Like most legume seeds, Vigna subterranea seeds contain antinutritional factors such as trypsin inhibitors, phytates and tannins, including condensed tannins in the seed coat. Tannin content is correlated with coat colour, the cream-coloured seeds containing less tannin than brown or black seeds. Tannins can be deleterious to livestock performance though they have beneficial nutritional properties in some cases (by-pass protein in ruminants, anthelmintic effect).
Vigna subterranea seeds are reported to have higher antitryptic activity than soybean and the level of activity depends on the variety (landrace) (Tibe et al., 2007). Heat treatments such as boiling or roasting are usually effective in destroying trypsin inhibitors.
Phytates are found in high proportions in Vigna subterranea seeds and are known to reduce cation availability (Ca particularly) (Nwanna et al., 2005).
Cooking and other forms of processing (e.g. soaking, milling, hulling, germination, fermentation) reduce the concentration of antinutritional factors. However, processing does not always increase the feeding value (Nwanna et al., 2005; Oloyede et al., 2007).