Fuel ethanol industry has advanced in SSF technology by incorporating the yeast propagation (from active dry yeasts) in the fermenter during initial saccharifi­cation, a process called simultaneous saccharification, yeast propagation, and fermentation (SSYPF), as indicated in Figure 9.1. High sugar concentrations are not achieved in the fermenter avoiding the inhibition of the enzymatic hydro­lysis that is characteristic for the amylases. Due to this, the bacterial growth is inhibited because of the lack of substrate caused by the immediate conversion of glucose into ethanol. Maintaining the pH, nutrients, and sterility in relation to bacteria, the complete conversion of available starch into ethanol is accom­plished. SSYPF technology has been utilized in several plants in North America mainly employing corn, milo (a variety of sorghum), and wheat (Madson and Monceaux, 1995).

The objective of this configuration is to increase the contact time between the mash and yeasts in order to reduce the bacterial growth. This allows reaching higher yields, an earlier ethanol production (that also reduces the contamination), and reducing the need of handling large amounts of yeasts (Novozymes & BBI International, 2005). At the beginning of an SSYPF process, pH is adjusted to 5.2 for favoring the growth of microorganisms and, as the cultivation goes forward, pH is diminished to 4.5 at the end of fermentation (Madson and Monceaux, 1995).