Why do potatoes turn pink?

A person sent me this picture with a question:

Q: Why do potatoes turn pink? Can we eat these pink-tinged potatoes?

Krishna: There are various reasons why some potatoes turn pink.

Potatoes can turn pink inside due to a chemical reaction that occurs when the potato's cell walls are damaged and its enzymes and phenols meet oxygen. This reaction is similar to what causes apples and bananas to turn brown and mushy.

Main reasons why potatoes might turn pink:

Cutting: Cutting into a potato damages its cell walls, exposing the phenols and enzymes to oxygen.

Alternating temperatures: Cool nights and hot days can cause potatoes to turn pink.

Soil: Dry soil or soil rich in nitrogen can cause potatoes to turn pink.

Potato leaf roll virus (PLRV): This virus can cause potatoes to turn pink.

Here are some tips to prevent potatoes from turning pink: Use a sharp knife or potato peeler to reduce damage to the potato. Use the potatoes right after cutting them. Submerge the potatoes in cold water to create a barrier between the potato and the air.

They are okay to eat when not infested with virus, bacteria or fungus. 

(Others reasons 're given below, please check them)

However, experts say  some types of of pink colouration of the flesh sometimes occurs in cultivars such as Bintje or Rosa when infected by Potato leaf roll virus (PLRV). The second type affects mainly cv. Roseval. The pigment responsible for this colouration is not toxic and is related to those found in wine and red cabbage.

And there are varieties like Kerrs pink potatoes (  Solanum tuberosum ), pic below.  The skin is semi-rough, firm, and light brown, covered in patches of pink blush with a few, medium-set red-pink eyes scattered across the surface. Underneath the skin, the flesh ranges in color from pale yellow to ivory and is fine-grained, dry, and starchy. When cooked, Kerr’s Pink potatoes have a floury texture creating a soft, fluffy consistency with a mild and earthy flavour.

Kerrs pink  potatoes

Pink eye is a disorder of potato tubers that can cause costly storage losses for potato growers and can reduce tuber quality to the point where tubers will be rejected by potato processors.  Pink eye not only directly affects tubers, but also makes tubers more susceptible to diseases such as Pythium leak, bacterial soft rot (see UW Plant Disease Facts D0010, Bacterial Soft Rot), pink rot, and Fusarium dry rot.  These diseases cause additional storage losses and reduction in quality.

Pink eye is characterized by a pink discoloration of the skin of potato tubers.

External pink eye symptoms are often accompanied by brown patches in the tuber flesh immediately underneath the skin.  Browning due to pink eye can resemble browning due to other disorders such as internal brown spot or heat necrosis, but these latter disorders tend to occur deeper in the tuber (i.e., inside the vascular ring), rather than just underneath the skin.

Pink eye is a physiological disorder (i.e., an abnormality in plant growth), rather than a true disease that involves a disease-causing microorganism.  Pink eye arises during periods of excessive soil moisture and warm temperatures, especially during the later stages of tuber development.  Pink eye symptoms typically appear within seven to 10 days after excessive rain.  Excessive soil moisture coupled with high soil temperature causes a lack of oxygen around potato tubers, leading to damage of cells in the tuber skin.  This cell damage contributes to pink eye development.  Environmental conditions that lead to pink eye also promote tuber infections by the pathogens that cause Pythium leak, bacterial soft rot (see UW Plant Disease Facts D0010, Bacterial Soft Rot), pink rot, and Fusarium dry rot (all diseases associated with pink eye in storage).

Once pink eye symptoms develop, they are permanent.  If symptoms are minor, tubers may still be usable.  However, when pink eye symptoms are severe, symptomatic tubers will be rejected and discarded.

Pink rot: 

Symptoms of potato pink rot in potato tubers inoculated with the Kv10 isolate of P. drechsleri (right), as compared with the healthy check (left)

Pink rot appears as a decay of tubers that usually begins at or near the stem end of potatoes in the field or through eyes of potatoes in storage. Infected tissue becomes somewhat rubbery but not discolored. When an infected tuber is cut, the rotted portion is delineated by a dark line at its margin. With exposure to air, the surface of the decay turns a salmon pink color, which later turns to brown and then finally black. Roots and lower stems may also rot, causing a wilt and early dying of plants. The spread of pink rot may continue in storage.

Phytophthora root rot caused by Phytophthora drechsleri Tucker is one of the most devastating  diseases in tropical areas. 

You cannot eat the pink  potatoes when infected with virus , bacteria or fungus. 

If the colour  is the result of just an enzymatic  reaction with oxygen, it is okay to eat. 

But how can a lay man decide this? 

Check for any infection  spots on the tuber. Smell the tubers. After reading this article, if  your gut feeling says something is not right, just throw the potatoes away. 

Cut the potatoes with a sharp knife. This will not damage the cells of the potatoes much. Then  the phenols and enzymes will not get exposed to oxygen and the potatoes don't turn pink. If infected with pathogens, this will not work. The potatoes will be pink anyway. That way you can roughly decide what the real cause is.