In Africa, food scientists say several species of the custard apple family could help provide alternative foods in times of scarcity. 

One of them, the junglesop tree [Anonidium mannii], may be medium-sized, but not its fruit: it can be as long as a person's forearm and, at up to six kilograms, about as thick as a human leg. The junglesop is  a tree common to Central Africa.

The tough and leathery brown skin of the fruit yields to soft yellow-orange flesh that can be either sweet or sour, depending on the maturity of the seed and the genetics of the tree it came from. The fruit can be an acquired taste, so rich in flavor that food scientists say some people can only eat a little at a time.

It's much enjoyed by the people of the Central African rainforest, who call it "bobo." For the pygmies, it provides an extra source of calories.

Roy Danforth is an Evangelical Covenant Missionary who (with his colleague Paul Noren) been documenting the junglesop and other fruits for three decades:

"The pygmies know all about the junglesop and absolutely love the fruit," says Danforth.

"There's no need to promote it among them. They know what time of year they produce; they are an excellent resource on which trees to go to.

"The junglesop," he continues, "needs research [for example, on how to create] improved varieties by grafting genetic material from the trees with fruit that is seedless or [sweet] tasting all the time. Only 30 percent [of the trees produce fruit that tastes good]: there is a balance between the sourness and sweetness of the fruits; some are almost all sour. But the ones that are sweet [have] an excellent flavor that rivals a peach."

But it may be years before junglesop is grown on a large scale. Agriculturalists say work is needed in learning how to fight the fungal diseases that affect the trees. Danforth says scientists also need to develop plants that can produce fruit within years. He says some that he has developed have not produced fruit in two decades. 

Food scientists say the junglesop is just one of several varieties of custard apple fruits that are grown in Africa but have been ignored by Western food scientists.

In southern Africa, another is the yellow or reddish colored groundsop [Annona stenophylla], a fruit who pumpkin colored flesh is eaten both raw and cooked.

Villagers in other parts of the region make jam from the pulp of the small scarlet fruits of a shrub called baboon's breakfast [Hexalobus monopetalus]. Seeds from the tree, which grows well in both dry and moist climates, make a spicy condiment. 

Other neglected fruit of the custard apple family is the bright red dwaba-berry [Monanthotaxis caffra] of northern KwaZulu Natal in South Africa and the scarlet-colored fruit called monkey-fingers [Friesodielsia obovata], which are said to look like sausages hanging from the tree. The fruit can be stewed or made into jelly and wine.

Monkeyfingers, dwaba-berries, junglesop and groundsop are just a few of indigenous fruit documented in the series Lost Crops of Africa by the National Research Council.