Natural toxins in vegetables and beans

Natural toxins are more common in foods than people might think. Some vegetables and beans that we commonly eat contain potentially harmful natural toxins. Find out the risks and how you can limit your exposure by following some simple practices.


Protect yourself from plant toxins

Toxins can be in plant-based foods for several reasons. In some plants, a toxin may function as a naturally-occurring pesticide to ward off insect attack. Or a toxin may protect the plant from spoiling when damaged by weather, handling, UV light or microbes.

Whether you have an adverse reaction to a toxin depends on your own sensitivity and the strength of the toxin.

Use the 'quick guide' to help prepare food safely

Our guide will help you quickly see what common foods may contain natural toxins and the easy things you can do to limit your exposure.

More information follows this guide:

Quick guide for safe vegetable preparation
Vegetable Suggested preparations
Potatoes
  • Remove green or damaged parts before cooking
  • Do not eat potato sprouts
  • Do not eat if it tastes bitter after cooking
Kumara
  • Remove damaged parts before cooking
  • Do not eat if it tastes bitter after cooking
Parsnip
  • Peel before cooking and remove damaged parts
  • If boiling, discard used water
Kidney beans
  • Soak beans for 5 hours or longer, then briskly boil for at least 10 minutes
  • Tinned beans can be used without further cooking
Rhubarb
  • Do not eat the leaves
Zucchini (courgette)
  • Do not eat if it has a strong unpleasant smell or tastes bitter
Cassava
  • Peel and slice before cooking thoroughly
Bamboo shoots
  • Slice in half lengthwise, peel outer leaves away and cut off fibrous tissue at the base
  • Thinly slice the fresh shoots and boil in lightly-salted water for 8-10 minutes

More about the toxins

Potatoes

All potatoes contain natural toxins called solanines (which are glycoalkaloids), generally in low levels but higher concentrations are found in potato sprouts and bitter-tasting peel. The plants produce the toxins in response to stresses like bruising, UV light and microorganisms and attack from insect pests and herbivores.

It's unusual to have a bad reaction, but overseas there have been reports of severe stomach ache and even death from glycoalkaloid poisoning. Cooking doesn't destroy the toxins, so don't eat sprouts, and remove any green or damaged parts from potatoes before cooking.

If you come across a bitter tasting or green potato or potato chip, it's best not to eat it, and remember to store potatoes in a dark, cool and dry place.

Kumara

A member of the sweet potato family, kumara can produce toxins in response to stress, including injury and insect attack. The most common toxin, ipomeamarone, can make the kumara taste bitter. There have been reports of cattle death after they have eaten mouldy kumara.

Toxin levels are normally highest near damaged areas of the kumara, so remove these parts before cooking and don't eat the kumara if it tastes bitter after cooking.

Parsnip

Parsnips commonly contain a group of natural toxins called furocoumarins, which are probably produced to protect the plant when it has been stressed. The toxin is normally concentrated in the peel and the surface layer of the plant, as well as around any damaged areas.

One of the toxins can cause stomach ache and may cause a painful skin reaction when contact with the plant is combined with UV rays from the sun.

Peel parsnip before cooking and remove any damaged parts. The levels of furocoumarin toxins drop when the vegetable is cooked by baking, microwaving or boiling – but discard the used cooking water after boiling.

Kidney beans

Many types of beans contain toxins called lectins, but kidney beans have the highest concentrations – especially red kidney beans. As few as 4 or 5 raw beans can cause severe stomach ache, vomiting and diarrhoea.

When using raw beans, soak them for at least 5 hours then boil briskly in fresh water for 10 minutes to destroy the toxins. Do not cook them at a low temperature – for instance, in a slow cooker – as poorly-cooked beans can be more toxic than raw ones. Tinned beans are good to use without further cooking.

Rhubarb

Rhubarb contains naturally-occurring oxalic acid – almost all in the leaves.  Poisoning can cause muscle twitching, cramps, decreased breathing and heart action, vomiting, pain, headache, convulsions and coma.

Do not eat the leaves as this is where the toxin is most concentrated.

Zucchini (courgette)

Occasionally, zucchini contains a group of natural fungicide toxins called cucurbitacins, which give the vegetable a bitter taste. The plant produces cucurbitacins in response to attack by fungi. Bitterness is common in wild zucchini but rare in commercially-grown types.

Eating bitter zucchini can cause vomiting, stomach cramps, diarrhoea and collapse. Do not eat zucchini that has a strong, unpleasant smell or a bitter taste.

Cassava and bamboo shoots

Raw or unprocessed cassava and bamboo shoots have natural toxins called cyanogenic glycosides that can expose you to hydrogen cyanide.

Cassava is also called yucca, tapioca, gaplek or manioc. Bamboo shoots come from the underground stems of the bamboo plant.

To avoid toxin exposure, sweet cassava should be peeled and sliced before being cooked thoroughly, either by baking, boiling or roasting. Do the same thing if the cassava has been frozen.

When preparing fresh bamboo shoots, slice them in half lengthwise, peel away the outer leaves and trim off any fibrous tissue at the base. Then the remaining fresh shoots should be thinly sliced into strips and boiled in lightly-salted water for 8 to 10 minutes.

Who to contact

If you have questions about plant toxins, email info@mpi.govt.nz.

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