Travel mystery #4

It's really difficult to extract the cyanide from cassava. You have to ferment, drain, dry and then cook...

 Sometimes you see the fermented, chopped pieces of cassava on plastic outside homesteads to dry, but in parts of Angola you see these roadside patches of drying cassava. 

You get a real sense of the effort and industry that goes into cassava eating.

OF course this raises another mystery. Who worked out this is how to extract cyanide from cassava. 

Lots of these on the side of the road
Angola

 

Hint