Natural toxins are toxic compounds that are naturally produced by living organisms. These toxins are not harmful to the organisms themselves, but they can be toxic to people when we eat food contaminated with them.
Some toxins are produced by plants as a natural defence mechanism against predators, insects or microorganisms; as a consequence of infestation with certain microorganisms; or in response to climate stressors like drought or extreme humidity.
Other natural toxins that can affect human health are those produced by microscopic algae and plankton in oceans and freshwater. When people eat fish or shellfish contaminated with aquatic biotoxins, food poisoning can rapidly follow.
Some of the more common natural toxins that pose a risk to human health are described below.
There are two main types of aquatic biotoxins: algal toxins and ciguatoxins.
Algal toxins are produced by microscopic algae. When shellfish (particularly bivalve shellfish like oysters, clams and mussels) ingest toxin-producing algae, toxins can build up in their tissues.
Eating shellfish with high levels of algal toxins can lead to serious and potentially fatal illnesses in people — such as paralytic shellfish poisoning (PSP), a very serious illness which can cause death in as little as two hours.
Symptoms of shellfish poisoning may include: diarrhoea, vomiting, tingling, disorientation and paralysis.
Ciguatoxins are primarily produced by marine plankton. Ciguatera toxin tends to accumulate in large predator fish like barracuda, black grouper, eel, sea bass, dog snapper and king mackerel.
When people eat fish contaminated with ciguatoxins, they can get ciguatera fish poisoning (CFP). Symptoms of CFP may include: nausea, vomiting, tingling, numbness, muscle pain, dizziness and vertigo.
Aquatic biotoxins have no taste or smell and are not eliminated by cooking or freezing. Food Handlers must be aware of these risks and know how to prevent illness from toxins in seafood.
Cyanogenic glycosides are toxic chemicals produced by plants — including a wide range of fruits, vegetables and plant-based foods in Australia and New Zealand.
Cassava, sorghum, bamboo roots, bitter almonds, raw apricot kernels and some stone fruits — including apricots, cherries, peaches, pears and plums — are known to contain cyanogenic glycosides.
In the case of stone fruits, cyanogenic glycoside is contained in the pit; if ingested, cyanogenic glycoside breaks down into hydrogen cyanide.
The flesh of the fruit itself is safe and even if you were to eat the pit (raw or otherwise), it’s unlikely that the concentration of toxin would be enough to cause any symptoms (though it is certainly not recommended).
For the most part, acute cyanide intoxication can be prevented by proper handling and preparation of cyanide-producing fruits and vegetables; requirements for handling certain plants are specified in the Australian Food Standards Code.
Food Standards Australia New Zealand (FSANZ) provides the following resources:
Clinical signs of acute cyanide intoxication can include: rapid respiration, dizziness, headache, stomach pain, vomiting, diarrhoea, mental confusion, cyanosis (bluish or grey skin, nails or lips), and convulsions followed by terminal coma.
Mycotoxins are naturally occurring toxic compounds produced by certain species of mould. Different types of mycotoxins include: aflatoxins, ochratoxin, trichothecene and fumonisins.
Moulds that can produce mycotoxins grow on a variety of food, such as cereals, dried fruits, nuts and spices. Mould growth can occur before or after harvest, during storage or on / inside foodstuffs under warm, damp and humid conditions.
Most mycotoxins are chemically stable and survive food processing.
The effects of food-borne mycotoxins can be acute — meaning symptoms or even death occur very quickly after eating highly contaminated food — or they can cause long-term health conditions like cancer or immune deficiency.
Aflatoxins, which can be found in grains, nuts, legumes and milk products, are particularly potent and can be very harmful to human health.
Solanine and chaconine
All solanaceae plants, which include tomatoes, potatoes and eggplants, contain natural toxins called solanine and chaconine (which are glycoalkaloids).
Both solanine and chaconine can cause vomiting, abdominal pain, diarrhoea, headache, flushing, confusion and fever.
While levels are generally low, higher concentrations of these toxins can be found in potato sprouts, peels and green parts.
To reduce the production of solanine and chaconine in potatoes, be sure to:
- store potatoes in a dark, cool and dry place
- wash potatoes before cooking
- peel or cut away green areas prior to cooking
- discard potatoes with pronounced greening or damage
Glycoalkaloids are not destroyed by cooking; elevated levels of the toxins may cause a bitter taste or a burning sensation in the mouth.
Wild mushrooms may contain a number of different toxins, such as muscimol and muscarine, which can cause vomiting, diarrhoea, confusion, visual disturbances, salivation and hallucinations.
Australia is home to many poisonous varieties of fungi, including the most lethal mushroom in the world, Amanita phalloides (or ‘death cap mushroom’). In April 2019, eight people were hospitalised after eating death cap mushrooms in New South Wales.
Symptoms usually begin between six and 24 hours after ingestion, but they can present after 24 hours. Symptoms may include: severe abdominal pain, vomiting, diarrhoea and intense thirst.
If toxins damage the kidneys, liver or central nervous system, poisoning can be fatal.
Food businesses and related organisations, as well as urban foragers, must understand that cooking or peeling does not inactivate toxins, so it’s very important not to eat or prepare wild mushrooms. Always buy commercially available mushrooms from a trusted supplier.
Food safety training to prevent poisoning risks
It’s important to be aware of the risk of natural poisons in foods; if you operate a food business or prepare food for sale to the public, you must ensure that all reasonable precautions have been taken to prevent food poisoning from natural toxins.
The best way to reduce food safety risks in a commercial food business is to ensure that everyone who handles food has completed food safety training. Nothing could be easier — Food Handlers can enrol in and complete an online food handling course in just a few hours.
A Food Safety Supervisor is also a tremendous asset when it comes to maintaining food safety. As the person responsible for overall food safety in the business, Food Safety Supervisors ensure that Food Handlers are following established food safety procedures and protocols.
Food Safety Supervisors are a legal requirement in most states and territories in Australia; they must complete a nationally recognised Food Safety Supervisor course to qualify for the position.
In New South Wales (NSW), Food Safety Supervisors must also obtain a Food Safety Supervisor certificate (‘green certificate’) issued by a Registered Training Organisation (RTO) approved by the NSW Food Authority, such as the Australian Institute of Food Safety.