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 micromol of gallic acid
equiv/g of grain) of the grains tested, followed by wheat (7.99 +/- 0.39 micromol
of gallic acid equiv/g of grain), oats (6.53 +/- 0.19 micromol of gallic acid equiv/g
of grain), and rice (5.56 +/- 0.17 micromol 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 micromol of vitamin
C equiv/g of grain), followed by wheat (76.70 +/- 1.38 micromol of vitamin C equiv/g
of grain), oats (74.67 +/- 1.49 micromol of vitamin C equiv/g of grain), and rice
(55.77 +/- 1.62 micromol 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.