NUTRITION
June 9, 2009

UV Boost for Antioxidants

Darker leafy greens like kale and spinach are more nutritious than lettuce. But the antioxidant content of lettuce can be given a boost by...

Dark green leafy vegetables like spinach and kale are more nutritious than light green ones such as lettuce. But Americans like lettuce. When was the last time you heard of a bacon, kale and tomato sandwich?

All lettuce is not created equal. Red leaf lettuce has red tinges because it contains polyphenols. Polyphenols are strong antioxidants...

If Americans are going to eat lettuce, they might as well eat healthier lettuce. While not much can currently be done to increase the vitamin content of lettuce, there appears to be a way to increase its antioxidant content. Hit it with some extra ultraviolet light.

All lettuce is not created equal. Red leaf lettuce has red tinges because it contains polyphenols. Polyphenols are strong antioxidants; these particular ones happen to be red, like the ones in apple skin. Several studies have suggested that increased consumption of antioxidants may be beneficial for humans. From the standpoint of the lettuce, the antioxidants help absorb the ultraviolet light in sunlight which otherwise might damage the plant's DNA. It's the plant's way of trying to make its own sunscreen. Researchers from the U.S. Department of Agriculture wondered what would happen if red leaf lettuce was exposed to extra ultraviolet light.

What happened was that the lettuce produced more polyphenols and became redder. It's self−defense on the part of the lettuce.

The researchers used ultraviolet emitting LEDs to increase the lettuces' UV exposure. These diodes put out short wavelength ultraviolet light (UVB). The dosage used was about what a beachgoer would get on a sunny day. After 43 hours of exposure, the lettuces were noticeably redder than those exposed only to white light, just like the beachgoers would be.

These experiments were only preliminary. It's not yet known what the best length of exposure is − whether giving lettuce a mild tan or really crisping it works best. So far, the more UVB, the redder the lettuce. UVB of wavelength 282 or 296 nanometers is most effective.

If boosting the UVB exposure of lettuce above normal turns out to have undesirable effects, there will be other uses for UVB. Crops grown in winter receive lower than normal UVB, as do crops grown in greenhouses (glass is a potent absorber of ultraviolet light). Using UVB emitting diodes on these crops would merely be returning the level to what the crops would normally receive outdoors in a sunny clime.

Previous experiments have shown that picked apples exposed to UV maintain their color longer. This suggests future studies to see if UV can increase the storage life of picked fruits and vegetables or help maintain their nutrient quality for longer than is currently possible.

The results of the study were presented June 2 at the 2009 Conference on Lasers and Electro Optics/ International Quantum Electronics Conference (CLEO/IQEC) in Baltimore.

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