The Effects of Dietary Raw and Heat-Treated Cowpea (Vigna unguiculata) on Growth and Intestinal Histomorphometry of Pigs

In vitro digestion of tropical legume starch is influenced by the combination of heating and soaking treatments

Raw legumes have in general low starch digestion, which limits their use in animal nutrition. This study aimed to determine the effect of different thermal (raw, autoclaving and boiling for 5 and 20 min) and soaking (without or with) treatments on the in vitro degree of hydrolysis (DH) of starch for selected grains of tropical legumes (Canavalia brasiliensis, CB; Lablab purpureus, LP; pink, red and white colour hulls Vigna unguiculata, PVU, RVU and WVU). Indigested residues collected after in vitro sequential pepsin-pancreatin (120–240 min) digestion were used to determine the starch DH and the estimated digestible starch content. The starch content was in general higher when the legumes were treated (e.g. autoclaving increased 28% the starch content of CB). Autoclaving and boiling for 5 min increased the DH for both unsoaked and soaked CB and RVU (+6–16% units), whereas they reduced the DH for unsoaked boiled LP, PVU, WVU and unsoaked autoclaved LP (P < 0.05). Extending the boiling from 5 to 20 min reduced the DH for CB, PVU, RVU and WVU (–8–22% units), whereas it increased 17% units the DH of LP (P < 0.05). In contrast, extending the autoclaving increased DH for soaked LP, PVU and WVU (+4–13% units) (P < 0.05), but did not affect the other legumes (P > 0.05). Similar trends were observed for the estimated digestible starch content, despite the changes in starch content. In conclusion, the effect of soaking and heating treatment combinations on the DH of starch and estimated digestible starch content varied among legumes. Conditions for starch determination may be adapted for legumes.