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There are several reasons potatoes may discolor and blacken when cooked:

1) Cooking in aluminum or iron pots.

Aluminum and iron are 'reactive' - they can react with the chemicals present in foods and discolor them.   
Always use stainless steel, ceramic or glass when cooking potatoes.

2) If potatoes are stored below 40 degrees F they may turn black when cooked, especially if they have been handled roughly or dropped while cold.

3) Blackening can also occur because of the conditions they were grown (climate, soil composition, fertilizers, etc) - it is impossible to tell which potatoes will be affected. Some of the chemical compounds in the potatoes react to the heat and exposure to oxygen, causing them to darken.

This type of discoloration sometimes occurs in cooked potatoes and appears as a gray-blue-black area as the cooked potato cools. Any discolored area can simply be cut away. Some potatoes are more susceptible to this discoloration and are influenced by the soil and climate in which the potatoes were grown.

American Journal of Potato Research Jan/Feb 2004
"After-cooking darkening (ACD) is caused by the oxidation of the ferri-chlorogenic acid in the [cooked] potatoes. The severity of the darkening is dependent on the ratio of chlorogenic acid to citric acid concentrations in the potato tubers. Higher ratio normally results in darker tubers. The concentration of the chlorogenic and citric acids is genetically controlled and influenced by environmental conditions.
     After-cooking darkening occurs when potatoes are exposed to air after cooking, including boiling, baking, frying, or dehydration. After-cooking darkening has been reported from every potato-growing area in the world and is one of the most widespread, undesirable tuber traits, even though ACD does not affect the flavor or nutritional value of the potatoes.
     It is generally accepted today that ACD is caused by a non-enzymatic oxidation reaction. During cooking, a ferrous-chlorogenic acid complex is formed which, on exposure to air, oxidizes to a bluish-gray compound ferri-dichlorogenic acid.  Although the dark color is generated by the formation of ferri-dichlorogenic acid, changes in pH affect the formation of the ferri-chlorogenic acid from a 1:1 to a 1:3 complex."



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