A sharpened knife has been revealed as the culprit in the mystery of an orange that turned purple in Brisbane earlier this month.
Resident Neti Moffitt asked for an investigation after slices of an orange she cut up for her two-year-old son turned purple hours later.
Pieces he'd sucked on, and uneaten pieces Ms Moffitt had left on the kitchen bench, all turned part-purple.
"It looks like someone's dipped it on an ink pad, which I guarantee you we haven't," Ms Moffitt told the ABC at the time.
A Queensland Health officer took the discoloured orange slices, a knife used to cut it, a sharpener that had been used on it recently and other items for forensic testing.
Scientists have now revealed the discolouration was due to a natural reaction between the fruit and the sharpened knife, the BBC reports.
Queensland Health's chief chemist, Stewart Carswell, said numerous tests were conducted to determine the cause of the colour change.
The results revealed that anthocyanins - a naturally occurring antioxidant in oranges - had reacted with iron particles from the newly sharpened blade, he said.
"We see samples that range from blood, urine, water, soil, fish and foodstuffs. So to have an orange come through was really different for our team."
The Queensland government said it had assured the family the orange was not a health risk.
Following the incident, Ms Moffitt found one other case on the internet of an orange turning purple, also in Queensland in 2015, the ABC reported.
A Nine News report at the time had said forensic testing had ruled out artificial colouring, and no iodine was found.