Food Safety Focus (208th Issue, November 2023) – Article 1
Phytohaemagglutinin Poisoning
Reported by Dr. John LUM, Scientific Officer,
Risk Assessment Section, Centre for Food Safety
Beans are one of the most versatile and commonly eaten nutritious foods throughout the world. However, the consumption of common beans (e.g. green beans, French beans) and other beans (e.g. red kidney beans, white kidney beans) without proper processing may cause food poisoning due to the naturally present toxin phytohaemagglutinin. This article will give a brief introduction on phytohaemagglutinin poisoning.
Phytohaemagglutinin Toxin in Beans
Lectins are widely occurring, sugar-binding proteins that perform a variety of biological functions in plants and animals. However, some of them may become toxic at high levels. Among the lectins known to have toxic effects is phytohaemagglutinin, which occurs at relatively high levels in the seeds of legumes (i.e. beans). Phytohaemagglutinin is involved in defence against plant pests and pathogens.
Phytohaemagglutinin, as its name implies, can agglutinate many mammalian red blood cells and interfere with cellular metabolism. Moreover, phytohaemagglutinin is an antinutrient, which can interfere with the absorption of minerals, particularly calcium, iron, phosphorus and zinc.
Phytohaemagglutinin is found in many beans, but the level varies among different species of beans. The concentration of phytohaemagglutinin is the highest in red kidney beans (Phaseolus vulgaris). White kidney beans, another variety of P. vulgaris, contain about one-third the amount of toxin as does the red variety. On the other hand, broad beans (Vicia faba) contain only 5% to 10% of the amount of phytohaemagglutinin that red kidney beans contain. Some commonly consumed beans in Hong Kong, including soya beans (大豆), green beans (四季豆) and yard-long bean (豆角) have been reported to cause phytohaemagglutinin poisoning in other places. To avoid food poisoning, all beans should be cooked properly before consumption since various cultivars of the same species of bean might have significantly varying levels of the phytohaemagglutinin toxin.
Figure 1: Some commonly consumed beans in Hong Kong that have been reported to cause phytohaemagglutinin poisoning in other places
Clinical Presentation of Phytohaemagglutinin Poisoning
Symptoms of phytohaemagglutinin poisoning include severe stomach-ache, vomiting and diarrhoea. Some of the characteristics of phytohaemagglutinin poisoning are summarised in the table below:
Characteristics of Phytohaemagglutinin Poisoning | |
Mortality: | Generally not life-threatening. |
Toxic dose: | As few as four or five raw beans can trigger symptoms. |
Onset time: | Usually begins with extreme nausea and vomiting within 1 to 3 hours after ingestion of the product, with diarrhoea developing later within that timeframe. |
Symptoms: | Upper and lower gastrointestinal illness. Vomiting may become severe. In addition to vomiting and diarrhoea, abdominal pain has been reported by some people. |
Duration: | Recovery usually is rapid, within 3 to 4 hours after the onset of symptoms and spontaneous, although some cases have required hospitalisation. |
Susceptible | All people, regardless of age or gender, appear to be equally susceptible. The severity is related to the dose ingested. |
Figure 2: How to prevent phytohaemagglutinin food poisoning from consuming raw beans
Methods of Toxin Reduction
Cooking with moist heat can reduce the toxicity of phytohaemagglutinin. When compared with fully cooked beans, raw kidney beans could have phytohaemagglutinin levels that are hundreds of times higher. Therefore, after sufficient cooking, the use of phytohaemagglutinin-containing beans as food in human diets is not a cause for concern. Special attention, however, should be paid when the phytohaemagglutinin-containing food is prepared at high altitudes where the boiling point is reduced, when low heat cooking methods are employed or in situations where heat transfer is uneven.
To destroy the phytohaemagglutinin toxin, beans should be soaked and boiled thoroughly in fresh water (e.g. soaked for at least 12 hours and then boiled vigorously for at least 10 minutes in water). Previous studies showed that the phytohaemagglutinin toxin remained active after the beans had been cooked even at 85°C for an hour. Therefore, beans should not be cooked at a low temperature, for example in a crock pot or slow cooker, since it may not destroy the toxin. On the other hand, commercially tinned/canned beans are safe to eat without further cooking as they have been subjected to thorough heat treatment.
Food Safety Limit on Phytohaemagglutinin
Phytohaemagglutinin has not been evaluated by food safety regulatory authorities including the Joint FAO/WHO Expert Committee on Food Additives (JECFA), and a health-based guidance value has not been established for risk assessment. Moreover, there is no relevant food safety standard established by Codex. Nevertheless, it has been reported that ingestion of as few as four or five raw beans can trigger symptoms. Crucially, cooking with moist heat can remove the toxicity of phytohaemagglutinin. Consumers should not eat raw or inadequately cooked beans.
Key Points to Note
- Many types of beans contain the natural toxin phytohaemagglutinin. Consumption of raw beans can cause phytohaemagglutinin poisoning.
- Cooking with moist heat can remove the toxicity of phytohaemagglutinin. Beans should be soaked and boiled thoroughly in fresh water (100°C). Beans should not be cooked at a low temperature, for example in a crock pot or slow cooker, since it may not destroy the toxin.
- Canned beans can be consumed safely without further cooking.
Advice to Consumers and Trade
- Soak and cook beans thoroughly to destroy the phytohaemagglutinin toxin.
- Beans should not be cooked at a low temperature, for example in a crock pot or slow cooker, since it may not destroy the toxin.
- Do not consume raw or inadequately cooked beans.
