I always heard that light can destroy nutrients in food. My husband disagrees, saying he has heard just the opposite is true. What is correct?
Actually, you are both correct, at least in some cases.
It's long been known that light can destroy some vitamins in foods. The same can be said for exposure to air and heat. Vitamin C is so vulnerable to these types of exposure that it's often used as a measure of total nutrient degradation during storage, transportation and processing.
Other vitamins also are susceptible. For decades, studies have shown that light exposure decreases both riboflavin and vitamin A in milk.
Similarly, a Canadian study examining the effectiveness of fortifying vegetable oil with vitamin A showed that oil stored in clear bottles retained only 17 to 33 percent of the vitamin after being exposed to light, while oil stored in brown bottles retained 72 to 88 percent of the vitamin A. When completely protected from light, the oil retained up to 91 percent of its vitamin A after six months of storage.
So, when a study examining vitamin content in spinach was published in the Journal of Agricultural and Food Chemistry in 2010 showing that vitamin content actually increased in spinach leaves when they were exposed to light, lots of people (perhaps even your husband) took notice.
For the study, researchers compared vitamin content in spinach leaves stored for nine days in plastic disposable tubs with snap-on lids, like the kind you find at the grocery store. They were refrigerated at 4 degrees C (about 39 degrees F). Some of the spinach was exposed to light similar to the 24-hour artificial fluorescent light at stores, while some was enclosed in a two-layer thick grocery store brown paper bag -- simulating conditions you might find in the back of the produce case.
The researchers found that the spinach leaves exposed to light had significantly higher levels of carotenoids (which transform into vitamin A in the body) as well as vitamins C, E, K and B9 (or folate). The increases were likely due to the fact that these vitamins are found in the chloroplasts of the leaves, where photosynthesis occurs. Although the spinach leaves have already been harvested, reactions in the chloroplasts, when exposed to the right kind of light, continue to occur.
After three days, flat spinach leaves showed signs of wilting, but that wasn't apparent in crinkle-leaf spinach. Though the wilting might be a turn-off for consumers, the vitamin content continued to increase.
It's not clear if this kind of reaction occurs in other types of produce. But for now, there's no need to worry about the effect of light on spinach.
Chow Line is a service of Ohio State University Extension and the Ohio Agricultural Research and Development Center. Send questions to Chow Line, c/o Martha Filipic, 2021 Coffey Road, Columbus, OH, 43210-1044, or firstname.lastname@example.org.
Editor: This column was reviewed by Amber Riggin, a dietetic intern with Ohio State University Extension's community nutrition programs.