Keeping salad in the dark may make it healthier

You’d be amazed what plants get up to at night. US researchers have found that by manipulating the circadian clocks of fruits and vegetables, they were able to improve the amount of antioxidants they contain.

“Vegetables and fruits don’t die the moment they are harvested,” said Rice University biologist Janet Braam, lead researcher of the study published in Current Biology.

“They respond to their environment for days, and we found we could use light to coax them to make more cancer-fighting antioxidants at certain times of day.”

In a collaboration with the University of California at Davis, Braam’s team used light to simulate day-night cycles to control the internal clocks of fruit and vegetables including cabbage, carrots, squash and blueberries.

The research follows on from Braam’s award-winning study into how plants use their circadian clocks to defend themselves against insects. They found that Arabidopsis thaliana begins to increase production of insect-fighting chemicals several hours before sunrise, the time that insects begin to feed.

These chemicals are known to be beneficial to human health, Braam said, so there could be health benefits to eating vegetables when they’re producing these chemicals.

The team began by ‘entraining’ cabbages’ internal clocks. Entrainment is similar to the process of overcoming jetlag, where the circadian clock resets itself over several days to the day-night cycle of a new location.

Using controlled light, study lead author Danielle Goodspeed found she could entrain the circadian clocks of postharvest cabbage in the same way she did in the 2012 Arabidopsis study. Having succeeded with cabbage, Goodspeed and co-authors John Liu and Zhengji Sheng studied spinach, lettuce, zucchini, carrots, sweet potatoes and blueberries.

“We were able to entrain each of them, even the root vegetables,” Goodspeed said. Storing fruits and vegetables in dark trucks, boxes and refrigerators may reduce produce’s ability to keep daily rhythms, the researchers suggested.

“We cannot yet say whether all-dark or all-light conditions shorten the shelf life of fruits and vegetables,” Braam said. “What we have shown is that keeping the internal clock ticking is advantageous with respect to insect resistance and could also yield health benefits.”

The researchers found they could manipulate cabbage leaves to increase their production of anti-insect metabolites at certain times of day. One of these, an antioxidant called glucoraphanin, or 4-MSO, is a known anticancer compound that has been previously studied in broccoli and other vegetables.

Braam’s team has received support from the Bill and Melinda Gates Foundation to further research whether light and other stimuli such as touch could be used to enhance pest resistance in food crops in developing countries.

“It’s exciting to think that we may be able to boost the health benefits of our produce simply by changing the way we store it,” Goodspeed said.