Antioxidant Activity of Grains

Epidemiological studies have shown that consumption of whole grains and grain-based products is associated with reduced risk of chronic diseases. The health benefits of whole grains are attributed in part to their unique phytochemical composition. However, the phytochemical contents in grains have been commonly underestimated in the literature, because bound phytochemicals were not included. This study was designed to investigate the complete phytochemical profiles in free, soluble conjugated, and insoluble bound forms, as well as their antioxidant activities in uncooked whole grains. Corn had the highest total phenolic content (15.55 ± 0.60 μmol of gallic acid equiv/g of grain) of the grains tested, followed by wheat (7.99 ± 0.39 μmol of gallic acid equiv/g of grain), oats (6.53 ± 0.19 μmol of gallic acid equiv/g of grain), and rice (5.56 ± 0.17 μmol of gallic acid equiv/g of grain). The major portion of phenolics in grains existed in the bound form (85% in corn, 75% in oats and wheat, and 62% in rice), although free phenolics were frequently reported in the literature. Ferulic acid was the major phenolic compound in grains tested, with free, soluble-conjugated, and bound ferulic acids present in the ratio 0.1:1:100. Corn had the highest total antioxidant activity (181.42 ± 0.86 μmol of vitamin C equiv/g of grain), followed by wheat (76.70 ± 1.38 μmol of vitamin C equiv/g of grain), oats (74.67 ± 1.49 μmol of vitamin C equiv/g of grain), and rice (55.77 ± 1.62 μmol of vitamin C equiv/g of grain). Bound phytochemicals were the major contributors to the total antioxidant activity: 90% in wheat, 87% in corn, 71% in rice, and 58% in oats. Bound phytochemicals could survive stomach and intestinal digestion to reach the colon. This may partly explain the mechanism of grain consumption in the prevention of colon cancer, other digestive cancers, breast cancer, and prostate cancer, which is supported by epidemiological studies. Keywords: Phytochemicals; phenolics; grains; ferulic acid; antioxidant activity