At the moment, biofuel is mainly produced from high-value raw materials, such as rapeseed, corn or sugar beet. Fuel therefore competes with food production. In addition, large areas of agricultural land are required for cultivation. The production of biodiesel is therefore viewed critically. A more sustainable alternative is to use yeast cells to produce fat from cellulose waste, which can then be converted into biodiesel.
In a project funded by the Austrian Science Fund FWF, a research group from Graz is currently studying how fat can be produced for the production of biodiesel using genetically modified yeast cells. Possibly they can even excrete biodiesel directly. The research is led by molecular biologist Klaus Natter.
High lipid content
Natter and his group benefit from the fact that yeast cells have already been studied very well. This certainly applies to baker’s yeast and oily yeasts. Baker’s yeast is widely used in foodstuffs such as bread, beer and wine. Research has been done since the middle of the last century. The DNA has been fully mapped and almost all of the metabolic processes in the cell are known. Oil or fat yeasts are only recently in use, but are interesting because of the high lipid content.
Fatty yeasts retain in natural state up to 20 percent of their total weight in fat, baker’s yeast between five and ten percent. Via genetic modification, the fat levels can be increased up to between 50 and 60 percent, according to computer models and practical experiments of the research group. Surprisingly enough, that turns out to be possible for both types of yeast. This makes baker’s yeast the most promising candidate for the production of lipids.
Natter emphasizes that the modified yeast cells are viable and only grow at a slightly slower rate than their natural counterparts. But still ten times faster than the only serious competitor in terms of future biodiesel production: algae. Algae are attractive because they can convert sunlight and CO2 directly into fat. However, large areas are required for cultivation and they grow much slower.
Diesel secreting cells
There are still a number of obstacles for the industrial implementation will become viable. “In order to make the process sustainable, waste products should be used as a nutrient for these yeasts,” says Natter. Cellulose that is broken down by enzymes would be a good candidate. This can be obtained, for example, from agricultural residues. Collecting them must be economically feasible.
An important aspect is how the fat can be removed from the cell. The extraction of fat drops from the yeast cell is a complex process. It would be easier if the cells could be adjusted so that they secrete fat themselves. Alternative approaches are even aimed at allowing the yeast cells to produce and excrete biodiesel directly.
Although according to Natter this is possible in principle, the yield will be much lower than with fat storage in the cell. Further research is needed to better understand these processes. “Fundamental research into the production of fat from yeast has already reached a very advanced stage,” confirms Natter. “Now it’s time to put it into practice.”