SSF technology born in the 1970s was assimilated by the starch-processing industry for ethanol production obtaining high and sustainable yields on the

order of 0.410 L/kg of corn (Madson and Monceaux, 1995). In the case of the saccharification step when starch is used as feedstock, glucoamylase experiences the inhibitory effect caused by glucose released as the hydrolysis of this bio­polymer advancement. This effect is more pronounced at high conversions of starch into ethanol. In contrast, integration by SSF makes it possible for yeasts to consume the glucose immediately as it forms under the action of the amylases on starch. In addition, the risk of bacterial contamination of the wort is dras­tically reduced because of the low level of glucose in the medium during the SSF process. The elimination of the external step of saccharification, the major source of infection by bacteria, also contributes to the reduction of contamination (Madson and Monceaux, 1995). In the same way, capital costs are reduced as a consequence of the increase in the compactness of the system (fewer numbers of units). Moreover, low glucose concentrations in the medium decrease the osmotic pressure over the yeasts because the use of concentrated solutions is avoided (Bothast and Schlicher, 2005). Energy costs can also be reduced considering that the SSF process is operated at temperatures less than those of the separate sac­charification process; this implies the reduction in the steam consumption. All these synergic features have allowed gains of ethanol yields higher than those of the SHF process.

The main disadvantage of the SSF process is that the optimum temperature of glucoamylase (65°C) does not coincide with the optimum temperature for yeast growth (30°C). Fortunately, starch saccharification can be carried out at 30 to 35°C although at a slower rate. For this reason, higher enzyme dosages are required. Finally, processing times for batch SSF are longer than the correspond­ing times for batch SHF.

Most ethanol production facilities utilizing the corn dry-milling technology employ batch SSF processes. The duration of this process is 48 to 72 h achiev­ing final ethanol concentrations in the medium of 10 to 12% by volume (Bothast and Schlicher, 2005). A number of modifications of the SSF of starchy materials have been proposed in order to decrease the production costs. Some of them are included in Table 9.1. Montesinos and Navarro (2000) have studied the possibility of utilizing raw wheat flour during the batch SSF process with the aim of reducing costs attaining a decrease in the process time. On the other hand, the SSF per­formed at a temperature above 34°C using a thermotolerant yeast, which enabled the reduction of cooling requirements and the improvement of the conversion process, as claimed in the patent of Otto and Escovar-Kousen (2004).