*4.1.1. Differences by grain hardness*

*3.2.1. Ferulic acid and diferulates*

222 Phenolic Compounds - Natural Sources, Importance and Applications

*3.2.2. p‐coumaric acid*

of p‐coumaric acid [29].

**4.1. White maize grain**

Most of the phenolic acids in maize grain are found in the bound form and in this fraction, the ferulic acid (4‐hydroxy‐3‐methoxycinnamate) predominates. It is found esterified to arabi‐ noxylans from the hemicelluloses in the cell wall of the various structures of the maize kernel. Most of this acid in its free form is present in the germ, while its bound form is concentrated in the grain pericarp [18]. As noted above, the ferulic acid is mainly esterified to cell wall com‐ ponents. In the free form, it can be also found conjugated to different molecules, among which

After ferulic acid, the second most abundant phenolic compound in maize grain is p‐cou‐ maric acid. It is present in both forms, the soluble phenolic fraction and the insoluble, with greater presence in the latter. In the insoluble fraction it is mainly linked to lignin, and to a lesser extent to polysaccharides embedded in the cell wall [28]. In the soluble fraction of purple maize the value reported for this acid is 34.1 mg/100 g DW, and for the phenolic insol‐ uble fraction is 573.4 mg/100 g DW. Surprisingly, ferulic acid values reported in the insoluble fraction of this maize were significantly lower (154.2 mg/100 g DW) compared to the amount

The content and type of phenolic compounds in the maize grain will vary depending on the color of the grain, the genetic origin, and the extraction method used. In the latter factor is determining if technologies such as ultrasound and microwave to favor extraction are used, in addition to the type of solvent used to extract them. **Table 2** contains information regard‐ ing the amount of free or bound soluble and insoluble phenolics found in maize grain. The data show very large differences in the reported values by different authors for the same maize grain color. This is undoubtedly due to the genetic variability of the biological material itself. Furthermore, in some works as that of Montilla et al. [29], the extraction of the insoluble phenolic was performed sequentially with three different hydrolysis methods and finally the phenolic compounds recovered from each were added. This method differs from what has been done, for example, by López‐Martínez et al. [30], who performed alkaline hydrolysis and recovered the phenolics by liquid‐liquid extraction with ethyl acetate, which is the most common method applied. Under this method, the authors reported very different values in

In white maize grain, differences in total phenolic content (solubles + insolubles) due to grain hardness are present. These differences are supported by the role of ferulic acid to provide strength to the cell wall of different grain structures. But also, each botanical grain structure has a content and a particularly phenolic profile, which is associated with its functionality.

the most common are simple sugars and some amines [15, 16].

**4. Phenolic compounds in maize by grain color**

four samples of purple grain of different genetic background.

Few studies have studied the relationship between the hardness of the maize grain and its phe‐ nolic content. The hardness of the grain is a widely studied aspect, because of the importance that this feature has for the different processes to which maize grains are subject to obtain the myriad of products derived from it [32]. Among the literature on the matter, Cabrera‐Soto et al. [33] analyzed the soluble and insoluble phenolic fractions in the grain of seven maize hybrids of different hardness. The authors observed a significant positive correlation between the values of soluble esterified phenolics in the three grain structures (pericarp, germ, and endosperm) and the grain hardness. In the study by Chiremba et al. [34] in maize and sorghum, a higher content of phenolic acids in grains of greater hardness was observed, however, the correlation between the content of these phenolics and hardness was higher in maize grains than in sorghum grains.

### *4.1.2. Differences by grain botanical structure*

Phenolic compounds are distributed in all maize grain structures, however, their concentra‐ tion and type varies among them. Several studies using fluorescent techniques have reported that the pericarp is the structure with the highest concentration of phenolics, followed by the germ and the endosperm [15].

**Pericarp**. The pericarp of maize grain is a rich source of phenolics, mainly in their bound form. However, a minor amount of phenolics can also be present in their free form. The main phe‐ nolic in the bound form is ferulic acid, followed by p‐coumaric and sinapic acids. Several free phenolic acids reported in this structure are ferulic (in its conjugated form), vanillic, caffeic, p‐coumaric, and p‐hydroxybenzoic. However, the abundance of each one varied according to the maize type [18].

Most of the ferulic acid in the pericarp of the maize grain is linked by ester bonds to cell wall polysaccharides [35]. Nonetheless, they are also present in the form of dehy‐ drodimers originating from the oxidative coupling of ferulate esters by means of the enzyme peroxidase. The diferulates that have been identified in maize grain are 8,5′‐dife‐ rulic acid, 8,O,4′‐diferulic acid, 8‐8‐diferulic acid, 4‐O‐5‐diferulic acid, and 5.5′‐diferulic acid [35], of which the most abundant is the first one [26]. The diferulates are linked to the arabinoxylans of the polymers that form the cell wall [36]. The resistance of the pericarp cells is attributed to the presence of these compounds and their abundance has been associated to resistance toward the development of fumonisins [37, 38] and tolerance to warehouse pests [15, 39]. The presence of ferulic acid dehydrotrimers was reported by Rouau et al. [40] in maize bran, which is a fraction that is composed of rem‐ nants of pericarp and aleurone layer. Until now, seven different ferulic acid dehydro‐ trimers have been identified [35]. The different ways in which ferulic acid is integrated into the cell wall components contributes to the formation of networks that support the resistance of this structure whose main function is to isolate and protect the grain from external agents.

The type of maize affects the values reported for total soluble phenolics in the grain peri‐ carp. In dent maize grain, Cabrera‐Soto et al. [33] reported a variation of 232.4–334.0 mg EAG/g DW in seven different maize varieties. In cornpop, Das and Singh [18] observed a value of 13.1 ± 0.66 µmol of FAE/g of DW, while the value for QPM maize was of 15.9 ± 0.28. Insoluble phenolic content (IPs) in this structure is between 18 and 21 times that of soluble phenolics (SPs). Das and Singh [13] reported values of SPs of 11.9 and 10.4 µmol of FAE/g of DW in the pericarp of dent and crystalline maize types, respectively: the values of IPs were 218.6 and 219.4 µmol of FAE/g of DW, for the same maize grains. In the SPs fraction, the phenolic acids presented were vanillic, caffeic, ferulic, and p‐coumaric, with predominance of the latter; in IPs, ferulic acid represented between 40 and 50% of this fraction.

In maize grain with presence of anthocyanin pigments in the pericarp, the SPs content is commonly higher than the values observed in white maize grain. This is due to the presence of anthocyanins that occurred mainly in soluble form, because the amount of bounded antho‐ cyanins reported is very low [29].

**Germ**. The germ concentrated the highest content of soluble phenolics of the maize grain structures. In seven varieties of white maize grain, Cabrera‐Soto et al. [33] reported in the germ values of 499.1–689.2 µg GAE/g DW; in the pericarp, the values were of 232.4–334.1 µg GAE/g DW, while in the endosperm, they reported values of 124.1–194.0 µg GAE/g DW. Meanwhile, Das and Singh [13] reported 14.2 ± 0.3 for germ, 11.9 ± 1.1 for pericarp, and 0.4 ± 0.02 µmol of FAE/g of DW, in one sample of dent maize grain. In the germ, the phenolic acids: 3‐hydroxy‐ benzoic acid, caffeic, p‐coumaric, ferulic in itsr cis and trans forms, and salicylic have been reported. Ferulic acid predominates in its esterified soluble form [41].

**Endosperm**. The concentration of phenolic compounds in this structure is very low. Cabrera‐ Soto et al. [33] reported values of 124.1–194.0 µg GAE/g DW. The amount of IPs is also mar‐ ginal since it is less than 2% of that present in the pericarp and 3.56% of that contained in the germ. However, the endosperm represents 80–85% of the total weight of the grain, so its total contribution is close to that of the germ.
