhibit strong absorption in the UV spectral region, and some colorful phenolic compounds absorb in the visible region as well [8].

Phenolic compounds can be classified in various ways, one proposed by Harborne and Simmonds [9] considers the number of carbons contained in their molecule. According to this criterion a total of 20 different groups of phenolic compounds are considered, where simple phenols are the simplest and the phlobaphenes are the most complex group [10]. Another criterion is used by Shahidi and Naczk [6], which classifies groups of compounds based on their complexity. **Figure 1** illustrates this classification.

**Figure 1.** General classification of phenolic compounds (adapted from Shahidi and Nazck [6]).
