SINGAPORE - Sweet potatoes that turn green are safe to eat, the Agri-Food & Veterinary Authority of Singapore (AVA) clarified on Monday in response to a widely circulated message that said the root vegetables imported from Vietnam are toxic.

The message, which spread on Facebook since last week, said that Japanese sweet potatoes grown in Vietnam and imported here, turned green after they were kept in the refrigerator overnight and placed in the microwave oven.

The post quoted an unnamed doctor as saying that the sweet potatoes were grown in soil contaminated by Agent Orange.

During the Vietnam War, the Americans used Agent Orange to defoliate the forests. The toxic herbicide caused many Vietnamese babies to be born with severe deformities.

AVA said in a statement on Monday: "We would like to assure the public that 'Agent Orange' is not known to cause sweet potatoes to turn green."

AVA said that sweet potatoes contain flavonoids (a type of anti-oxidant), and water-soluble pigments which may cause colour changes.

"The greenish colour can be due to a natural occurrence, when the water soluble pigments in the cooked sweet potatoes are exposed to air," AVA said.

"If the cooked sweet potatoes are handled and stored properly, they should not pose a food safety concern."

AVA also said that food imports, including sweet potatoes, are regularly tested for chemicals and other forms of contamination "such as heavy metals, pesticides and drug residues".

"Any food products that do not meet our requirements are not permitted for sale in Singapore," the food safety agency said.

Here is AVA's full Facebook post below.