7.1.3.1 Conversion of Saccharified Corn Starch into Ethanol

Yeasts are not able to metabolize the starch, so this polymer should be hydrolyzed before fermentation, as was discussed in Chapter 5. Most cereal ethanol is pro­duced from corn.

To start the fermentation, the corn mash is cooled until reaching the cultivation temperature, usually 30° to 32°C, and then the cells of the process microorgan­ism (generally S. cerevisiae) are inoculated. The corn mash is obtained by dry­milling technology and contains, besides the starch, all the other components of the grain (proteins, oil, fiber, etc.) that remain unmodified during the fermentation process. As in the case of molasses, the culture medium is also supplemented with urea or even with ammonium sulfate. Recently, some proteases are being added to the mash to provide an additional nitrogen source to the yeast resulting from the hydrolysis of corn proteins (Bothast and Schlicher, 2005). In most corn dry­milling plants producing ethanol, fermentation is accomplished in batch regime though milling, liquefaction, and saccharification as well as the subsequent distil­lation and dehydration, which are performed in a continuous regime. To ensure a continuous flow of materials, three or more fermenters are used in such a way that at any given moment one apparatus is an filling-up with the mash, the starch is fermented in another, and a third one is being unloaded and prepared for the next batch. Fermentation time is about 48 h. At the end of fermentation, the culture broth (fermented mash or beer) is sent to a storage tank from which the constant supply of beer to the distillation step is guaranteed.

In the case of ethanol production plants employing the corn wet milling, the fermentation is mostly carried out in a continuous regime. The technology is called cascade fermentation and is applied in large-scale facilities. Thus, a con­tinuous cascade saccharification process is also employed. The effluent of this system feeds not only the bioreactors for yeast propagation, but also the prefer­mentation and fermentation trains. The fermentation train consists of a cascade of four continuous fermenters. The major infection source is concentrated in the saccharification step. To avoid the development of contaminating bacteria, the cultivation of yeasts is carried out at a pH of 3.5, which requires the construction of stainless steel bioreactors (Madson and Monceaux, 1995). The temperature is maintained below 34°C by recirculating the culture medium through an external heat exchanger. Airlift bioreactors are utilized in these technological configura­tions. The total residence time of the fermenters system is about 48 h and obtains an ethanol concentration in the effluent of the last tank of about 9% by weight (12% by volume; McAloon et al., 2000). Some ethanol-producing plants using the dry-milling technology also employ cascade fermentation; in those cases, higher yields are obtained by simultaneous saccharification and fermentation of corn mash.