Starch digestibility and morphology of physically modified jack bean (Canavalia ensiformis L.) seed flours

The effects of a drum drying treatment applied to jack bean (Canavalia ensiformis, JB) flour prior to roasting on starch morphology and digestibility were investigated. JB flours were assayed in both raw form and after mixing them with distilled water (1:1.5 flour:water proportion, w/v) followed by drum drying at 125 °C. The drum-dried flour was then roasted at 180 or 190 °C for 2 min starch morphology was determined by scanning electron microscopy (SEM) and in vitro digestibility of raw and modified flours were evaluated by enzymatic techniques. Additionally, a balance trial was conducted with 8 adult Rhode Island Red cockerels fed with the flours of each drum drying/roasting condition to evaluate the presence of indigestible starch granules in the feces collected over 72 h. Results showed no significant differences between total starch content of raw and modified flours. Potentially available starch content (starch that may be converted to glucose by sequential hydrolysis with α-amylase and amyloglucosidase) decreased from 310 g/kg to 250–280 g/kg on a dry matter basis, suggesting a process-related formation of retrograded resistant starch. In vitro α-amylolysis indicated that the overall digestibility increased after heat processing since starch digestion at 60 min reaction rose from 0.07 (raw flour) to 0.49–0.57 (drum-dried and roasted samples). SEM showed that modified flours contain larger amounts of fractured starch granules which made them more susceptible to digestive enzymes compared to the raw JB flour. The results indicate that drum drying used as a prior step to roasting improves starch digestibility and allows the use of lower roasting temperatures in an attempt to preserve the overall nutritive value of JB seeds.