Supermarket chain FairPrice has halted sales of vegetables marked as Shanghai Green and Japanese Green from importer Alamanda Singapore following photos posted online of blue stains found on them.
FairPrice said yesterday the stain was caused by "residual remnants of undissolved fungicide" which the Agri-Food and Veterinary Authority (AVA) said on Wednesday could be copper-based fungicide, based on the photos.
The Straits Times understands that FairPrice had received four complaints.
The AVA, which is investigating the incident, said such fungicides are approved for agricultural use, and "can be easily removed through thorough washing before consumption".
Food science expert William Chen, who is director of Nanyang Technological University's food science and technology programme, said: "Our body also needs copper. It is used for energy generation and prevents us from getting fungi infection. In fact, copper can be found naturally in a lot of greens."
Prof Chen said washing should get rid of the residue.
At least three consumers have shared photos of the affected vegetables online.
Copper-based fungicides are used in agriculture to protect crops from fungus growth, which can affect production yield.
FairPrice said all its food products "comply with stringent regulatory requirements and have been approved for sale by the authorities".
"Nonetheless, we acknowledge that better care should have been taken while harvesting the vegetables before they were packed," said a FairPrice spokesman.
The spokesman added that it has been in contact with customers who have reported the matter to the supermarket chain.