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REPLACING carbon-rich fossil fuels with more environmentally friendly alternatives should slow global warming. As part of that drive, both America and Europe have embraced biofuels—liquids derived from plants that can be used to power cars and other vehicles. By their very nature, biofuels cannot be carbon-free because carbon is essential to life on Earth. Burning biofuels does indeed release carbon dioxide, a greenhouse gas. The attraction is that the volume of gas released exactly matches that taken up by the plant when it was growing. So overall (and with the huge proviso that you do not count the fossil fuel used to farm the stuff) biofuels are carbon-neutral.

In America the Department of Energy has set a target for 30% of the 2004 gasoline demand for vehicles to be met by biofuels by 2030. The European Union wants 25% of transport fuels to be derived from biofuels by the same date. At present, the most widely used substance is ethanol, which can be made from sugar cane, sugar beet and maize (or corn, as it is called in America). But ethanol does not pack a particularly powerful punch. It is also susceptible to absorbing water, further diluting its oomph. It takes days to ferment the stuff. A biofuel that did not suffer from these limitations would be welcome.

That is what a team led by James Dumesic of the University of Wisconsin-Madison claims to have developed. The researchers think they have devised a biofuel that has a 40% higher energy density than ethanol, that repels water and that can be made relatively speedily.

One of the most frustrating aspects of biofuels is the stark contrast between what exists in nature and what you can put in the tank. Plants are rich in carbohydrates, a group of organic compounds based on carbon and water, itself a combination of hydrogen and oxygen. These carbohydrates take the form of chains of thousands of sugar units; each unit contains six carbon atoms and a similar number of oxygen atoms. An ideal fuel, on the other hand, should lack oxygen. Its molecules should also be small, that is, they should contain few carbon atoms. Creating an efficient fuel from plants thus presents a headache.

To date, scientists have approached the problem by taking one of two routes. The chemists have tinkered with heat and metal catalysts to refine their materials. Biologists, meanwhile, have recruited enzymes and microbes to do the job.

The team led by Dr Dumesic combined the two methods. It was thus able to break down long carbohydrate chains to form small, useable molecules while simultaneously removing the oxygen and maintaining the energy content of the biofuel. The researchers began by using enzymes to snip the carbohydrates into fragments that were then rearranged to form a sugar called fructose, which is found in fruits.

They doused the fructose in acid, which catalysed a chemical reaction expelling oxygen atoms as water molecules. The researchers immediately added a second catalyst and some hydrogen, which eliminated more oxygen. The result was a fuel called “2,5-dimethylfuran”. The process is written up in this week's issue of Nature.

The new biofuel can be made directly from fructose, which is present in fruits such as apples, pears, berries and melons as well as some root vegetables. It can also be manufactured from the large polymer chains found in cereals, grasses and trees. Perhaps the most promising method, from a biochemical point of view, would be to use glucose, a sugar common in food.

The resulting biofuel is not only energy-rich and water repellent but it also has a higher boiling point than ethanol. Keeping it liquid in a vehicle's fuel tank should therefore be straightforward.

But this biofuel shares a disadvantage with ethanol: its raw material is food. Ideally, biofuels would be made from waste farm products rather than crops. That way, the chaff could be used to produce biofuel for transport and the wheat could be used for people.

Unfortunately for science, nature conspires against this. Plants have evolved chemical and structural properties that make it difficult to break them down. One possibility would be to use genetic modification to create plants that are more amenable to such manipulation. Another is to use existing chemical or biological techniques, or to combine the approaches, rather as Dr Dumesic and his colleagues have done. Whichever works best, the second generation of biofuels is coming down the pipeline.