Thanks for the interesting example of the time taken in the preparation of cassava leaves. In the case of cassava leaves, grinding has a purpose, as it removes cyanogens, which are often present in high concentrations.
The grinding thus prevents cyanide poisoning.
A recent report of a simpler preparation method also refers to the literature on nutrient content and preparation of cassava leaves:
Bradbury JH, Denton IC (2011) Mild methods of processing cassava leaves to remove cyanogens and conserve key nutrients. Food Chemistry 127: 1755-9.
Cassava roots may also contain high concentrations of cyanogens.
For both cassava roots and leaves, removal of cyanogens is an important part of food preparation.
Links and resources:
Indigenous Peoples’ food systems: the many dimensions of culture, diversity and environment for nutrition and health
Comparative Assessment of Indigenous Methods of Sweet Potato Preservation among Smallholder FArmers: Case of Grass, Ash and Soil based Apporoaches in Zimbabwe
FSN Forum discussion: Looking back to effective rural practices ... Did we miss something?