*3.1.1. Phenolic acids*

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**.

218 Phenolic Compounds - Natural Sources, Importance and Applications

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

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

role of phenolic amines in maize grain.

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

**3.1. Soluble phenolic compounds**

Phenolic acids can be found in maize grain in soluble and insoluble form. The soluble fraction is also known as free, although in a strict sense, in this fraction phenolic acids can be present in their free soluble forms, glycosylated or esterified. This fraction is very small compared to the insoluble or bound fraction. However, the fraction of soluble phenolic acids is more diverse than the insoluble, particularly in the grains containing anthocyanin‐type pigments. Phenolic acids that occur in maize grain are derived from both benzoic acid and cinnamic acid. The chemical structures of the most common are shown in **Figure 3**.

**Figure 3.** Phenolic acids in the maize grain. (A) Benzoic acid derivatives and (B) cinnamic acid derivatives.

Among the phenolic acids present as soluble form in the maize grain are p‐hydroxybenzoic, vanillic, and protocatechuic, all derived from benzoic acid. The first two have been reported in the pericarp of popcorn and in the germ of QPM, as well as in baby corn, with greater abun‐ dance of p‐hydroxybenzoic [18]. In the grain of purple maize the presence of protocatechuic, vanillic acid, and p‐coumaric has been reported in amounts of 14.61 ± 0.08 and 8.46 ± 0.09 mg equivalents of ferulic acid/g sample for the former two acids, whereas the latter was found only in trace amounts [19]. Meanwhile, Sosulski et al. [20] identified in yellow maize flour, in addition to the previously mentioned acids, p‐coumaric, ferulic, and syringic acids in the free phenolic fraction. This fraction was much smaller than the soluble esterified phenolic fraction, in which ferulic and syringic acids predominated. **Table 1** shows the reported concentrations of some phenolic acids in maize grain of different colors.

### *3.1.2. Flavonoids*

They are phenolic compounds that form a very large group of which more than 5000 differ‐ ent members have been identified [21], many of which possess important biological activities such as antioxidant, antimutagenic, and microbicidal. Different types of flavonoids have been identified in colored maize grain with anthocyanin‐type pigments. They can be found free


**Table 1.** Concentrations of some phenolic acids in maize grain with different colors. or in conjugated forms [22]. These compounds are very susceptible to hydrolysis and some losses may occur during their analysis. The presence of these phenolic has been reported in maize grains with anthocyanin‐type pigments and the flavonoids reported are anthocyanins, flavonols, and proanthocyanins (flavan 3‐ols).
