In this section, we will discuss different fermentation processes appli­cable for ethanol production. Fermentation processes, as well as other biological processes, can be classified into batch, fed-batch, and contin­uous operation. All these methods are applicable in industrial fermen­tation of sugar substances and starch materials. These processes are well established, the fed-batch and continuous modes of operation being dominant in the ethanol market. When configuring the fermentation process, several parameters must be considered, including (a) high ethanol yield and productivity, (b) high conversion of sugars, and (c) low equipment cost. The need for detoxification and choice of the microor­ganism must be evaluated in relation to the fermentation configuration.

Presentation of a variety of inhibitors and their interaction effects, e. g., in lignocellulosic hydrolyzates, makes the fermentation process more complex than with other substrates for ethanol production [17, 21].

In fermentation of this hydrolyzate, the pentoses should be utilized in order to increase the overall yield of the process and to avoid problems in wastewater treatment. Therefore, it is still a challenge to use a hexose-fermenting organism such as S. cerevisiae for fermentation of the hydrolyzate.

When a mixture of hexoses and pentoses is present in the medium, microorganisms usually take up hexoses first and produce ethanol. As the hexose concentration decreases, they start to take up the pentose. Fermentation of hexoses can be successfully performed under anaerobic or microaerobic conditions, with high ethanol yield and productivity. However, fermentation of pentoses is generally a slow and aerobic process. If one adds air to ferment pentoses, the microorganisms will start utilizing the ethanol produced as well. It makes the entire process complicated and demands a well-designed and controlled process.