Enhancing the Iron Nutrition of Beans: Utilizing the Slow and Non-Darkening Traits to Reduce Proanthocyanidins in Multiple Market Classes of Dry Beans

Objectives: Reducing proanthocyanidin concentrations (inhibitors of iron absorption) in bean seed coats via alleles in the non-darkening (j gene) or slow darkening (Psd) gene will enhance iron (Fe) bioavailability across multiple market (color) classes of dry beans. Methods: Dry beans (Phaseolus vulgaris L.) with slow-darkening (SD), non-darkening (ND), and regular darkening (RD) traits were grown at research sites in North America (U.S. and Canada) and Africa (Zambia). The Caco-2 cell culture bioassay was used to determine the iron bioavailability of cooked beans. Mineral analysis was conducted by ICP-ES and phytate measured with a Megazyme™ kit. Flavonoids ( >25 compounds) were measured using a newly developed Acetone-Methanol (acidified) extraction methodology before analysis with UPLC/MS. Results: SD pinto beans from North Dakota provide 2-7x more bioavailable iron than RD pinto varieties. Iron and phytate concentrations were similar between SD and RD pinto beans, however, flavonoid analysis revealed 4x more proanthocyanidins were detected in RD pintos after cooking. Similar findings were demonstrated in ND yellow, cranberry and purple beans produced in North America and Africa. The absence or low levels of proanthocyanidins in the seed coats of ND beans correlated with 5-7x more bioavailable iron than RD beans. The greatest enhancement was observed in the yellow bean market class with a ND variety exhibiting 200% of a white kidney bean reference control. There was no relationship between iron bioavailability, phytate or iron concentrations in cooked beans. However, there was a strong (P < 0.001) association between iron bioavailability and proathocyanidins across all market classes. Conclusions: In addition to a brighter seed coat appearance after storage, this study reveals that downregulating the synthesis of proanthocyanidins with the slow darkening (Psd) or non-darkening (j) gene could be a novel, as well as a sustainable strategy to improve the iron bioavailability of dry beans, especially those susceptible to darkening, including yellow, pinto, purple and cranberry beans. Funding Sources: U.S. Department of Agriculture, Agricultural Research Service.