Corn

While some processing methods can improve the nutrition of food, some can also detract from it. To see how the nutrients in corn are impacted as corn is processed into cornflakes breakfast cereal, researchers analyzed the nutrient content at 5 points throughout the process (whole kernel, flaked grit, cooked grit, baked grit, and toasted cornflake). The scientists found that a large drop-off in phenolic acid (healthy phytochemicals that have antioxidant properties) occurred after the whole kernel was milled into flaked grits, when the bran and germ were removed. Smaller losses occurred at other points in the processing method as well, but were not as staggering.
Journal of Visualized Experiments. 2018 Jun 16;(136). (Butts-Wilmsmeyer C et al.)

While fruits and vegetables are known as sources of healthy antioxidants and phenolic compounds, research increasingly shows that whole grains contain them too. In this review, researchers analyzed the total phenolic contents, phenolic acid profile and antioxidant activity of several whole grains, including wheat, corn, rice, barley, sorghum, rye, oat and millet. The review shows that whole grains contain a number of phytochemicals (including antioxidants) and significantly exhibit antioxidant activity. Researchers conclude that the consumption of whole grains is considered to have significant health benefits including prevention of chronic diseases such as cardiovascular disease, diabetes, and cancer because of the contribution of phenolic compounds. 
Critical Reviews in Food Science and Nutrition. 2014 July 30. [epub ahead of print] (Van Hung P et al.)