Red kidney beans contain a natural toxin called phytohaemagglutinin. This toxin can cause severe gastrointestinal issues if the beans are not properly prepared before eating. Boiling the beans is not sufficient to deactivate the toxin – the beans must be boiled for at least 10 minutes to destroy the phytohaemagglutinin.
What is phytohaemagglutinin?
Phytohaemagglutinin is a lectin protein found in many varieties of beans, but is particularly concentrated in red kidney beans. Lectins are carbohydrate-binding proteins that Plants produce them as a defense mechanism against insects and other predators.
When consumed by animals, the lectin binds to carbohydrate molecules on the gut wall. This disrupts the digestive process and damages the intestinal tract. Symptoms of phytohaemagglutinin poisoning include severe nausea, vomiting, and diarrhea.
Why doesn’t boiling destroy the toxin?
Phytohaemagglutinin is incredibly resistant to heat. Short boiling times of 1-5 minutes are not sufficient to deactivate the toxin. This is because the lectin’s structure allows it to refold and regain function after heating.
To successfully destroy phytohaemagglutinin when cooking kidney beans, the beans need to be boiled for at least 10 minutes. The high heat for an extended period denatures the proteins in the lectin, permanently preventing refolding and deactivating its toxic effects.
What happens if you eat undercooked kidney beans?
Consuming inadequately prepared kidney beans can cause severe gastrointestinal distress. Symptoms generally begin 1-3 hours after ingestion and include:
- Nausea and vomiting
- Abdominal pain and cramping
- Dehydration due to fluid loss
In severe cases, improperly prepared kidney beans can cause:
- Electrolyte imbalances
- Kidney dysfunction
- Respiratory issues
- Death (in very rare cases)
Who is most at risk of toxicity?
Those most susceptible to phytohaemagglutinin poisoning include:
- Young children
- The elderly
- Those with compromised immune systems
Healthy adults can also experience adverse reactions if they consume very high quantities of improperly prepared beans. Cases of poisoning have occurred from eating undercooked kidney beans at potlucks or other group meals.
How to properly prepare kidney beans
To safely consume red kidney beans, they must be boiled for at least 10 minutes to neutralize phytohaemagglutinin. Other preparation methods include:
- Soaking and discarding water: Soak beans overnight, drain soaking water which contains toxins, rinse beans, then boil for at least 10 minutes.
- Pressure cooking: Pressure cook at 15psi for at least 15 minutes. The high pressure facilitates toxin breakdown.
- Slow cooking: Only safe if cooking time exceeds 10 hours. Toxins gradually breakdown but some remain if undercooked.
Canned kidney beans are safe to consume, as the canning process involves high heat for an extended period. When using canned beans, it is still advisable to boil for at least 10 minutes as a safety precaution.
Can you eat just a few undercooked beans?
It’s not recommended to eat even a small number of undercooked kidney beans. As few as 4-5 beans can cause a toxic reaction in sensitive individuals. Cooking for the full 10 minutes is necessary to destroy all lectin activity throughout the entire batch of beans.
Signs of properly prepared beans
When kidney beans are fully cooked and safe to eat, they will exhibit the following characteristics:
- Soft, tender texture throughout
- Easily mashed between fingers
- No longer firm or “crunchy” feeling
- Inner white color is uniform
Beans that are still firm, retain their red color, or seem crunchy/raw inside should be cooked further to degrade toxins.
Substitutes for kidney beans
Other varieties of beans make good substitutes for kidney beans in recipes. Safe options include:
- Black beans
- Pinto beans
- Navy beans
- Cannellini beans
- Great northern beans
These bean varieties do not contain high levels of phytohaemagglutinin, so they do not require the same preparation methods as red kidney beans. However, it is still recommended to cook any beans thoroughly before consuming.
Red kidney beans contain the dangerous lectin phytohaemagglutinin. Consuming inadequately prepared beans can cause severe gastrointestinal distress and toxicity. To safely eat kidney beans, they must be boiled for a full 10 minutes to destroy heat-resistant lectins.
Undercooking beans, even for just a few minutes, allows some toxin to remain active and potentially cause illness. Pressure cooking and prolonged slow cooking also deactivate the lectin. For optimal safety, always boil kidney beans for at least 10 minutes before serving.