Renewable jet fuel is green and orange-scented

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Biochemical engineer Tim Brennan is conducting research into a sweet-smelling bio-jet fuel.

The hunt is on for alternatives to fossil fuels. Electric cars are taking off. This technology won’t fly for commercial aeroplanes until batteries become much lighter – but renewable fuels might work.

At the University of Queensland I have been working with Boeing and Virgin Australia on a project that makes use of the wonderfully versatile baker’s yeast (think of fermenting beer and rising dough) to brew up bio jet fuel. In this case the yeast is genetically engineered so that when we feed it sugar, it makes a chemical called limonene. 

Limonene is responsible for the distinctive scent that hits you when you peel an orange. It can also be readily turned into a biofuel with great jet fuel properties – including high energy density and a low freezing point. The fuel has already been proved in a test flight.  As a bonus, it smells great. 

Part of our team takes limonene-forming genes from citrus-scented plants such as oranges and lemons and transfers their DNA into yeast. But there’s a problem: yeast finds limonene to be extremely toxic. We won’t ever make bulk quantities of bio jet fuel if the fuel kills the yeast in the process. That’s where I come in.

My research is focused on overcoming the toxicity issue. I have tried two very different approaches.

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Renewable fuels might hold the answer to environmentally sustainable air travel. – istockphoto

First, I redesigned the yeast’s habitat to remove the limonene as soon as it’s produced.  The yeast grows in a “bio-reactor” which is a watery medium.  Limonene is an oily substance, so my trick was to add another oil into the reactor that is non-toxic to the yeast. It forms a separate layer floating on the water’s surface. Limonene moves into the oil layer as fast as the yeast can produce it. By continually siphoning off the oil layer, the fuel is immediately removed from the reactor and recovered.

Second, for something completely different, I used biology’s oldest tool – evolution – to find a genetic mutation that helps the yeast cells tolerate higher levels of limonene. 

Combining these approaches, we successfully increased the fuel production capability of our bio-reactor 700-fold! The next step will be to genetically engineer yeast strains that produce more limonene.

Papers: Alleviating Monoterpene Toxicity Using a Two-Phase Extractive Fermentation for the Bioproduction of Jet Fuel Mixtures in Saccharomyces cerevisiae. Biotechnology and Bioengineering, 2012, vol. 109, p2513-2522 / Brennan, T. C. R., J. O. Krömer, and L. K. Nielsen. 2013. Physiological and transcriptional responses of Saccharomyces cerevisiae to d-limonene show changes to the cell wall but not to the plasma membrane. Appl. Environ. Microbiol. 79:3590-3600.

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