Aberia caffra.

Botanical name: 

Aberia caffra Harv. & Sond. Bixineae (Flacourtiaceae). Kai Apple. Kau Apple. Kei Apple.

South Africa. The fruits are of a golden-yellow color, about the size of a small apple. They are used by the natives for making a preserve and are so exceedingly acid when fresh that the Dutch settlers prepare them for their tables, as a pickle, without vinegar.

Sturtevant's Edible Plants of the World, 1919, was edited by U. P. Hedrick.