Purple corn is fast approaching classification as a functional food—an integral component of the diet that provides energy and essential nutrients. Researchers in the fields of food and nutrition are intensely searching for functional foods in almost every corner of the world and from a diversity of plants. Examples include purple corn (Zea mays L., Poaceae), green tea (Camellia sinensis [L.] Kuntze, Theaceae), soy isoflavones (Glycine max [L.] Merr., Fabaceae), various nuts, plus various other natural substances in the human diet containing antioxidant and other substances with alleged or proven potential disease-preventive properties.
Purple corn (frequently referred to as blue corn) is botanically the same species as regular table corn. Yet by a twist of nature, this corn produces kernels with one of the deepest shades of purple found anywhere in the plant “kingdom.” Research has shown that purple corn contains cell-protecting antioxidants with the ability to inhibit carcinogen-induced tumors in rats. Many plant-derived substances are believed to show these properties, but few have also demonstrated anti-inflammatory capabilities and the potential to help prevent obesity.
The kernels of purple corn (maiz morado in Spanish) have long been used by the people of the Peruvian Andes to color foods and beverages, a practice just beginning to become popularized in the industrialized world. They also make a fermented/alcoholic drink from the kernels which they call chicha morada.1
The source of this natural alternative to synthetic food dyes is the largest group of natural, water-soluble pigments in the plant world, known as “anthocyanins.”2 (The word anthocyanin is derived from the Greek terms, anthos, meaning flower, and kyanos, meaning blue.3) Anthocyanins are responsible for the purple, violet, and red colors attending many plants. Anthocyanins belong to an even larger class of plant chemicals known as flavonoids and are found in diverse plants, including many food plants.4
Researchers at the Horticultural Sciences Department of Texas A&M University in College Station, Texas, recently determined that the mean anthocyanin content of whole, fresh purple corn from Peru was 16.4 mg/g, which was much higher than fresh blueberries (1.3-3.8 mg/g). On a dry weight basis, the mean content of purple corn was comparable to blueberries (17.7 and 9.2-24.0 mg/g, respectively). The kernel pericarp held by far the greatest concentration of anthocyanins, contributing 45% of the total content. More intriguing, the in vitro antiradical capacity of purple corn extract against the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was greater than that of blueberries (Vaccinium corymbosum L., Ericaceae), which have shown higher antioxidant values than many other commercial food plants.5
Digging deeper, the most abundant anthocyanin found in purple corn, called “C3G” (3-O-? -D-glucoside6,7), also known as cyanidin-3-O-?-glucopyranoside,8 has been keeping researchers very busy lately. In a number of tests designed to assess the potential health benefits of this anthocyanin, one study after another has proven its antioxidant strength. Like other anthocyanins, C3G is found in a wide variety of food plants and is actually the most common anthocyanin found in nature. C3G is the most abundant anthocyanin in some foods, such as the juice of ruby oranges (Citrus sinensis [L.] Osbeck “Blood orange,” Rutaceae)8 and blackberry (Rubus allegheniensis [L.] Bailey, Rosaceae) extract.9 Red wine also contains appreciable amounts,10,11 but other anthocyanins predominate.12
C3G displays significant in vitro antioxidant activity. In one study C3G came out on top when compared to 13 other anthocyanins in the ORAC (oxygen radical absorbance capacity) assay, which tests for antioxidant activity. The strength of C3G was 3.5 times that of Trolox® (a synthetic and potent antioxidant analogue of vitamin E).13 To date, the radical scavenging/antioxidant capacity of C3G has been demonstrated in at least a dozen different assays.8,14-20 In one in vitro study, C3G was tested for the potential to prevent damage caused by ultraviolet (UV) light. Its ability to inhibit the oxidation of fat cells induced by UVB (280-315 nm) light was at least 40 times that of vitamin E; however, vitamin E is a weak inhibitor of UVB-induced lipid oxidation because it rapidly breaks down under UV light.19 Oxidative stress and immune suppression caused by UV light are well-known for their role in the induction of skin cancers.20
Oxidative stress is described as a state in which there is an excess of oxygen-based free radicals. To avoid the damage they can cause to cells, the body produces antioxidants to inactivate these free radicals. If they prove insufficient, however, the body suffers from oxidation of lipids, proteins, and nucleotide bases. In models of oxidative stress using oxidative injury to the liver, male rats fed a diet containing 0.2% C3G (2 g/kg of feed) for 2 weeks beforehand showed significantly less liver injury compared to the control group.21 A similar study in rats fed C3G in liquid form (0.9 mmol/kg) also found significant hepatoprotective effects.22
In a study on the anti-inflammatory potential of C3G, male rats administered the anthocyanin orally in liquid form (0.9 mmol/kg) prior to chemically-induced acute inflammation showed significantly less inflammation and significantly attenuated levels of pro-inflammatory cytokines (interleukin-6, interleukin-? , and tumor necrosis factor-? , and inducible nitric oxide [iNOS] expression) and nitric oxide (a free radical).23 Based on these results, it is possible that this plant pigment may also suppress the inflammatory response in diseases marked with inflammation.
Could the anthocyanin pigment also help prevent some types of cancer? That question was put to the test in rats first treated with a carcinogen (1,2-dimethylhydrazine) and then fed a diet containing a known environmental carcinogen (PhIP or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) that also targets the mammary gland, prostate, and large intestine in rats and causes colorectal cancer. Incidentally, the carcinogen used in the study, known as a heterocyclic amine, is the most abundant of around 20 other types found in cooked meats and fish. Both the early signs of colorectal cancers and the numbers of malignant and benign tumors that formed in the colons of rats that had the purple pigment in their diet (5% of feed for 32 weeks; a nontoxic dose based on previous carcinogenicity studies of PCC) were significantly reduced, and there were no adverse effects. The authors of the study note that extract or juices of plants that contain high amounts of anthocyanins have previously been reported to inhibit mutagenesis induced by heterocyclic amines.24
The oxidation of fats or lipids in blood serum contributes to the condition known as atherosclerosis. When male rats were fed a diet containing a high amount of C3G (0.2% of feed for 2 weeks) in place of sucrose content in the control diet, their blood serum showed a significantly lower level of oxidation along with a significant decrease in the susceptibility of their serum lipids to undergo oxidation, yet their body’s natural antioxidants (serum levels of vitamins C and E, glutathione, and uric acid) remained unaffected. Another intriguing discovery in this study was that the rats with C3G in their feed also showed significant decreases in levels of total cholesterol—about 16% less.25
What would happen if rats were fed C3G as part of a high-fat diet? To find out, researchers in Japan compared the body weights of male mice fed a high-fat (HF) diet with another group fed the same HF diet but with the addition of purple corn color (PCC) which provided C3G (0.2% or 2 g/kg of feed). Results were also compared to 2 control groups: one fed a normal diet and one fed a normal diet with C3G. After 12 weeks, the results were obvious: mice in the PCC-HF group showed significantly less signs of developing obesity, yet exhibited no significant difference in food consumption compared to the control groups with or without the PCC in their feed. When related to the primary control group (no HF diet or PCC), the adipose tissue weights of the PCC-HF group were not significantly different. In addition, fatty tissue in HF-diet group was found to be growing in size but showed no increase in the PCC-HF group. The HF-diet group also developed a state of hyperglycemia along with an over-production of insulin. Interestingly, this was not observed in the PCC-HF group in which both pathologies were completely normalized. In conclusion, the researchers stated that their tests of PCC provide a nutritional and biochemical basis for the use of the pigment or anthocyanins as a “functional food factor”—one that may be beneficial for helping to prevent diabetes and obesity.26 It now remains for future studies to determine the possible contributing effects of other substances from purple corn which are extracted along with PCC.
More recent efforts to determine the potential anti-obesity mechanisms of purple corn pigment have focused on the effect of C3G on fat cell dysfunction, fat cell-specific gene expression, and the regulation of chemical messengers (adipocytokines) secreted by fat cells, such as the fat-derived hormone adiponectin. After feeding male mice a diet containing PCC to provide C3G (2 g/kg of feed for 12 weeks), gene expression levels of adiponectin in white fatty tissue was upregulated 1.7-fold compared to the control group not fed the food colorant.27 Plasma and gene expression levels of adiponectin are decreased in obese humans and mice and in insulin resistant states.27,28 When adiponectin was administered intravenously to mice fed high-fat/sucrose diets, weight gain was significantly inhibited. Adiponectin (i.v.) also lowered plasma glucose levels in lean mice fed a high-fat meal.28
Rich in C3G (approximately 70 mg/g), about 50,000 kg of PCC is used in Japan as a food color for confections and soft drinks annually.26 So far, PCC remains to be officially approved for use as a food colorant by the U.S. Food and Drug Administration. However, approval seems likely because “grape skin color” and “grape skin extract” (“enocianini” or “enocyanin”)2 made from Concord grapes29 (Vitis vinifera L., Vitaceae) are also rich in anthocyanins2 and both are FDA-approved for use in beverages and non-beverage foods.29
Kenneth Jones is a medical writer specializing in the field of medicinal plants. He is the co-author of Botanical Medicines: The Desk References for Major Herbal Supplements by McKenna, Jones, and Hughes (Haworth Herbal Press, 2002). He has no affiliation with any commercial producers of purple corn or any of the other products mentioned in this article.
1. Brack-Egg A. Diccionario Enciclopédico de Plantas Útiles del Perú. Cuzco, Peru: Imprenta del Centro Bartolomé de Las Casas; 1999:537-538.
2. Bridle P, Timberlake CF. Anthocyanins as natural food colours — selected aspects. Food Chem. 1997;58(1-2):103-109.
3. Kong J, Chia L, Goh N, Chia T, Brouillard R. Analysis and biological activities of anthocyanins. Phytochemistry. 2003;64:923-933.
4. Mazza G, Miniati E. Anthocyanins in Fruits, Vegetables, and Grains. Boca Raton, FL: CRC Press; 1993.
5. Cevallos-Casals BA, Cisneros-Zevallos L. Stoichiometric and kinetic studies of phenolic antioxidants from Andean purple corn and red-fleshed sweet potato. J Agric Food Chem. 2003;51(11):3313-3319.
6. de Pascual-Teresa S, Santos-Buelga C, Rivas-Gonzalo J C. LC-MS analysis of anthocyanins from purple corn cob. J Sci Food Agric. 2002;82(9):1003-1006.
7. Nakatani N, Fukuda H, Fuwa H. Major anthocyanin of Bolivian purple corn Zea mays. Agric Biol Chem. 1979;43(2):389-392.
8. Amorini AM, Fazzina G, Lazzarino G, et al. Activity and mechanism of the antioxidant properties of cyanidin-3-O-b-glucopyranoside. Free Radic Res. 2001;35:953-966.
9. Rossi A, Serraino I, Dugo P, et al. Protective effects of anthocyanins from blackberry in a rat model of acute lung inflammation. Free Radic Res. 2003;37(8):891-900.
10. Waterhouse AL. Wine phenolics. Ann N Y Acad Sci. 2002;957:21-36.
11. Burns J, Gardner PT, O’Neil J, et al. Relationship among antioxidant activity, vasodilation capacity, and phenolic content of red wines. J Agric Food Chem. 2000;48(2):220-230.
12. García-Beneytez E, Cabello F, Revilla E. Analysis of grape and wine anthocyanins by HPLC-MS. J Agric Food Chem. 2003;51:5622-5629.
13. Acquaviva R, Russo A, Galvano F, et al. Cyanidin and cyanidin 3-O-? -D-glucoside as DNA cleavage protectors and antioxidants. Cell Biol Toxicol. 2003;19(4):243-252.
14. Wang H, Cao GH, Prior RL. Oxygen radical absorbing capacity of anthocyanins. J Agric Food Chem. 1997;45(2):304-309.
15.Espin JC, Soler-Rivas C, Wichers HJ, Garcia-Viguera C. Anthocyanin-based natural colorants: A new source of antiradical activity for foodstuff. J Agric Food Chem. 2000;48(5):1588-1592.
16. Kähkönen MP, Heinonen M. Antioxidant activity of anthocyanins and their aglycons. J Agric Food Chem. 2003;51(3):628-633.
17. Stintzing FC, Stintzing AS, Carle R, Frei B, Wrolstad RE. Color and antioxidant properties of cyanidin-based anthocyanin pigments. J Agric Food Chem. 2002;50(21):6172-6181.
18. Tsuda T, Watanabe M, Ohshima K, et al. Antioxidative activity of the anthocyanin pigments cyanidin 3-O-? -D-glucoside and cyanidin. J Agric Food Chem. 1994;42(11):2407-2410.
19. Tsuda T, Shiga K, Ohshima K, Kawakishi S, Osawa T. Inhibition of peroxidation and the active oxygen radical scavenging effect of anthocyanin pigments isolated from Phaseolus vulgaris L. Biochem Pharmacol. 1996;52(7):1033-1039.
20. Morazzoni P, Bombardelli E. Vaccinium myrtillus L. Fitoterapia. 1996;67:3-29.
21. Tsuda T, Horio F, Kitoh J, Osawa T. Protective effects of dietary cyanidin 3-O-b-D-glucoside on ischemia-reperfusion injury in rats. Arch Biochem Biophys. 1999;368(2):361-366.
22. Tsuda T, Horio F, Kato Y, Osawa T. Cyanidin 3-O-b-D-glucoside attenuates the hepatic ischemia-reperfusion injury through a decrease in the neutrophil chemoattractant production in rats. J Nutr Sci Vitaminol. 2002;48(2):134-141.
23. Tsuda T, Horio F, Osawa T. Cyanidin 3-O-b-D-glucoside suppresses nitric oxide production during zymosan treatment in rats. J Nutr Sci Vitaminol. 2002;48(4):305-310.
24. Hagiwara A, Miyashita K, Nakanishi T, Sano M, Tamano S, Kadota T, Koda T, Nakamura M, Imaida K, Ito N, Shirai T. Pronounced inhibition by a natural anthocyanin, purple corn color, of 2-amino-1-methyl-6-phenylimidazol[4,5-b]pyridine (PhIP)-associated colorectal carcinogenesis in male F344 rats pretreated with 1,2-dimethylhydrazine. Cancer Lett. 2001;171:17-25.
25. Tsuda T, Horio F, Osawa T. Dietary cyanidin 3-O-? -D-glucoside increases ex vivo oxidation resistance of serum in rats. Lipids. 1998;33(6):583-588.
26. Tsuda T, Horio F, Uchida K, Aoki H, Osawa T. Dietary cyanidin 3-O-? -D-glucoside-rich purple corn color prevents obesity and ameliorates hyperglycemia in mice. J Nutr. 2003;133(7):2125-2130.
27. Tsuda T, Ueno Y, Aoki H, et al. Anthocyanin enhances adipocytokine secretion and adipocyte-specific gene expression in isolated rat adipocytes. Biochem Biophys Res Commun. 2004;316:149-157.
28. Fruebis J, Tsao TS, Javorschi S, Ebbets-Reed D, Erickson MR, Yen FT, Bihain BE, Lodish HF. Proteolytic cleavage product of 30-kDa adipocyte complement-related protein increases fatty acid oxidation in muscle and causes weight loss in mice. Proc Natl Acad Sci USA. 2001;98:2005-2010.
29. FDA, Dept. of Health and Human Services. Code of Federal Regulations. Part 73. Listing of Color Additives Exempt from Certification. Sec. 73.169. Sec. 73.170. April 1, 2003. Available at: http://www.access.gpo.gov/cgi-bin/cfrassemble.cgi?title=200321.