When cereals are used for producing fuel ethanol, the feedstock enters the pro­cess in the form of grains, which need to undergo some preliminary operations like washing and milling. As pointed out in Chapter 3, the milling of cereal grains like corn, wheat, and barley can be carried out by either the wet-milling or dry­milling process. In wet-milling technology only the starch enters the process for fuel ethanol production once it is separated from the rest of the grain components. In the case of corn, these components represent value-added products employed mostly for animal feed and human food. Moreover, part of the starch can be devi­ated toward the production of sweetening syrups like high-fructose corn syrup (HFCS). This type of milling technology was discussed in Chapter 3 for corn. Currently, 67% of ethanol produced in the United States is obtained in plants using the corn wet-milling technology. The ethanol yields for this process reach 403.1 L EtOH/ton corn (Gulati et al., 1996).

During the dry milling of cereals, the whole grain enters the ethanol produc­tion line, which means that the rest of its components are processed along with starch. The nonutilized components are accumulated in the bottom of the first distillation column and are concentrated as a co-product employed in animal feed. In the United States, 33% of ethanol is produced in plants employing the dry milling of corn, although other grains are used to a lesser degree. While the

dry-milling process generates co-products with lower value than the wet-milling process, this technology offers higher ethanol yields that can be in the range

419.4 to 460.6 L/ton (Gulati et al., 1996). Furthermore, the dry milling has lower capital and labor costs.

The dry milling of corn is made up of washing and milling the grains until they reach 3 to 5 mm. Then, some impurities are removed (Cardona et al., 2005). The milling is generally accomplished with the use of hammer mills. The starch from the ground grain should be gelatinized since the granules of native starch are not susceptible to enzymatic attack. To carry out this process, a starch suspension con­taining no more than 45% solids is prepared and cooked. To start the cooking pro­cess, the suspension is prepared in hot water (88°C). Madson and Monceaux (1995) emphasize that one of the key issues during the design and operation of production processes for ethanol production from starch is the elimination of contaminating bacteria which requires the maintenance of sterile conditions along the production line until the fermentation step. The decisive factor during the design of cooking systems is not that the starch cooks itself, but the elimination of bacteria. The over­all process for cooking and liquefying ground corn grains is presented in Figure 4.1, which is based on information provided by Madson and Monceaux (1995).

Considering that the conditions needed to reach the sterility are different from the cooking conditions, the preliminary cooking of the suspension of ground grains should be accomplished with minimum solubilization of the potential fermentable substances in order to avoid undesired reactions. These substances should be released only during the subsequent steps of liquefaction, saccharifica­tion, and fermentation. This is explained taking into account that a premature solubilization can lead to the risk of undesired reactions involving fermentable substances, such as sugars contained in the corn fiber. These secondary reactions can provoke the retrogradation of starch (crystallization of soluble starch after
cooling the gelatinized starch) or reactions between carbohydrates and amino acids causing the fermentable compounds to convert into nonfermentable com­pounds (Madson and Monceaux, 1995). In addition, these reactions also contrib­ute to an increased infection risk. Considering these issues, the size of the ground grain should be selected taking into account that the starch and sugars contained in the matrix of the grain particles have minimal mobility, but ensuring, in turn, a suitable hydration of such particles.

The resulting slurry undergoes instantaneous cooking in order to complete the gelatinization process, i. e., to reach the total solubilization of starch components (amylose and amylopectin) and the release of all fermentable substances. For this, jet cookers are employed. In these units, the initial heating is carried out by inject­ing steam directly to the slurry. In the jet cookers, the slurry is maintained at 105 to 110°C for 10 to 15 sec (Lopez-Munguia, 1993). The high temperatures and the mechanical forces allow a fast gelatinization by breaking up the starch granules. The maximum availability of these substances for fermentation with yeasts can be achieved if the process manages to keep the fermentable substances within the matrix of grain particles just until the moment in which the liquefaction is started.

During the liquefaction, thermoresistant a-amylase is used in order to hydro­lyze the starch slurry in a preliminary way that allows abruptly decreasing the viscosity and improving the system operation. This process is carried out in a tank at 80 to 90°C. Moreover, this hydrolysis process allows for the avoid­ance of starch retrogradation, which is a latent danger since the amylose and amylopectin are dissolved in the system. With the aim of preventing undesired secondary reactions, the liquefaction is accomplished in such a way that a mini­mum amount of starch is hydrolyzed, thereby avoiding its conversion into other nonfermentable substances before the fermentation is started (Madson and Monceaux, 1995). Among the products of the liquefaction step are the dextrins, which are oligosaccharides formed as a consequence of the partial breakdown of amylose and amylopectin chains. The stream leaving the liquefaction tank undergoes cooling to reach the optimal temperature for the following steps (sac­charification and fermentation).

Another variant of the process involves a preliquefaction step that is carried out during the cooking step. For this, 10% of the a-amylase dosage is added to the cooking tank. When the slurry is sent to the jet cooker, while the starch gran­ules are broken down, part of the starch chains begin their hydrolysis process. The gelatinized (cooked) corn slurry is cooled to 80 to 90°C, then the remaining dosage of a-amylase is added and the hydrolysis is kept at least 30 min in the liq­uefaction tank (Bothast and Schlicher, 2005). When the corn wet-milling technol­ogy is used, the starch obtained undergoes cooking and liquefaction in a similar way as the ground corn grains, though process conditions can vary slightly.

From the viewpoint of process synthesis, the different schemes for pretreat­ment of starchy materials can cause the evaluation of multiple alternative process configurations derived from a set of several combinations of methods and tech­nological procedures. For this, the simulation tools are of paramount importance. Unfortunately, mathematical models describing starch cooking and liquefaction

have not been effectively published in the open literature. This fact limits the quality of the related simulations and the possibility of involving a more detailed description of these steps during the application of process synthesis procedures to elucidate the best conditions for fuel ethanol production.