ITHACA, N.Y. -- Cooking tomatoes -- such as in spaghetti sauce -- makes the fruit heart-healthier and boosts its cancer-fighting ability. All this, despite a loss of vitamin C during the cooking process, say Cornell food scientists. The reason: cooking substantially raises the levels of beneficial compounds called phytochemicals.
Writing in the latest issue of the Journal of Agriculture and Food Chemistry (April 17), Rui Hai Liu, M.D., Cornell assistant professor of food science, notes, "This research demonstrates that heat processing actually enhanced the nutritional value of tomatoes by increasing the lycopene content -- a phytochemical that makes tomatoes red -- that can be absorbed by the body, as well as the total antioxidant activity. The research dispels the popular notion that processed fruits and vegetables have lower nutritional value than fresh produce."
Tomato samples were heated to 88 degrees Celsius (190.4 degrees Fahrenheit) for two minutes, a quarter-hour and a half-hour. Consistent with previous studies, vitamin C content decreased by 10, 15 and 29 percent, respectively, when compared to raw, uncooked tomatoes. However, the research revealed that the beneficial trans-lycopene content of the cooked tomatoes increased by 54, 171 and 164 percent, respectively. Levels of cis -lycopene (which the body easily absorbs) rose by 6, 17 and 35 percent, respectively; and antioxidant levels in the heated tomatoes increased by 28, 34 and 62 percent, respectively. Antioxidants protect the human body from cell and tissue damage, which occurs when harmful molecules called free radicals, released as oxygen, are metabolized by the body.
Lycopene, a carotenoid responsible for the red color in tomatoes and other fruits, has long been known as a powerful antioxidant that decreases cancer and heart-disease risk. Carotenoids, along with phenolic acids and flavonoids, are all phytochemicals, the nutritionally beneficial active compounds found in every fruit and vegetable.
While the antioxidant activity in tomatoes is enhanced during the cooking process, vitamin C loss occurs when the food's ascorbic acid is oxidized to dehydroascorbic acid and other forms of nutritionally inactive components.Lycopene is the most-efficient single oxygen quencher, and devours more than 10 times more oxygenated free radicals than vitamin E. "This makes lycopene's presence in the diet important," says Liu.
"While these findings go against the notion that processed fruits and vegetables have lower nutritional value, this may create a new image for processed fruits and vegetables," says Liu. "Ultimately, this could increase consumers' intake of fruits and vegetables and could possibly reduce a person's risk of chronic disease."
Liu's coauthors on the research paper, "Thermal Processing Enhances the Nutritional Value of Tomatoes by Increasing Total Antioxidant Activity," are Cornell graduate students Veronica Dewanto and Kafui K. Adom, and a visiting fellow in Liu's laboratory, Xianzhong Wu. The research was funded with Hatch funds from the U.S. Department of Agriculture's Cooperative State Research, Education and Extension Service.