Why did this orange turn purple?!?
What you see: An orange slice that looked normal and, many hours later, turned purple What it is: A pigment in the orange reacted with iron on the knife used to chop it Eat or toss: Chances are you discovered the orange had turned purple after you ate some of it. Don't worry—you didn't ingest anything that could harm you.
The story: Today we’re traveling to Australia, to the kitchen of Neti Moffitt, on a September day in 2018 when she served her two-year-old son some freshly cut orange slices.
He ate them. Neti discarded the chewed up rinds and saved the remaining slices.
But the next morning, she was alarmed to discover that parts of the slices turned a deep shade of indigo. Frightened for her son’s well-being, she contacted her local health department, who later determined that the culprit was some unusual—but perfectly safe—chemistry.
Oranges contain pigments called carotenoids, which make them, well, orange. But some varieties also contain anthocyanins, which can create red, purple, blue and even green shades, depending on their surroundings. Anthocyanins are the pigments coloring many berries, as well as red onions and red cabbage. If you’ve ever seen a Moro blood orange with a sinisterly beautiful reddish purple color, you’ve seen the work of anthocyanins in citrus.
But, Neti’s orange, seen above in a photo courtesy of Queensland Health, looks like it started out as a straightforward orange color. The type of orange was never identified, but I’m theorizing that it may have some overlap with a Tarocco blood orange, which contains anthocyanins, but doesn’t have the intense look of a Moro blood orange.
During cold storage, the anthocyanin content in a Tarocco orange can increase significantly. The orange may look simply orange, with perhaps a rosy blush, but the anthocyanins are very much present.
Anthocyanins are master color changers—in alkaline environments they turn shades of blue and green (for a fun demonstration of that color change try this kitchen science experiment) they also can react with metals to form unexpected colors. That is what happened in Neti’s kitchen.
A freshly sharpened knife
Her husband had recently sharpened the knife she used to cut the orange, which led analysts at Queensland Health to believe that metal particles still on the knife reacted with the anthocyanins and created that inky stain. Indeed, when they treated some of the still-orange areas with iron, they were able to recreate the purple color.
And if you’re wondering about any risk from ingesting the tiny bits of metal from the freshly sharpened knife, scientist Kimberley R. Miner, whose specialties include toxicology, says not to worry. While too much iron can be harmful, you’re unlikely to take in a problematic amount from standard sharpening of a normal metal knife. Miner said she would be more concerned about ingesting plastic or other compounds from a coated knife.
Don't plan your next party trick just yet
But before you get some oranges, sharpen your knives and promise your family you can turn an orange purple, some more context: this was a rare event, impacted by the type of orange, the orange’s storage time and environment, and the type of knife. You probably won't be able to replicate Neti's experience.
“It was a special combination of factors,” wrote Jim Carter, senior chemist in Organic Chemistry, Forensic and Scientific Services for Queensland Health.
Kimberley R. Miner, Research Assistant Professor, University of Maine.
Short Cold Storage Enhances the Anthocyanin Contents and Level of Transcripts Related to Their Biosynthesis in Blood Oranges. Tiziana Crifò, Goffredo Petrone, Luca Lo Cicero, Angela Roberta Lo Piero. Journal of Agricultural and Food Chemistry. Dec. 12, 2011.
Anthocyanin Based Blue Colorants. Neda Ahmadiani. Master’s Thesis. Graduate Program in Food Science and Nutrition. The Ohio State University. 2012 Changes in Anthocyanin Production during Domestication of Citrus. Butelli E, Garcia-Lor A, Licciardello C, Las Casas G, Hill L, Recupero GR, Keremane ML, Ramadugu C, Krueger R, Xu Q, Deng X, Fanciullino AL, Froelicher Y, Navarro L, Martin C. Plant Physiology. 2017.