For many, nothing is better on a hot day than biting into an ice-cold slice of watermelon.

But scientists now say the juicy summer fruit is most nutritious when stored and served at room temperature.

Reporting in the Aug. 9 issue of the Journal of Agriculture and Food Chemistry, the researchers based their premise on a tally that compared the levels of key antioxidants in whole watermelons that were either refrigerated or stored at room temperature for two weeks.

"What we found was very surprising," said study author Penelope Perkins-Veazie, a plant physiologist at the South Central Agricultural Research Laboratory with the USDA Agricultural Research Service in Lane, Okla.

"The amount of lycopene in watermelons went up about an average of 20 percent when we left them out uncut at room temperature, while beta carotene actually doubled," she said.

Perkins-Veazie noted that, like tomatoes, the red flesh of watermelons owes its coloring to an abundance of lycopene, an organic pigment from the carotenoid family that ranges in shade from pale yellow to deep red.

Beta carotene -- another carotenoid -- is also a nutritional feature of watermelons, although at far lower levels.

Antioxidants gobble up cancer-causing free radical molecules that can damage cells.

While it is known that light, temperature and moisture changes which occur during harvesting and packaging can alter a watermelon's lycopene content by 10 percent to 20 percent, the researchers realized that little was known about the impact storage can have once the heavy fruit is in the kitchen.

To fill in the blanks, Perkins-Veazie and her USDA colleague Julie K. Collins focused on three popular seeded and seedless varieties of watermelon.

All were described by their Oklahoma harvesters as "fully ripe" when acquired. Whole, uncut samples of each of the melons were kept in a cooler for one night at 68 degrees Fahrenheit (F) before being cut up and sampled for color, condition and carotenoid content.

Twenty samples of each melon variety were then weighed and stored in coolers set at either 41, 55, or 70 degrees F.

After two weeks, the researchers found that lycopene levels were dependent on storage temperature.

Compared to measurements taken at picking, carotenoid levels in melons stored at room temperature were up between 11 percent and 40 percent, depending on the variety.

As visual proof of this biochemical development, the authors observed that, after the 14 days of storage, the flesh of all three varieties of watermelons kept at room temperature was darker than they had been when they were freshly picked -- a sign of increased pigmentation from the lycopene boost.

These room-temperature melons also had thinner rinds, a sign of continued ripening.

The flesh of melons stored at either of the below-room temperature levels, by contrast, had not experienced any gains in carotenoids.

Such melons either lost color or maintained the same color as when picked, with no change in rind thickness.

The researchers posited that a drop in carotenoid enzyme activity at the colder temperatures might have halted a ripening process that continues the build-up of beneficial antioxidants.

"But we don't want people to think they can take cut watermelon and just leave it out in room temperature, because that's a safety issue," cautioned Perkins-Veazie.

"If it's cut, you want to leave it in the fridge," she advised. "If it's uncut, it's perfectly alright to leave it on the counter for a day or two, and if you like cold watermelon -- which a lot of people do -- it's perfectly alright to put it in the fridge to let it cool down a little before eating it."

Lona Sandon, an assistant professor of clinical nutrition at the University of Texas Southwestern Medical Center at Dallas, agreed that food safety takes precedence over antioxidant concerns.

"But you can leave certain fruits sitting out," she added. "If you're not going to eat it right away, they don't have to be taking up space in your fridge. In fact, there are several fruits that ripen better when left out -- peaches, bananas -- that not only end up having better nutrient quality but also perhaps better taste."

Connie Diekman, director of nutrition at Washington University in St. Louis, did not think the study considered all the nutritional factors related to the temperature issue.

"I could not find where they looked at some of the water-soluble nutrients in the melon which we know are very sensitive to light and air," Diekman said. "So, I would be curious to know if there were changes to any of those on a negative side that might have offset this positive."