UNHEALTHILY pale lettuce can make even the best salads look wan and unsavoury. But a couple of days under weak ultraviolet light before you pick it can make the lettuce more palatable.
Tests at a US Department of Agriculture (USDA) lab in Maryland show that an LED radiating just a few milliwatts in the UV-B band stimulates red-leaf lettuce to make the antioxidants that give it its characteristic colouring.
Growing crops in greenhouses, done to supply fresh greens all year round, blocks the UV in sunlight that normally prompts lettuce and other crops to make antioxidants outdoors.
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Fluorescent lamps could be used to provide the missing UV, but that would require vast arrays of lamps containing mercury, which could contaminate food, says Steve Britz of USDA.
Curious to see if cheap, mercury-free UV LEDs could do the job instead, he suspended one a few centimetres above a red-leaf lettuce plant. After 48 hours, the leaves were a deep red, but others away from the UV LED remained green, Britz will tell a laser conference in Baltimore in June.
His team is now trying to work out which wavelength in the UV-B band does the job best.
