Fermentation I — Microorganisms
Fermentation is the biochemical transformation by which sugars are converted to ethanol, and carbon dioxide is also formed as a byproduct in this reaction. This step can be applied in the cellulosic ethanol production process in two different ways.
1. Separate Hydrolysis and Fermentation (SHF)
This classic configuration involves a sequential process where the hydrolysis of cellulose and fermentation are carried out in different units as shown in the scheme in Figure 8.1. In this route the first enzymatic or acid hydrolysis can be used to produce biomass hydrolyzate or C-5, C-6 sugar solution, as presented in Chapters 6 and 7. Secondly this sugar solution is subjected to fermentation in a separate vessel and this configuration is known as separate hydrolysis and fermentation (SHF).
Figure 8.2 Simultaneous saccharification and fermentation (SSF).
2. Simultaneous Saccharification and Fermentation (SSF)
In this alternative route, pretreated biomass is subjected to hydrolysis and fermentation in a single unit as shown in the scheme in Figure 8.2. This method can be carried out in two different ways: (1) using a mixture of biomass saccharification microorganisms and fermentation microorganisms, or, (2) using genetically engineered microorganisms that can do both saccharification and fermentation.