Phytohaemagglutinin (PHA) is a lectin, a protein found in certain plants that binds to carbohydrates. Its presence in some foods can lead to adverse effects if not properly prepared, making understanding PHA important for food safety.
What is Phytohaemagglutinin?
PHA is a protein found in plant seeds, serving as a natural defense against pests and pathogens. It binds to specific carbohydrate structures.
PHA is present in many types of beans, with varying concentrations. Red kidney beans (Phaseolus vulgaris) have the highest concentration, while white kidney beans contain about one-third the amount. Broad beans (Vicia faba), such as fava beans, contain only 5% to 10% of the PHA found in red kidney beans. Other legumes like cannellini beans, green beans, soybeans, and lentils also contain PHA.
How Phytohaemagglutinin Impacts Health
When consumed, PHA can cause a range of symptoms, typically within 1 to 3 hours after ingesting improperly prepared beans. Common symptoms include nausea, vomiting, diarrhea, and abdominal pain.
PHA binds to cells lining the gastrointestinal mucosa, interfering with digestion and nutrient absorption. It also has the ability to agglutinate, or clump together, mammalian red blood cells. The severity of symptoms is related to the amount of PHA ingested; as few as four or five raw beans can trigger symptoms. While recovery is usually rapid, occurring within 3 to 4 hours, some cases have required hospitalization.
Safe Preparation for Consumption
Proper preparation of foods containing PHA is necessary for safe consumption. Soaking dried beans is a recommended first step, often overnight, and the soaking water should be discarded before cooking. This initial soaking helps to reduce the toxin levels.
Boiling is a method for inactivating PHA. Beans should be boiled in fresh water at 100°C (212°F) for at least 10 minutes, with some recommendations suggesting at least 30 minutes to ensure complete destruction of the toxin. This sustained high temperature denatures the protein structure of PHA, rendering it harmless.
Undercooked beans pose a risk because insufficient heat may not fully destroy the toxin. Slow cookers, for instance, may not reach temperatures high enough, often only around 75°C (167°F), to effectively neutralize PHA. Therefore, it is important to avoid using low-temperature cooking methods for dried beans unless they have been pre-boiled thoroughly. Commercially canned beans are generally safe for consumption without further cooking, as they have already undergone sufficient heat treatment during their processing.