Farmer's Weekly (South Africa)
The potato menace
38 years ago
If poorly packaged or exposed to excess sun, the humble potato can become highly toxic, causing chronic enteritis or even death.
The humble potato is a potential killer, because it contains certain alkaloids which, in their concentrated form, are deadly poisonous. Although these alkaloids do not pose any threat to human health in correctly handled potatoes, certain methods of handling and presentation for marketing can and do promote the development of these toxins.
All members of the potato family produce these alkaloids or toxins, according to Mr Theuns Duvenhage, formerly of the National Potato Work Team. Included in the potato family are tobacco, tomatoes, chillies, eggplant, gooseberries, nightshade and stinkblaar. Not only are the tubers and fruits of these plants to a greater or lesser degree poisonous, but so is the foliage.
In the fruits of these plants, toxicity usually decreases as the fruits ripen. For example, the green fruits of nightshade are extremely poisonous, but the ripe berries are edible, says Mr Duvenhage.
There are more than 10 different alkaloids present in the wild potato species. Depending on the use to which they are put as parent breeding material, for breeding resistance to disease, eelworm and insect pests, such resistant cultivars may well tend to produce alkaloids under certain conditions.
Potato breeders throughout the world are trying, through the use of repeated back crossings, to breed out those genes that are responsible for producing alkaloids, he says.
It will be possible to eliminate certain alkaloids from potatoes in this way. If the crop is correctly handled, the alkaloid content may be kept down to a level at which it has minimal effect on animals.
There are a number of factors that promote alkaloid formation in potato tubers, the most important being exposure to light.
Potato tops are poisonous to all animals, according to Mr Duvenhage.
In some overseas countries, Canada for example, commercial cultivars contain genetic material derived from parents containing deadly alkaloids. If these particular potatoes are allowed to green, they can be lethal.
No green potato should ever be eaten, cautions Mr Duvenhage. The alkaloid content is high enough to cause chronic enteritis, as well as having other adverse effects.
Exclude Light
A number of practices are recommended by Mr Duvenhage to prevent or keep alkaloid formation in the tuber to a minimum.
In the late season the farmer should ridge up the crop to prevent tubers being exposed to sunlight. At lifting, potatoes should not be
left lying on lands in the sun for longer than half an hour. Potatoes are often left lying in the lands for a day or longer, particularly where grading and packing take place in the lands rather than in sheds. Even in sheds they should be packed as soon as possible, he says.
The introduction of 15kg brown-paper pockets has done much to improve the situation. These pockets effectively exclude light and the housewife should insist on buying potatoes packed in this manner.
No green (sub-standard) potatoes are permitted to be graded for human consumption, according to Mr Duvenhage.
One of the main places of deterioration is in supermarkets where potatoes are packed in net bags because “the housewife wants to see what she is buying”.
Sharp taste
According to Mr Duvenhage, many housewives are under the misapprehension that potatoes should be kept in open vegetable racks to avoid rotting in the bags. A simple solution is to smell the bag every few days – the smell of rotting potatoes is unmistakable.
The housewife should also ensure that potatoes are stored under dry conditions. They should not be kept in plastic bags as this promotes rotting, he says.
The formation of alkaloids is accelerated during the greening process and little advantage is gained by removing a greater thickness when peeling potatoes. The toxins are often spread in large amounts throughout the potato.
Also, it should be remembered that most of the vitamins and minerals are concentrated in and just under the peel.
Alkaloids do not break down with cooking and are characterised by a sharp taste at the back of the tongue and throat.
The most important toxin present in South African potato cultivars is solanine, while in some overseas varieties other alkaloids such as chaconine, commersine and B-chaconine occur.
The National Potato Work Team is contemplating closer investigation into alkaloids in potatoes. The possibility of reducing or eliminating alkaloids through breeding programmes, the amendment of cultural practices (excessive nitrogenous fertilization, for example) and the use of certain chemicals to prevent the formation of alkaloids could be considered.
Mr Duvenhage offers a few hints for the housewife to help her keep the alkaloid content of her potatoes to a minimum. Buy potatoes in
15 kg paper bags, he says. If necessary, open the bag in the shop and inspect the contents – do not buy if not satisfied with the contents.
At home, the potatoes should be stored in the paper bag and the bag kept closed. Tubers showing the least sign of greening should be thrown away. Alternatively, they can be left to sprout and used for planting in the vegetable patch.