The United Nations has just reported that its index of food prices had reached its highest level in the 20 years of tracking. And now some experts are suggesting that the growing demand for biofuels - fuels made from plant material - is helping drive that increase.

Around the world, governments are urging and often mandating that a larger percentage of the fuel supply be comprised of biofuels. In the United States, where government policies help subsidize biofuels created from corn, almost 40 percent of the corn crop is now destined for fuel use. While many other factors, such as weather patterns and fuel prices, can impact crop prices, that rise in demand undoubtedly played a role in the whopping 73 percent jump in the price of corn in the last half of the year.

Other countries, and other traditional food crops, face similar and even greater pressure form the biofuel boom. The New York Times reports that China has turned to the cassava root as a biofuel source, gobbling up 98 percent of Thailand's cassava root crop last year and contributing to a doubling of the root's price.

It's a real quandary. Biofuel has been positioned as an environmentally-more-friendly alternative to traditional fossil fuels, but at what point do concerns about its impact on food prices force a rethinking of the biofuel movement?

The Times article suggests that such a reckoning is already happening. China's appetite for cassava root took off after that government outlawed the use of grain in biofuels. That edict was a response to food shortages and price increases that threatened the country's food supply.