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Cooking Potatoes FAQ

What causes potatoes to turn green?

Greening is a natural process for potatoes that have been over exposed to light on the farm, in the supermarket, or in the home storage.  It is not normally found in potatoes that have been grown and stored under proper conditions. 

The bitter taste of green potatoes is due to glycoalkaloids which can be found in the skin or in the tissue under the skin.  Glycoalkaloids are not destroyed by cooking and may cause burning sensations in the mouth, stomach upset, or headaches.

If greening has occurred, the potato may be consumed after cutting away and discarding the green portion before cooking.  If the greening is sever, discard the whole potato.

What causes potatoes to have brown centers or a hole inside?

“Hollowheart” is a discoloured cavity in the centre of an otherwise healthy potato.  It can be caused by rapid growth or possibly by sudden temperature changes early in the growing season.  This condition doesn’t affect taste or nutritional value of potatoes. These potatoes can be eaten but the hollow piece should be cut out.

Why do potatoes darken when cooked?

There are three possibilities:

  1. “After cooking darkening” occurs in some potatoes and is the result of an imbalance of naturally occurring acids. Cholorgenic acid combines with the iron in the potato to produce the grey or dark appearance. Adding lemon juice during cooking prevents this reaction and eliminates the problem.
  2. Darkening may also be caused by a natural reaction with the iron in the potato.  To prevent, add a little lemon juice or vinegar to the cooking water.
  3. When potatoes are stored at refrigerator temperatures the starch changes to sugar.  When cooked, this sugar “caramelizes” or darkens.  Store potatoes at cool, not cold temperatures.

What causes black spots in a potato?

Black spots are usually bruises which are caused by rough handling.  One type of bruising happens when the skin is broken.  The potato forms a thicker layer of skin to protect and heal the wound.

The second type of bruising shows up just under the skin or deeper inside the potato as a blackspot.  This happens when potatoes are moved at cold temperatures or dropped more than six inches.  Remove any blackspots before cooking potatoes.

Why isn’t the potato variety printed on the potato bag?

There are no regulations requiring potato variety names to be placed on bags; however, the flesh colour and shape should be shown, i.e. Yellow Flesh Potatoes.  To help consumers make purchasing choices, many producers now label by variety.

Why do French fries sometimes go dark?

If potatoes are stored at cold temperatures, part of the starch in the potato changes to sugar.  The high temperatures needed for frying causes the sugars on the surface to darken before completely cooking inside.  Blanching potatoes in hot water 90C (170F) for several minutes removes most of the sugars from the surface of the fries allowing them to brown more evenly.  Warming potatoes to 16C (60F) for a number of days before frying reduces the sugar level.