The oca or oka is a perennial plant grown in the central and southern Andes for its starchy edible tuber, used as a root vegetable. Its leaves and young shoots can be eaten as a green vegetable as well. Introduced to Europe in 1830 as a competitor to the potato and to New Zealand as early as 1860, it has become popular in that country under the name New Zealand yam and is now a common table vegetable.
The flavor is slightly tangy, and texture ranges from crunchy (like a carrot) when undercooked, to starchy or mealy when fully cooked. Though the original Andean varieties are widely variable in color from purple to yellow, the standard NZ variety is a fleshy pink.
Ocas need a long growing season, and are day length dependent, forming tubers when the day length shortens in the fall. In areas with harsh winter climates, the cold weather that accompanies shorter days may kill the plant before tubers have a chance to form. Likewise in tropical areas where the days are uniformly longer, the oca will not set a crop successfully, since the days are never short enough.
Ocas are fairly high in oxalates, concentrated in the skin, and traditional Andean preparation methods were geared towards reducing the oxalate level of the harvested vegetable. This is done by exposure to sunlight which increases the glucose content and sweet taste of the oca. Recent oca cultivars have a lower oxalate content, and have also been selected for more flexibility in day lengths.
The oca can be prepared like most root vegetables by being boiled, baked or fried. In the Andes it is part of stews and soups; served like potatoes or can be served as a sweet. Oca is eaten raw in Mexico with salt, lemon and hot pepper.