Toward Standardized Measurement of Active Phytohemagglutinin in Common Bean, Phaseolus vulgaris, L

Common bean (Phaseolus vulgaris, L.) is the most widely consumed grain legume globally. The seeds of common bean are a rich source of protein, but one of the seeds' storage proteins is phytohemagglutinin (PHA), a lectin whose consumption in raw or inadequately cooked bean seed or products into which the seed is milled results in acute symptoms of food poisoning. Given that demand for incorporating common bean ingredients into foods is expanding, there has been a call for regulatory agencies to formulate more robust guidance on allowable levels of active PHA in beans and bean ingredients and for establishing standardized methodology for measuring active PHA. Herein, detailed protocols are provided for extraction of PHA from beans and for the use of digital image analysis in the traditional hemagglutination assay. Results are compared to an ELISA assay. Given reports that ingestion of four to five soaked raw dark red kidney bean (DRK) seeds can induce food poisoning, our focus was on this market class of bean. By ELISA assay, estimated concentration of active lectin in DRK was 223 ± 0.07 mg/g dry weight and the total amount of PHA contained in four seeds was 544 mg. Commercially cooked canned beans had >99% reduction in PHA (4.9 µg/g dry weight). Consumption of an entire can (1.5 cups, equivalent to 94 g dry matter) would equal 0.46 mg PHA which is approximately 1000-fold lower than the amount estimated to be associated with food poisoning. It is hoped that this report stimulates continued interest in standardizing methodology across laboratories and in setting standards of identity for active PHA in bean products.