African yam bean

African yam bean is a climbing, twining perennial legume grown as an annual, with trifoliate leaves, purplish-pink flowers and long, slender pods that can reach 30 cm. The pods hold up to about 30 small seeds, which are smooth, rounded to kidney-shaped, and range from cream to brown, speckled or mottled. Underground, the plant forms small, edible tubers in some varieties.

The seeds have a pleasant, mild bean flavour but are notoriously slow to cook, a trait known as hard-to-cook. They are sold dried and whole in West African markets and used cooked whole, ground into flour or paste, or boiled and added to stews.

Sphenostylis stenocarpa is native to tropical Africa and was domesticated in Africa, with the greatest diversity found in West Africa, especially Nigeria, as well as in Central and East Africa. It has long been grown on small farms, often in mixed cropping systems with yams and cereals, and sometimes staked on poles or trees.

The genus name Sphenostylis is Greek for wedge-shaped style, describing a feature of the flower, and stenocarpa means narrow fruit, for the slender pod. The common name refers to the plant's edible tubers, which are used like small yams.

African yam bean is part of a group of traditional African crops that have been grown for generations by local farmers but were largely overlooked by commercial agriculture and international research. In Nigeria, it is known among the Igbo as azama and the Yoruba as sese, and cooked seeds are used in dishes eaten at feasts and festivals. In other regions, cooked tubers are eaten as a vegetable.

Because the seeds need long cooking times and the crop is slow to mature, it was displaced in many places by more convenient legumes such as cowpea. This hard-to-cook trait, along with long growing time and uneven yields, has limited its spread.

Since the late 20th century, research centres including IITA, the International Institute of Tropical Agriculture in Nigeria, have collected and characterised African yam bean varieties, and there is renewed interest in it as a nutritious, climate-resilient neglected crop that could improve diets and farm resilience. Efforts focus on shortening cooking time, improving yield and keeping genetic diversity.

Boiled seeds are eaten whole as a main dish or mixed with other foods, in stews and soups, or ground into flour for porridge and cakes in Nigeria and neighbouring countries. Seeds are sometimes fermented or sprouted before use.

The tubers can be boiled or roasted, and young pods and leaves are used as vegetables in some areas. The plant also fixes nitrogen, improving soil for following crops, and its foliage can serve as fodder. Cooked seeds are often combined with maize or plantain in one-pot meals, which helps balance the amino acids of cereal-based diets.

The seeds are a rich plant protein source and contain fibre, minerals and B vitamins, so they could help diversify diets in regions where animal protein is scarce. Like other pulses, they contain antinutrients such as trypsin inhibitors, tannins and phytates, which are reduced by soaking, long cooking, fermentation or sprouting. Human health trials on this specific legume are lacking.