Engineering Yeast Can Make Biofuels Commercially Viable
by John Timmer (Ars Technica) …Cellulose is a very long polymer of sugar molecules. Yeast will happily use those sugars to produce ethanol, but it can’t get at them while they’re trapped in cellulose. As a result, a suite of enzymes from other species are first used to break the cellulose down into individual sugars, a step that adds to the cost and complexity of producing the ethanol. A Berkeley-Tianjin collaboration has now cut down on the steps required to get yeast feeding on cellulose.
The initial step in digesting a cellulose molecule involves enzymes that break the long chains into short pieces; these short pieces are later broken down into individual sugars that the yeast can digest. A different fungus, Neurospora crassa, grows happily on cellulose, in part because it can import the short pieces of cellulose and break them down inside the cell. The authors identified the genes responsible for both the import and chemical reactions, and transferred them into yeast cells. READ MORE and MORE (MIT’s Technology Review) Abstract
From Ars Technica Comments: There’s still a lot of issues with the whole cellulose to ethanol plan:
- Separating the ethanol from the fermentation broth. Distillation the current standard process is energy intensive, and fermentation broth would foul up a column quite nicely. Other technologies like pervaporation aren’t there yet.
- Dealing with the issue of revertants
- Having an organism that produces ethanol at a reasonable rate and concentration. This work has only looked at yield
- Separating the cellulose from the hemicellulose and lignin in the plant matter. If you don’t use the other components you’re throwing away ~40% of the plant.
- Some of the above compounds (lignin in particular) can be inhibitory to enzymes/organisms. Separating them out cleanly and cheaply is not trivial.
- Processing the plant matter to the point where it’s suitable to throw into a fermenter. It sounds stupid, but grinding the stuff up isn’t a trivial problem.