Although organic acids are welcome in the mixed-acid fermentation pathway, the opposite is true for lipid accumulation, a second pathway van Walsum researches.
The pathway starts again with liquid extract, but here it is concentrated and cleaned of organic compounds — additional steps that add to the cost of the process. The microbes that are key to the pathway prefer a clean foodstock without organic matter.
UMaine researchers use a strain of microbes developed at the Massachusetts Institute of Technology.
In their normal life cycle, the microbes grow and multiply, then they start over again. But as lipid organisms, the microbes actually have a different phase imposed on them. In the first part of their life, they grow; in the second, researchers change their nutrient mix so that the microbes start to store energy and accumulate lipids, or fat molecules.
“That’s what we want them to do,” van Walsum says. “They don’t want to do that. They’d rather keep multiplying because it’s in their interest to multiply and propagate their DNA. So we let them grow for a little while, but then we cut off the nutrients they need to make more DNA so they can’t make more cells. But they can store energy so that when the food disappears, they can survive longer. It’s a survival strategy.”
When the microbes have stored enough fat molecules, a stage known as lipid fermentation, their cell bodies are separated and ruptured. While the cell bodies themselves are waste and could be used as compost or a similar nutrient source, the accumulated lipid — similar in appearance to vegetable oil — is the basis for biofuel.
With hydrogen upgrading, the lipid oil takes on the properties of jet fuel.
“Hydrogen upgrading is the downstream part, and it’s relatively easy,” van Walsum says. “The challenge has been to get the microbes to grow quickly and without having to clean up everything too much. So far, we’ve only fed the microbes very clean, very well-treated extracts. And the act of cleaning it up to get all the different contaminants out probably makes it too expensive to be viable. But we like it because it’s been demonstrated to make jet fuel.”
There may be ways to bring down costs for lipid accumulation. Van Walsum says the microbes are very adaptable and because of that, researchers may be able to find ways to make the process go faster. There are also cyclic fermentation processes van Walsum has done in the past that can increase productivity two to three times.