In this study the main focus was on production of bioethanol from rape straw, but there are a lot of other possible feedstocks suitable for bioethanol production in general, all cellulosic material can be converted by physical/chemical pretreatment followed by enzymatic hydrolysis into glucose. It is also possible to produce bioethanol from sugar and starch from crops such as corn, wheat, sugarcane, and sugar beet, but since sugar is a food source, using it could decrease food availability for future generations. Using food sources or available agricultural land for pure energy production is generally classified as a 1st generation technology, and is not normally regarded as a sustainable energy source. To compare the different feedstocks Table 4 is produced. However, there are numerous ways to co-produce food and feedstocks for bioenergy when utilizing the lignocellulosic residues from agricultural production as shown in table 4.

As table 4 shows, bast fibers have a very high cellulose content (60-63%) and a low lignin content (3-4 %) which should make them ideal for producing bioethanol, but bast fibers as a feedstock would fall under the category of 1st generation bioethanol, because the production of bast fibers requires land that could otherwise be used for food production. Rape straw has a low cellulose content compared to other straw fibers (32%) but in return the hemicellulose (14%) and lignin (18%) content is also low compared to wheat straw (20%) and corn stover (19 — 21%). Low lignin content is good for the enzymatic hydrolysis, since lignin can denaturize cellulase enzymes (Thygesen et. al 2003). Low hemicellulose content will result in a slightly lower concentration of microbial growth inhibitors derived from oxidation of the hemicellulose. Sugarcane bagasse seems to be the ideal 2nd generation feedstock with its high cellulose content (43%) and low lignin content (11%) but sugarcanes require high temperatures and a lot of rainfall to grow and are therefore only energy efficient when grown in tropical regions, which limits the amount of ethanol produced from sugarcane bagasse worldwide. It is possible to produce bioethanol from wood fibers, like waste wood from carpentry or

willow, which can grow on land not suitable for agriculture, using pretreatment methods such as steam explosion (Soderstrom et. al. 2002). The high lignin content in wood fibers increases the amount of enzymes needed and the time period of the fermentation. Furthermore wood fibers have other uses and can easily be burned to produce electricity and heat in a cogeneration plant.

Marine biomass has the advantage that it does not use the same space as agriculture and even though it is not a waste product from food production it is still a viable feedstock because it does not reduce food availability. Chaetomorpha linum has very low lignin content (6 — 8 %) and cellulose content similar to straw fibers (34 — 40 %). C. linum and other types of useable macroalgae are easy to grow in most of the world, and is therefore a suitable candidate for expanding the bioethanol production to more than what can be obtained from waste products (Schultz-Jensen et. al. 2011).

4. Conclusion

The amount of yeast needed for SSF of pretreated rape straw is dependent on the DM content, despite the fact that enzymes continue to be the primary rate-determining factor. The positive feedback effect from the yeast lowering the sugar concentration can have high relevance when running SSF with high DM content. After prolonged testing of Turbo yeast, the optimal temperature of the SSF is found to be 37°C. Furfuryl alcohol and possibly other growth inhibitors as well, show a positive effect on the rate of fermentation when added in small dosages, since yeast will increase its metabolism under stress. The positive effect of growth inhibitors is so strong that the fermentation rate in sugar media is lower than the fermentation rates in a medium produced from wet oxidized rape straw (filter cake), given the DM concentration does not exceed critical levels.

5. Acknowledgement

The Danish Research Council, DSF is gratefully acknowledged for supporting the research project: Biorefinery for sustainable reliable economical fuel production from energy crops (2104-06-0004). The European Union is acknowledged for supporting the EU-project: Integration of biology and engineering into an economical and energy-efficient 2G bioethanol biorefinery (Proethanol nr. 251151). Efthalia Arvaniti is acknowledged for academic advice. Tomas Fernqvist, Ingelis Larsen and Annette Eva Jensen are thanked for technical assistance and HTX Roskilde for providence of microscope cameras.