Food prices have been declining since 2011; when these guys wrote the article it made sense based on the trend at the time, but prices have backed off a bit since then.
If you look at the nations where food riots took place, they are almost without exception places where the people spend more than 35% of their income on food, with many of them spending 40-45%. So when their food prices double, its really life and death for them.
The other side of that coin is, I don’t see food riots erupting in nations where the food budget is less than 20%. That comprises the industrialized western world.
What makes food prices jump? My guess is crude oil:
More statistics on the various nations involved here: http://mdbriefing.com/food.shtml