POTATOES' NATURAL DEFENCES

(In an article in the June issue of "The Grower", Dr. S.T. Ali-Khan, describes the potatoes' defense system. Here are some highlights with regard to human health.)

All living beings have some means of natural defense. Potatoes contain chemicals called glycoalkaloids (GA). The two GA found in potatoes are solanine and chaconine, and are collectively referred to as total glycoalkaloids (TGA). These TGA can cause serious gastro-intestinal disturbances, and even neurological abnormalities in certain people. The intensity of their effect varies greatly with the sensitivity of the individual.

Even though the maximum tolerable level of TGA is 0mg./100g., most potato varieties contain between 2-15mg./100g. of unpeeled fresh weight. In an effort to hit a safe point between tolerable and toxic levels, international potato breeders now work only with varieties having TGA levels of 10mg./100g. or less. One variety, Lenape, (now off the market) had normal TGA levels of 27 mg./100g., which rose as high as 65mg. under stress conditions.

There are several stress factors which increase levels of TGA in potatoes. Exposure to sunlight or to near-freezing temperatures at harvest time are two main ones. Even damage such as skinning and bruising increases these levels. Exposure to light (especially if it causes greening) or to high temperatures (5-10C is ideal) during storage also increases TGA levels.

Levels of TGA are low at harvest time, assuming tubers are mature, their full size "diluting" the TGA which form at an early stage. You can minimize TGA levels by taking care to avoid the stresses mentioned above in the growing and storing of potatoes. Also, since most of the TGA is concentrated in a 1.5 mm. (1/16") layer under the skin, peeling does remove most of these substances.

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