The Purple Orange???

In Brisbane, back in early September, an extremely unusual occurrence made headlines and provoked fear in some and avid curiosity in others.

A freshly-cut orange turned purple overnight.


The only other time such an occurrence had been noted was three years earlier by another Queenslander … doesn’t that raise such suspicion? Similar area, perhaps the same grower or conditions, perhaps it’s picked up fungus or lichen! What if it’s a reaction  with chemicals in the house? A child ATE it! What if it’s poisonous?

Welcome back to reality … it was a harmless chemical reaction that delightfully intrigued the scientists studying it.

The first piece of the puzzle was that the knife used to cut the orange had been freshly sharpened, meaning that some iron particles were exposed and hadn’t yet reacted with the air. Normally knives are steel of some variety and the combination of metals means that some form harmless metal oxides on the surface to prevent further corrosion. In this case, the knife had not had enough time to fully react and this left the far more reactive iron particles exposed. (Note that reactive does not mean it is explosively or violently so, only that it readily combines with other compounds).

Secondly, oranges contain anthocyanins. Anthocyanins are plant pigments present in many fruit and veg, mainly the red, blue and purple ones (purple cabbage, berries, etc..) and some of these are found in oranges. There are many different types of anthocyanins and many are under ongoing investigation for their health benefits.

But back to the orange, the anthocyanins in the orange reacted with iron particles from the freshly sharpened knife to go from an orange colour to the deep purple of ferric anthocyanin that was observed. This is actually a documented effect in other fruit, including the reactions of cyanidin-3-glucoside (a common anthocyanin)to produce purple ferric complexes when reacted with iron in peaches and nectarines. The purple colouration in oranges was reproduced when other oranges were treated with iron particles.

So that was the mystery solved. Merely a natural pigment reacting with iron particles from a normal steel knife. The whole scenario excited the scientists at the forensic department who had the opportunity to problem solve what might have happened and compare to the previous case 3 years ago. And they concluded that it was a “colourful but harmless incident” and it has helped communicate a little more science to everyone!


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