Danish scientists say a study of one biodiesel source plant -- the castor bean -- predicts problems if it's considered for large-scale use as a fuel source.

While countries like Brazil and India grow large quantities of castor oil beans that can be refined into bio-diesel, the beans contain allergens and also the extremely potent poison ricin, meaning bean pulp left after extraction of the oil cannot be used for animal feed and becomes a problematic waste byproduct.

"Therefore we are interested in finding out if it in some way could be possible to eliminate the allergenic proteins and the ricin from the beans, so that the pulp can be used for animal feed," said biochemist Peter Roepstorff at the University of Southern Denmark.

Roepstorff and a team of Danish/Brazilian colleagues have mapped thousands of proteins found in castor beans, with discouraging results.

While both ricin and the allergen 2S Albumin are only present in low amounts in the early development stages of the bean, they found, as the beans mature the content of ricin and 2S Albumin increases.

"Unfortunately, the mature beans also have the highest oil content and therefore an oil producer will not harvest the beans before they are fully mature," Roepstorff said. "The price of optimizing the oil production is that you also get a toxic and allergenic pulp that cannot be used for animal feed."

One option would be to eliminate the unwanted proteins by plant breeding, the researchers said. Another option is to remove them by genetic engineering.