Storing produce in a dark fridge could ultimately affect its nutritional qualities according to a study out of Rice University and the University of California, Davis.
Day-night cycles affect live plants, but now research from Rice University in Houston and the University of California, Davis, suggests that they also affect harvested produce, meaning the way we store it could make a difference.
"Vegetables and fruits don't die the moment they are harvested," says Rice biologist Janet Braam, lead researcher on the new study recently published in the journal 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."
Braam's team simulated day-night cycles of light and dark to control the internal clocks of fruits and vegetables, including cabbage, carrots, squash and blueberries. The research is a follow-up to her team's award-winning 2012 study of the ways that plants use their internal circadian rhythms to defend themselves from hungry insects. The first study found that Arabidopsis thaliana, a flowering plant widely used as a model in plant research, increases production of insect-fighting chemicals a few hours before sunrise, the time that hungry insects begin to feed.
Braam says the idea for the current research came from a conversation with her teenage son.
"I was telling him about the earlier work on Arabidopsis and insect resistance, and he said, 'Well, I know what time of day I'll eat my vegetables!'” Braam says. "That was my 'aha!' moment. He was thinking to avoid eating the vegetables when they would be accumulating the anti-insect chemicals, but I knew that some of those chemicals were known to be valuable metabolites for human health, so I decided to try and find out whether vegetables cycle those compounds based on circadian rhythms."
Arabidopsis and cabbage are both members of the Brassicaceae family, so Braam's team began their research by attempting to entrain the clocks of cabbage in the same way they had Arabidopsis. Entrainment is akin to the process that international travelers go through as they recover from jet lag; after flying to the other side of the globe, travelers often have trouble sleeping until their internal circadian clock resets itself to the day-night cycle in their new locale.
Using controlled lighting in a sealed chamber, Rice graduate student and study lead author Danielle Goodspeed found she could entrain the circadian clocks of postharvest cabbage just as she had those of Arabidopsis in the 2012 study. Following the success 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 says. She and Braam both say the findings suggest that storing fruits and vegetables in dark trucks, boxes and refrigerators may reduce their ability to keep daily rhythms.
"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."
In the cabbage experiments, the researchers found they could manipulate cabbage leaves to increase the production of anti-insect metabolites at certain times of day. One of these, an antioxidant called glucoraphanin, or 4-MSO, is a known anti-cancer compound that has been previously studied in broccoli and other vegetables.
Braam's team has already begun follow-up research, which is supported by the Bill and Melinda Gates Foundation, into whether light and other stimuli, like touch, can be used to enhance pest resistance of 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 says.
The research was supported by the National Science Foundation and by a 2011 Medical Innovation Award from the Rice University Institute of Bioscience and Bioengineering.