Steam-Cooking and Dry Heating Produce Resistant Starch in Legumes

Starch was isolated from either raw or steam-heated black, red, and lima beans. Isolates from steam-heated legumes were rich in indigestible (resistant) starch (19-31%, dmb), a fact not observed when raw seeds were used. Similarly, resistant starch measured directly in conventionally and high-pressure steamed beans was 3-5 times higher than in the raw pulses, suggesting retrogradation as the major mechanism behind the reduction in digestibility. Thus, steam-heating comes forth as an effective way to produce resistant starch in legumes. Prolonged steaming as well as short dry pressure heating decreased the enzymatically assessed total starch content of whole beans by 2-3% (dmb), indicating that these treatments may induce formation of other types of indigestible starch.