Summary

Bioethanol is currently the most important biofuel. It is being produced by sugar fermenting yeasts as a metabolic product, basically by the same process as for beverage alcohol production. The sugar source for the fermentation may originate from different plants or parts thereof:

1. Sugar containing plants: e.g. sugar cane, sugar beet, sweet sorghum
2. Starch containing plants: e.g. maize, wheat, rye, triticale, cassava
3. Cellulose containing plants: herbaceous plants, agricultural residues, wood

The technology for the processing of these plants depends mainly on the type of feedstock that is being used. Different technologies for pretreatment are used, in order to prepare the raw material for fermentation. They have in common that they provide a fermentable sugar solution to the fermentation step. While pretreatment of juices from sugar containing plants is mainly focussed on removal of impurities and disinfection, starch and cellulose containing raw material needs to be broken down to single sugar molecules. This preparation usually involves a combination of heat treatment and enzymatic treatment.

The sugar solution is then fermented, usually by the yeast Saccharomyces cerevisiae. Since the concentration of ethanol that can be reached in the fermentation is about 10% after the, ethanol has to be separated from the remaining stillage. This separation step is achieved by distillation, which is also common to all production facilities. Due to the achievable ethanol concentration in the broth, the amount of stillage is approximately 10 times that of ethanol. Depending on the feedstock and the local framework, different options for stillage utilization have been developed. For grain based ethanol production the stillage is usually used for fodder production. For sugar based ethanol production, recycling of stillage to the field as a fertilizer is quite common. An alternative utilization is the anaerobic digestion to obtain a biogas. The remains of the anaerobic digestion will still have to be disposed and can be used as fertilizer. A sound concept for the utilisation of the stillage is of eminent importance for the producers of bioethanol.

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