In a finding that contradicts the conventional wisdom on the processing of vegetables, Cornell University scientists recently showed that the antioxidant activity of cooked tomatoes is higher than in uncooked tomatoes. The findings were published online in the Journal of Agricultural and Food Chemistry on April 17.
“I think this is the first time that it’s been shown that antioxidant activity has increased after processing,” said Prof. Rui Hai Liu Ph.D. ’93, food science and toxicology, who was a co-author of the study.
“It does show that cooking may have a nutritional advantage over eating the tomato raw,” said Prof. Dennis Miller, chair of the food science department.
The scientists compared the antioxidant capacities and lycopene contents of tomato slurries that were uncooked, cooked for two minutes, fifteen minutes or thirty minutes. They found that both total antioxidant activity and lycopene content were significantly increased in cooked tomatoes.
Lycopene is a powerful antioxidant found in apricots, pink grapefruit, watermelon, rosehips and guava but is most abundant in tomatoes.
“Lycopene is heat stable. We chose [to measure it] because cooking might increase it,” Liu said.
Antioxidants, which are also called oxygen quenchers, work by donating their electrons to highly reactive oxygen free radicals, which posses an unpaired electron. These compounds, left unchecked, are a significant source of damage to cells and have been implicated as contributors to cancer and aging. Because antioxidants block these compounds, much recent research has been focused on examining their effect on health.
“Antioxidants have been associated with a reduced risk for cancer and heart disease,” Miller said.
The scientists speculated that the increased lycopene content may be attributable to the destruction of the cell matrix — to which lycopenes are bound — during heating.
“We expected heat processing would release lycopene from the matrix. Heating [thus] increases the pool of bioaccessible lycopenes,” Liu said.
“This would help the body in absorbing [lycopene],” said Veronica Dewanto grad, the first author of the study.
The scientists were still somewhat surprised at the finding.
“If you open a nutrition or food chemistry textbook, it will tell you that processing reduces nutrition,” Liu said.
“I was surprised because many of us have the assumption that the optimal nutrition is in the least processed foods,” Miller added.
Many of the original assumptions about the nutritive value of cooked fruits and vegetables were based on the knowledge that vitamin C, another antioxidant abundant in fresh fruits and vegetables, is unstable and is destroyed by heating. Indeed, vitamin C content decreased in the cooked tomatoes in this study. However, in an earlier experiment, scientists found that vitamin C contributed less than .4% to the total antioxidant activity of apples.
The main antioxidant activity in apples was provided by classes of chemicals called phenolics and flavonoids.
The scientists began this study by measuring the phenolics content of the tomatoes but were disappointed to find that while the phenolics were not destroyed, they also did not increase.
“We ran eight replicates but weren’t able to show phenolics went up [and] we were going to give up but we decided to look at something else,” Dewanto said. “Then we analyzed the lycopene and found it went up.”
While this study shows that the lycopene content of cooked tomatoes increases, it does not show that the body will necessarily absorb more.
“The next step will be to see how the body takes up these chemicals and how available they are to it,” Dewanto said.
Liu believes that this finding is good news for more than just food processors.
“From the economic point of view, it’s very good news for the tomato grower.”
According to Liu, this finding may increase the demand for processed tomatoes and make selling them for processing more attractive to growers.
“In the past the image of the processed vegetable was not good,” he said, which drove down the prices growers could obtain.
Liu was also optimistic about the importance of these findings to people’s dietary habits.
“It’s very good news … for consumers, because it will help them increase their servings of fruits and vegetables. Tomato, next to potato, is the most consumed vegetable in the U.S. and the majority of them are processed,” Liu said.
Liu did not want to de-emphasize the value of fresh produce, however.
“The take home message is you should get your fruits and vegetables however you can,” he added.
Archived article by Jennifer Frazer