Potato wars

Many a time I’ve picked up a potato when preparing a meal and asked myself is it too green? How green does a potato have to be before you discard it? A quick internet search soon reveals conflicting advice. The Food Safety Authority of Ireland gives a reassuring message that most of the toxins in a green potato are near the skin, so peeling will remove most of the harmful substances. However , if they still taste bitter once cooked, it would be best to discard them. Medline Plus the US National Institutes of Health’s Web site produced by the National Library of Medicine, the world’s largest medical library, takes a more cautious approach, advising never to eat potatoes that are spoiled or green below the skin, and to always throw away the sprouts.

Photo of potatoes, some green
Beware the green potato!

So, what is all the fuss about? The harmful substances we are concerned about in green potatoes are known collectively as glycoalkaloids (GAs). Although we mostly hear about the glycoalkaloid solanine it is accompanied by chaconine, which is more toxic. They are present throughout the plant and protect it from insect pests and fungal infections as well as deterring herbivores. This is definitely good news for the plants! The trick that potato growers need to pull off is to preserve these protective properties in the green shoots while keeping the GAs in the actual potato tubers (the bits we eat) as low as possible.

photo of a whole potato plant
Whole potato plant

One well-kept secret is that all potato tubers contain GAs, even the white ones. If you weigh 50kg, you would need to eat 100mg or more of GAs before you are at risk of developing symptoms of abdominal pain, vomiting, and diarrhoea. 100mg of GAs is typically present in only one kilo of non-green potatoes, although that is a lot for a small person to eat in a day. Peeling potatoes can remove half of the GAs, depending on how thick the peel is, for instance. The average potato consumer should be safe, although there are individual variations in susceptibility.

That is not the whole story. Some varieties of potatoes have much higher levels of GA than others. Behind the scenes of the supermarket shelves, scientists and farmers have been waging a quiet war against certain varieties of potato that have been found with dangerous levels of GA.  Sweden has outlawed one potato variety, which has over three times the recommended maximum levels of GA. GAs also fluctuate in all potato varieties according to growing and storage conditions. Any damage to the tuber causes a local rise in GAs.

A green potato is a signal of a GA ‘hotspot’, a protective response by the plant to exposure of the tuber above the soil. The green pigment itself, chlorophyll, is harmless. Removing the green parts of the tuber does indeed greatly reduce the amount of GAs left in the potato. Some GAs are also lost in cooking, but mainly though leaching into the cooking water as they need very high temperatures before they are destroyed. Unfortunately, the ‘lazy cook’s’ method of microwaving whole potatoes for a quick meal is a sure way of carefully preserving all the GAs present.

Am I still going to eat potatoes? Yes. Am I going to throw away any green potatoes? I might be more likely to in future.

Information sources:

  • Machado, R. M. D., Toledo, M. C. F. and Garcia, L. C. (2007) ‘Effect of light and temperature on the formation of glycoalkaloids in potato tubers’, Food Control, vol 18, no. 5, pp. 503-508.
  • Mensinga, T. T., Sips, A. J. A. M., Rompelberg, C. J. M., van Twillert, K., Meulenbelt, J., van den Top, H. J. and van Egmond, H. P. (2005) ‘Potato glycoalkaloids and adverse effects in humans: an ascending dose study’, Regulatory Toxicology & Pharmacology: RTP, vol 41, no. 1, pp. 66-72.
  • Valcarcel, J., Reilly, K., Gaffney, M. and O’Brien, N. (2014) ‘Effect of Genotype and Environment on the Glycoalkaloid Content of Rare, Heritage, and Commercial Potato Varieties’, Journal of Food Science, vol 79, no. 5, pp. T1039-T1048.

Image credits: Chase Studio / Photo Researchers and JANE SHEMILT / SCIENCE PHOTO LIBRARY, both of Universal Images Group

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In praise of Brassica oleracea

 

Cauliflower
Cauliflower

There can be few edible plant species that are quite as impressive as Brassica oleracea. It is difficult to appreciate that Brussels sprouts, cabbage, kale, cauliflower, brocolli and kohl rabi all belong to this single species. I sometimes liken it to the domestic dog, in all its glorious breeds and varieties, originating from the wolf, Canis lupus.

Brussels sprouts
Brussels sprouts

Brassica oleracea vegetables are a good source of Vitamin C, although the levels of this vitamin are heavily dependent on growing and storage conditions as well as the preparation method. Brocolli and sprouts retain their Vitamin C much better than kale during storage, an indication that growing your own kale and picking and eating on the same day are well worth it. Vitamin C is commonly accepted to possess therapeutic properties protecting against cardiovascular disease, cancer, supporting the production of collagen (e.g. in skin and bones), and promoting iron absorption.

Cabbage
Cabbage

In addition to Vitamin C, Brassica oleracea is also a good source of Vitamin E and carotenoids (the most well-known carotenoid is beta-carotene, the chemical that gives carrots their characteristic orange colour and gives us Vitamin A). All these vitamins act as antioxidants in the body, mopping up toxic compounds known as free radicals, and protecting against malignancy.

While the evidence in support of vitamin supplements is weak, the health-promoting effects of eating vegetables such as brassicas are gaining ground. Moreover, scientists are beginning to suggest that the unique chemical make-up of brassicas can boost health in ways other vegetables cannot achieve. Brassicas happen to be very well endowed with phytochemicals, which are substances derived from plants, that support human health. Phytochemicals include vitamins and a range of other substances that have active biological effects on our bodies.

Kohlrabi
Kohlrabi

One of the key health-promoting components of Brassica oleracea are glucosinolates, which are phytochemicals unique to brassica vegetables.  As with Vitamin C, glucosinolates can be lost or damaged during cooking by mechanisms such as breakdown by plant cell enzymes, heat degradation, and leaching into the cooking water. A recent investigation of cooking methods found that steaming, microwaving at high power and stir-frying helped to reduce the amount of these valuable compounds lost in cooking, in comparison to boiling or fermentation.

Stir-frying brocolli
Stir-frying brocolli
Purple sprouting brocolli Credit : Nigel Cattlin / Photo Researchers / Universal Images Group
Purple sprouting brocolli

Glucosinolates have a role in promoting the health of the liver and in protecting against cancer. Plant breeders and food scientists are beginning to work together to find ways of improving glucosinolate content, durability and better ways of processing our vegetables to further enhance their health-promoting potential.

If you are keen to grow your own, it’s a good idea to check your soil acidity first. You can obtain very inexpensive kits online or from your local garden centre. Brassica oleracea is susceptable to a fungal disease known as club root in acid soils. So, if your pH turns out to be less than 7, you should consider adding lime to the soil. The best time to do this is when you dig the soil over in the winter. Growing your own is highly rewarding, and the plants can look quite magnificent.

Kale growing Credit : Food and Drink Photos / Universal Images Group
Kale

Photo credits Universal Images Group: Brocollli and Kohlrabi – Nigel Cattlin;  Brussel sprouts – Michael P. Gadomski; Cabbage and cauliflower – Dorling Kindersley; Kale – Food and Drink Photos; brocolli stir-fry David Munns / Science Photo Library

Related articles

Forget the antioxidant pills https://theconversation.com/forget-the-antioxidant-pills-just-stick-with-veggies-45409

Brilliant brassica http://blog.tgac.ac.uk/brilliant-brassica/