Jatropha plant nuts consist of 30 percent to 40 percent oil that can be converted into biofuel. Air New Zealand, and other commercial airlines, have been trying them in a biofuel blend.

Biomass used to make biofuels must be carefully sourced, or the biofuels they produce may be no greener than conventional jet fuel.

That's according to a study that was published this week in the online version of Environmental Science and Technology and was conducted by a group of scientists at the Massachusetts Institute of Technology.

For the nearly four-year study, researchers conducted a life cycle analysis on 14 diesel and jet fuel sources made from feedstocks, and identified the key factors that make a difference in whether a biofuel is truly an environmental improvement over conventional jet fuel.

The team was led by James Hillman, principal research engineer, and a professor at the Department of Aeronautics and Astronautics at MIT.

"What we found was that technologies that look very promising could also result in high emissions, if done improperly. You can't simply say a biofuel is good or bad--it depends on how it's produced and processed, and that's part of the debate that hasn't been brought forward," Hillman said in a statement.

Biofuels made from jatropha oil, for example, can have a low carbon footprint because the byproduct husks, shells, and meal from jatropha plants can be used for fertilizer, animal feed, and electricity generation.

But the study found that many biofuel carbon footprints are based on where and how it's grown. As other studies on jatropha biofuel have shown, land-use in producing the necessary biomass is often a large factor. It can have a larger impact than the energy it takes to process the fuel from biomass, according to Hillman.

Some of the concrete statistics garnered from the study back up common sense ideas long suspected to be true.

For example, palm oil-based biofuel made from palms grown on chopped-down rainforest land produced 55 times more carbon dioxide than the same biofuel made from palms grown on fallow cropland.

The study suggested that easy-to-grow algae or salicornia, neither of which requires nutrient-rich soil, might be more effective biomass options for biofuels than crops requiring acres and acres of farmland.

The study, which included the work of MIT graduate students Russell Stratton and Hsin Min Wong, was funded by the Federal Aviation Administration and Air Force Research Labs.