*4.5. Sweet Corn*

Native to America, corn (*Zea mays* L.) has been cultivated in Central America by indigenous populations at least since 3500 BC [303]. When the European explorers arrived in North America, Iroquois Indians living in the region corresponding to the current state of Pennsylvania and New York grew a variety of sweet corn (*Zea mays* L. *saccharata* Sturt.) that turned blue upon ripening, along with other multi-colored varieties [304]. The first samples of maize seeds that arrived in Spain were described as characterized by different colors ranging from white to black [305,306]. Initially grown for curiosity, with the arrival of seeds from North America adapted to higher latitudes, and thus more suitable to the European photoperiod and climate conditions, the new cereal crop started to spread from Spain to the rest of Europe. The rapid diffusion of maize in Europe is also testified by the painter Giuseppe Arcimboldo, in his famous painting "Summer" made in 1573 in which a maize ear is visible [305]. With the process of selection initially conducted in Europe the new crop lost its multi-color pigmentation with the exceptions of a few local populations and assumed the typical white or yellow color of the varieties currently cultivated worldwide.

With over 1.134 million metric tons of corn produced at global level in 2017 [27], corn is by far the first cereal produced in the world, second in terms of acreage only to wheat. Among the different types of maize, sweet corn is particularly important as it is used for human consumption worldwide. In the US, first producer of sweet corn, in 2017 were harvested 464,600 acres (about 188.000 ha) of sweet corn generating a crop value of over \$892 million [307]. About 75% of the sweet corn is produced for the fresh market and the remaining portion is used for canned and frozen food processing, making sweet corn the second most important processing vegetable after processing tomato.

Sweet corn is derived from a natural genetic mutation of field corn (dent corn) that was first reported in Pennsylvania in 1770, although it was probably cultivated before by native Americans [308]. The mutant genotype *sugary* (*su*) accumulates more sugar in the endosperm (10.2%) than the standard starchy maize (3.5%) resulting in sweeter taste. Years later, several sugary sweet corn varieties have been selected. These traditional varieties characterized mainly by white or yellow kernels, are harvested before complete physiological ripening when the level of sugar is maximum and have short shelf-life because after harvest the sugar is rapidly converted to starch [309]. Subsequently, sweeter hybrid varieties have been developed by selecting *sugar-enhanced* (*se*) mutants characterized by double content of sugar (20–35%), tender kernels, creamy texture and good corn flavor. Yet, more recently *supersweet* varieties have been developed selecting *shrunken-2* (*sh2*) mutants characterized by even higher sugar content at the immature milky stage (29.9%) and extended shelf life, with lighter shrunken kernels. Lately high sugar hybrids are developed combining *su*, *se*, and *sh2* genes to obtain optimal combinations of sugar, texture, taste, and long shelf life [309,310]. Most of the sweet corn hybrids grown and consumed today at commercial level are characterized by yellow (60%), white (20%), or bicolor (20%, yellow and white) kernels. Nevertheless, in recent years, the interest in reviving ancient colored sweet corn varieties or developing new pigmented varieties characterized by high content of carotenoids [311] and especially anthocyanins is increasing [305,312–316] due to the potential functional properties of anthocyanin-rich genotypes [317,318], as well as to the increasing demand of natural colorants [22,319,320]. The biosynthesis of anthocyanins in maize aleurone (external

part of the endosperm) and pericarp (external part of the kernel) of the kernels or in plant tissues involve over twenty structural and regulatory genes that have been identified and functionally characterized as reviewed by Petroni et al. [305,321–323]. The wide range of colors observed in maize kernels is mainly due to the biosynthesis and accumulation of carotenoids and flavonoids. Carotenoids such as β-carotene, zeaxanthin and lutein which are lipid-soluble pigments and are responsible for the color of kernels ranging from yellow to deep orange [311,324]. Maize flavonoids include two main classes of pigments: (i) phlobaphenes which are water-insoluble 3-deoxyflavanoid pigments that accumulate in the pericarp of the kernels and the cob and are responsible for the development of kernel colors ranging from orange to brick red [315]; and (ii) anthocyanins which are water-soluble pigments responsible for the development of pink, red, purple, and blue color in the aleurone and pericarp of the kernels as well as in other plant tissues. Within the anthocyanins, the main class of pigments identified are cyanidin, peonidin, and pelargonidin derivatives with the first two providing bluish-red color and the latter being responsible for more orange-red color [22]. The combination of carotenoids and flavonoids in pericarp and aleurone generates an incredible variety of shades [22] and may be influenced also by the condensation with flavanols [319,320,325,326] or other modifications of the anthocyanin compounds. Analyzing the anthocyanin profile of the kernels of six different purple maize genotypes, González-Manzano et al. [326] identified the dimer catechin-(4a-8)-cyanidin-3,5-*O*-diglucoside and other flavanol-anthocyanin condensed pigments along with several related anthocyanin pigments such as cyanidin-3-glucoside, cyanidin-3-malonylglucoside, peonidin-3-glucoside, peonidin-3-malonylglucoside, pelargonidin-3-dimalonylglucoside, and other derivatives. Malonyl- and dimalonyl acylated anthocyanins are particularly interesting for their higher stability as natural colorants [22]. Analyzing the extract of corn cobs and kernels of a Chinese purple corn, Yang and Zhai [327] identified cyanidin-3-glucoside, pelargonidin-3-glucoside and peonidin-3-glucoside including their malonated derivatives. Besides the great potential of maize pigments as natural food colorants [22,319], there is great interest in the development of anthocyanin-rich maize functional food products [305,311,312,321,328]. Several studies have reported the potential health-beneficial properties of anthocyanin-rich maize [305,317,329]. Examining the phenolic and anthocyanin content of eighteen Mexican maize genotypes, Lopez-Martinez et al. found that total anthocyanins ranged from 1.54 to 850.9 mg of cyanidin-glucoside equivalents/100 g of whole grain flour. Purple genotypes rich in anthocyanins exhibited also the greatest antioxidant activity [330]. Andean purple corn had higher antioxidant activity and antiradical kinetics than blueberries and higher or similar level of anthocyanins [331]. Similarly, analyzing 49 lines of waxy corn (*Zea mays* L. var. *ceratina*) characterized by different colors Harakotr et al. [332] found a large variability of phenolic and anthocyanin compounds and a positive correlation coefficient between anthocyanins and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging ability (*r* = 0.94) and between anthocyanins and Trolox equivalent antioxidant capacity (TEAC, *r* = 0.88). Introduction in the diet of anthocyanin-rich corn may contribute to prevent also obesity and diabetes. Tsuda et al. [317] found that feeding high fat (HF) mice for 12 weeks with cyanidin 3-glucoside-rich purple corn significantly reduced HF body weight and white and brown adipose tissues compared to HF mice not receiving purple corn supplement. The HF diet induced also hyperglycemia, hyperinsulinemia, and hyperleptinemia, whereas the same physiological conditions were not observed in HF mice receiving the purple corn supplement. Moreover, an increase of tumor necrosis factor (TNF)-α mRNA level was observed in the HF-diet while it was not observed in the HF group receiving the purple corn supplement which suppressed the mRNA of enzymes involved in fatty acid and triacylglycerol synthesis and reduced the level of sterol regulatory element binding protein-1 mRNA in white adipose tissue. Using near-isogenic maize lines which differed only in the presence or not of anthocyanins, and feeding rats with anthocyanin-rich and anthocyanin-deprived maize for 8 weeks it was observed that in the group fed with anthocyanin-rich corn cardiac tissue damaged following ischemic conditions was reduced by about 30% compared to the group fed with anthocyanin-deprived corn [333]. An increase of myocardial glutathione and omega-3 fatty acids levels in blood indicated that diet supplementation

with anthocyanins regulated the cardiac antioxidant defense and the conversion of α-linolenic acid to omega-3 fatty acids [333,334]. In an in vivo study, using db/db mice fed with 10 mg/kg of purple corn extract for 8 weeks, Kang et al. [335] found that anthocyanin-rich purple corn extracts reduced glomerular angiogenesis of diabetic kidneys by inhibiting the induction of vascular endothelial growth factor (VEGF) and hypoxia inducible factor (HIF)-1a induced by hyperglycemic condition. Kang et al. results demonstrated that purple corn extract inhibited glomerular angiogenesis caused by chronic hyperglycemia and diabetes by disturbing the Angpt-Tie-2 ligand-receptor system linked to the renal VEGF receptor 2 (VEGFR2), suggesting that purple corn extract could be used to target abnormal angiogenesis in diabetic nephropathy leading to kidney failure [335]. Yet, in a recent study Mazewski et al. [336] found that anthocyanin-rich purple and red corn may potentially contribute to inhibiting human colon cancer cell proliferation by promoting apoptosis and suppressing angiogenesis.

The main pigments isolated in vegetables consumed for various plant parts are presented in Table 3.


**Table 3.** The main pigments isolated in vegetables consumed for plant parts other than fruit and leaves.


**Table 3.** *Cont.*
