**3. Phenolic compounds in maize grain**

response to stress conditions such as infections, injuries, ultraviolet radiation, among others [1, 2]. Furthermore, phenolic compounds serve as defense mechanisms since many of them display antifeedant and antipathogenic properties [3], which contribute to their adaptation to different environments. The presence of phenolic compounds in different plant foods contrib‐ utes to their distinctive characteristics and to its flavor and color. They can be found in solu‐ ble/free and bound/insoluble form. In maize grain the highest amount of phenolics (98.9%) is present in the insoluble fraction, and the remainder in the soluble fraction [4]. However, it is the soluble fraction that shows the greater chemical diversity, which depends on the color of the grain. The goal of this document is to provide a review of the various phenolic com‐ pounds present in maize grains of different colors, and the changes that occur when the grain is subject to nixtamalization processing for the elaboration of tortillas and all the diversity of

nixtamalized products consumed in Mexico and in many other parts of the world.

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

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

**2. The phenolic compounds**

216 Phenolic Compounds - Natural Sources, Importance and Applications

commenting on some flavonoids and phenolic acids.

Phenolic acids are the main phenolics in maize grain; however, other compounds like pheno‐ lic amines and some flavonoids have also been described [11]. The most abundant phenolic acids are ferulic and p‐coumaric, which may be in their isomeric form cis or trans, the most common being the trans form. Both acids are present in soluble form or bound to cell wall components. Ferulic acid (3‐methoxy‐4‐hydroxycinnamic acid) is the most abundant in the cell wall of monocots and is found in all fractions of the maize grain, but most abundantly in the pericarp and the aleurone layer. Chemically it is mostly ester‐linked to plant cell wall com‐ ponents, hemicellulose chains, mainly in the arabinose residues, but it can also be polymer‐ ized in lignin by ether linkages [12]. When ferulic acid is oxidized, it forms dimers or trimers, which after being hydrolyzed, are capable of forming gels when linked to two pentosans or protein molecules.

Among the flavonoids present in maize grain are the flavonols, anthocyanins, and proanthocya‐ nidins. Das and Singh [13] reported the presence of quercetin and kaempferol (flavonols) in the germ and pericarp of quality protein maize (QPM), popcorn and sweet corn. Meanwhile, Ramos et al. [14] reported the presence of kaempferol and morin in purple maize grains. The chemical structures of some phenolic compounds present in maize grain are shown in **Figure 2**.

**Figure 2.** Phenolic compounds identified in white and pigmented maize grains.

Phenolic amines were initially identified in the pericarp of white maize grain by Sen et al. [15], who associated their presence with tolerance to storage pests; recently, Collison et al. [16] reported that the phenolic amines: N‐N′‐dicoumaroylspermidine, N‐coumaroyl‐N′‐feruloylputrescine, and N‐N′‐diferuloylputrescine were the most abundant soluble phenolics in the methanolic extract of nixtamalized grains from red, blue, and purple maize grain. It is not known for sure what is the role of phenolic amines in maize grain.

### **3.1. Soluble phenolic compounds**

These compounds correspond to those obtained by treating a given sample size of ground maize grain to extraction with an organic solvent, typically aqueous solutions of methanol or ethanol. Quantification is performed by the Folin‐Ciocalteau method [17]. The identification of the different types of phenolic compounds is achieved by high‐resolution liquid chroma‐ tography (HPLC) when standards are available or by means of mass spectrometry (MS). The position in which the different moieties of the molecule are attached is elucidated by nuclear magnetic resonance (NMR) techniques.
