SSF Using Microorganisms that Can Do both Saccharification and Fermentation or Consolidated Bio-Processing (CBP)
A more recent development in simultaneous saccharification and fermentation (SSF) technique uses microorganisms that can do both saccharification and fermentation, thereby directly processing pretreated biomass to ethanol. This approach commonly involves four biochemical transformations:
1. Production of saccharolytic enzymes (cellulases and hemicellulases)
2. Hydrolysis of cellulose and hemicellulase present in pretreated biomass to sugars
3. Fermentation of hexose sugars (glucose, mannose and galactose)
4. Fermentation of pentose sugars (xylose and arabinose)
Since these four transformations occur in a single reactor and in one process configuration, this technique is known as consolidated bioprocessing (CBP). In recent years CBP has gained recognition as a promising bioethanol production system since the costs of capital investment, substance and other raw materials, and utilities associated with the production of cellulase enzyme can be avoided or reduced as these enzymes are generated by the same microorganism. However, one of the major drawbacks in the SSF process and CBP is the optimum temperature required for the saccharification and fermentation stages. The optimum temperature for saccharification with cellulolytic enzymes is around 50°C, while most fermenting microbes have a most favorable temperature for ethanol fermentation between 28°C and 37°C. In practice as well as following the current state of technology, it would be difficult to lower the optimum temperature of cellulases through genetic engineering. One possible answer to this problem could be the use of thermotolerant yeast strains that can ferment at higher temperatures as host for genetic manipulation of introducing saccharolytic enzyme producing genes.
There are two fundamental approaches for the construction of new microorganisms for consolidated bioprocessing (CBP) type simultaneous saccharification and fermentation process, which include:
1. Heterologous expression of cellulase genes in yeast.
2. Surface engineering of yeast strains to display cellu — lases on cell surface.
Interesting recent examples demonstrating these two approaches are discussed in Sections 8.7.3 and 8.7.4.