An orange had scientists in a bit of a quandary earlier this month when, for some bizarre reason, it turned purple. Now, after some digging, we have an answer to this mystery and it all comes down to a family of pigments called anthocyanins – confirming the theory some Twitter users and IFLScience commenters had all along.

Queensland Health investigated the strange occurrence when it was flagged up by Neti Moffitt, a mother of two, Brisbane resident, and purveyor of the fruit in question. Staff took samples of the orange as well as the knife and knife sharpener to study them in the Forensic and Scientific Services (FSS) laboratory at Coopers Plains, southern Brisbane.

The much-anticipated results of that analysis came out Tuesday, ABC News reports, confirming the cause of discoloration was a reaction between the orange's naturally occurring anthocyanins and traces of metals (iron and/or others) from the freshly sharpened knife.

“We put the orange through a variety of chemical and instrumental tests including simple chemical spot tests, liquid chromatography high resolution mass spectrometer, inductively coupled plasma mass spectrometry, and UV-Visible spectrophotometry looking for metals, natural and artificial colors and pigments, pesticides, and other contaminants,” Queensland chief chemist Stewart Carswell told ABC News.

As explained in the Certificate of Analysis sent to Moffitt, the chemists started by extracting the purple pigment from samples using a water and ethanol solution. They found that raising the extract's pH level changed its color from pink to blue to a greenish-yellow, which is characteristic of anthocyanins. Also characteristic of both anthocyanins and the extract is a maximum absorbance of roughly 573 nanometers.

Anthocyanins are “not known to represent any risk to human health”, the department emphasizes. In fact, they are responsible for the red and purple coloring of autumnal foliage, the dark purple hue in some flowers, and the red, blue, and purple shades of several popular foods, including eggplants, red cabbage, and sweet potato. They are also naturally present in oranges, though they occur more in blood oranges than any other.

Periods of cold storage may increase anthocyanin levels and exposure to iron can cause the pigment to discolor and turn purple, as seems to be the case here. 

The discolored areas in Moffitt's sample were analyzed and found to have above normal concentrations of iron and other metallic elements. This theory was confirmed when the chemists treated a piece of unaffected orange with iron – “It produced an intense blue color similar to that in the complaint samples,” the analysis explains.

Indeed, as Moffitt revealed on ABC News, her husband had sharpened the knife just “a night or two” before the incident. 

While it is a perfectly safe and natural reaction, purple oranges are still extremely rare. (Hence the headlines.) 

"Amazement, I think is the main thing I'm feeling at the moment, that this has happened, and it's still so rare," Moffitt added. "It was a matter of having all of our ducks lined up in a row for this to occur."

[H/T: ABC News]