Purple corn anthocyanins: chemical structure, chemoprotective activity and structure/function relationships

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Interest in purple corn (Zea mays L.) as a natural colorant has increased because of their potential health benefits. This study evaluated an anthocyanin-based purple corn extract as a natural colorant and chemoprotective source as compared to other anthocyanin sources. The structure/function relationship between anthocyanins and relative biological activity were investigated.

Purple corncob contained high monomeric anthocyanins concentration (290 to 1323 mg/100g DW) and acylated anthocyanins (35 to 54%). Obtaining a colorant from purple corn produces large amounts of a highly colored purple corn waste (PCW) with limited solubility. The limited solubility was associated with the complexation of anthocyanins with macromolecules (tannins and proteins) abundant in PCW. The purple corn pigment extraction procedure was modified to minimize waste production. Deionized water at 50 °C yielded high anthocyanin concentration with relatively low tannin and protein content. Application of a neutral protease during processing might decrease the level of the major protein (29KD) in purple corn and further reduce PCW.

PCW was soluble in neutral environment and tested as a natural colorant for milk. PCW provided an attractive purple color (hue: 324-347°) to milk. This color was more stable in milk than in a pH6.8 buffer, suggesting that milk components protected anthocyanins and color in an accelerated model (70 ºC).

Purple corn colorant showed higher inhibition of human colon carcinoma HT29 cell proliferation (GI50=~14μg/ml) than other six (chokeberry, bilberry, grape, purple carrot, radish, and elderberry) anthocyanin-rich extracts (ARE) (GI50 =31~130μg/ml). Anthocyanin-rich (2.29g/100g) PCW showed high antiproliferation (GI50 =21μg/ml), but lower than the purple corn colorant, suggesting that macromolecular complexes might trap monomeric anthocyanins reducing their bioavailability. An anthocyanin fraction separated from other phenolics in ARE played a major role on ARE’s chemoprotection. The interaction between anthocyanin and other ARE phenolics on chemoprotection was additive.

Anthocyanin chemical structure affected chemoprotection: cyanidin 3-glucoside had higher inhibitory effect than pelargonidin 3-glucoside. Anthocyanin monoglucosylation showed a higher inhibitory activity than the corresponding 3,5-triglucoside. Effect of acylation on chemoprotection was dependant on the type of aglycone and acylating acid. More research is needed to better understand the impact of anthocyanin structure on chemoprotection.

Pu Jing
Ohio State University