Experts estimate that our planet has only enough fossil fuel energy reserves left for up to 200 years. Alternatives are therefore urgently being sought. Biofuels are already being produced that could in future cover up to 20% of our fuel requirements. Although fuels made from renewable resources are in theory a good alternative to petroleum, they are criticised for potentially reducing food supplies when produced from feedstocks such as grain or sugar beet used for food production. However, producing biofuels from resources not suitable for human/animal consumption, and that do therefore not exacerbate the food/energy dilemma, is still costly and uneconomical. Prof. Dr. Ralf Kölling, a biotechnologist from the University of Hohenheim, and his team of scientists are working on a new, continuous method to produce bioethanol efficiently that could potentially overcome current drawbacks in biofuel production.
Bioethanol is a fuel made from renewable resources that is considered climate friendly and hence a good alternative to fossil fuels. Nevertheless, bioethanol production is still highly controversial. This is because the production of so-called first-generation bioethanol involves fermenting starch components of crops like grain and sugar beet, which could put food and fuel in direct competition for land and resources. However, using any other kinds of raw material that is not used for human/animal consumption as feedstock for fuel production makes the process uneconomical.
Prof. Dr. Ralf Kölling and his team of researchers from the Department of Yeast Genetics and Fermentation Technology at the University of Hohenheim are working on methods to produce second-generation biofuels in order to find a rapid solution for the problems related to bioethanol production. The biotechnologists are planning to develop a continuous method involving genetically modified yeasts that makes bioethanol production considerably cheaper than it currently is. This innovative method uses cellulose as feedstock, and, therefore, does not compete with plants used for food and animal feed. Second-generation bioethanol is produced from different raw materials than those used for first-generation biofuels. These new raw materials are based on cellulose-containing residues from straw, green plants, wood or other plant waste.
The University of Hohenheim department draws on a long tradition of more than 30 years of scientific expertise in the field of bioethanol research. Initially, the researchers used starch from grains or maize, which, in contrast to cellulose, is easily accessible and can subsequently be fermented into bioethanol by yeasts. “However, we consume these plant components and do not want to use them for fuel production,” says Kölling. “This is why we are now trying to use plant components that cannot be metabolised by humans and animals. Plant stems also contain sugar molecules, but they are not connected in the same way as glucose units in starch. Cellulose is found in the cell walls of plants, and lignin fills the spaces in the cell wall between cellulose. Lignin cannot be used for producing bioethanol but it can be used for producing combustibles. As lignin crosslinks different plant polysaccharides, it confers mechanical strength on the plant, and thus also requires methods to isolate the sugars that are different from those used for fermenting ethanol. “We need specific enzymes, but these are very costly. The production of second-generation ethanol cannot yet compete with that of first-generation ethanol,” concludes the biotechnologist.
The standard methods used for producing cellulosic ethanol work as follows: the plant material is pre-treated under high pressure, at high temperatures or using acids. In a second step, glucose is produced by enzymatic hydrolysis of cellulose involving cellulase enzymes. “This second step has a major impact on production costs,” says Kölling. “The enzymes are expensive and are marketed by big companies.” The glucose obtained is then fermented into bioethanol using yeasts.
The scientists from Hohenheim now plan to develop a completely new method for efficiently producing bioethanol. The project is funded by the Agency for Renewable Resources (FNR) and other organisations and will initially run for three years. “We are working on a continuous method in which the second and third steps will run simultaneously,” says the biotechnologist. The actual yeasts used for the process will produce the enzymes required. The scientists will use genetic engineering methods to modify yeast cells and make them present multi-enzyme complexes on their surface. Such complexes are called cellulosomes. They contain all the enzymes required for digesting cellulose. “They are like connector strips in a socket outlet into which the enzymes can be plugged when they are needed,” explains the professor. "The yeasts can then dock to the cellulose and degrade it into glucose as they start to make bioethanol. The great advantage of this is that many enzymes are present in one place and we can use the synergy effects for our method."
The biotechnologists are currently working on expressing cellulases from different cells in the yeasts. In addition to molecular research work, a group of researchers led by PD Dr. Thomas Senn from the same institute is concentrating on the technical implementation of the process. The group is working on the construction of a reactor as a continuously operating system, in which several biological events take place in a single bioreactor. This technology is known as consolidated bioprocessing (CBP). Creating a CBP bioethanol reactor to convert lignocellulose into the desired products in one step, is one of the primary objectives of the researchers from Hohenheim. “We want our bioethanol reactor to be as simple as possible, a system where we feed in the starting material on one side and remove the ethanol on the other,” says Kölling. The researchers aim to complete a prototype of a continuously operating bioreactor by the time funding comes to an end in three years.
Other institutes around the world are also working on ways to produce larger quantities of second-generation biofuel. “Quite a lot has happened in this field in the USA where huge bioethanol production plants have been built,” says Kölling. “However, these plants are not yet operative, and consequently only small quantities of cellulosic ethanol are produced. I presume that cellulosic ethanol production has been delayed by huge technical and other challenges. But details are not known. That said, the process is very sophisticated and not at all commercially viable. We, therefore, hope to be able to come up with a process that is both technically simple and economical.”