Why do some Yukon Gold Potatoes have purplish black spots and What causes the discoloration?

Yukon Gold potatoes, along with cabbage, onions, and yellow rice, get their yellow color from chemicals called anthoxanthins.

Anthoxanthins react with traces of metals such as iron and aluminum, which turn them blue-gray. A carbon steel knife can have that effect, so it’s best to cut and slice these vegetables with stainless-steel knives. Other varieties of potatoes contain smaller amounts of anthoxanthins and don’t stain as easily.

Storage at high temperatures can also turn anthoxanthins dark.

So if some of the potatoes are already stained upon purchase, seek out a market that stores its supply at cooler temperatures.

Speaking of anthoxanthins (now there’s a segue you won’t see every day), carrots contain small amounts of anthoxanthins, the colors of which can depend on the presence or absence of metal ions (charged metal atoms) such as iron and aluminum. People who bake carrot cakes in cast-iron or aluminum pans are sometimes startled to find that the carrots have turned green.

As in the case of the Yukon Golds, the reaction product of a carrot anthoxanthin with iron or aluminum can well be blue. And guess what blue plus the carrot’s yellow makes? Green!